skating - 1060.bloomfieldweb.com
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Skating
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Observations about Skating
When you’re at rest on a level surface, without a push, you remain stationary
with a push, you start moving that direction
When you’re moving on a level surface, without a push, you coast steady & straight
with a push, you change direction or speed
5 Questions about Skating
1. Why does a motionless skater tend to remain motionless?
2. Why does a moving skater tend to continue moving?
3. How can we describe the motion of a coasting skater?
4. How does a skater start, stop, or turn?
5. Why does a skater need ice or wheels in order to skate?
Question 1
Q: Why does a motionless skater tend to remain motionless?
A: A body at rest tends to remain at rest
This observed behavior is known as inertia
Question 2
Q: Why does a moving skater tend to continue moving?
A: A body in motion tends to remain in motion
This behavior is the second half of inertia
Newton’s First Law (Version 1)
An object that is free of external influences moves in a straight line and covers equal distances in equal times.
Note that a motionless object obeys this law!
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Question 3
Q: How can we describe the motion of a coasting skater?
A: The skater moves at a constant speed in a constant direction
Physical Quantities
1. Position – an object’s location
2. Velocity – its change in position with time
Both are vector quantities: Position is distance and direction from a reference
Velocity is speed and direction of motion, relative to a reference
Newton’s First Law (Version 2)
An object that is free of external influences moves at a constant velocity.
Note that a motionless object is “moving” ata constant velocity of zero!
Another Physical Quantity
3. Force – a push or a pull
Force is another vector quantity: the amount and direction of the push or pull
Net force is the vector sum of all forces on an object
Newton’s First Law
An object that is not subject to any outside forces moves at a constant velocity.
Question 4
Q: How does a skater start or stop moving?
A: A net force causes the skater to accelerate!
4. Acceleration – change in velocity with time
5. Mass – measure of object’s inertia
Acceleration is yet another vector quantity: the rate and direction of the change in velocity
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Newton’s Second Law
An object’s acceleration is equal to the net force exert on it divided by its mass. That acceleration is in the same direction as the net force.
Traditional form: net force = mass acceleration
F = ma
Cause
Effect Resistance to cause
About Units
SI or “metric” units: Position → m (meters)
Velocity → m/s (meters-per-second)
Acceleration → m/s2 (meters-per-second2)
Force → N (newtons)
Mass → kg (kilograms)
Newton’s second law relates the units:
Question 5
Q: Why does a skater need ice or wheels to skate?
A: Real-world complications usually mask inertia
Solution: minimize or overwhelm complications
To observe inertia, therefore, work on level ground (minimize gravity’s effects)
use wheels, ice, or air support (minimize friction)
work fast (overwhelm friction and air resistance)
Summary about Skating
Skates can free you from external forces
When you experience no external forces, You coast – you move at constant velocity
If you’re at rest, you remain at rest
If you’re moving, you move steadily and straight
When you experience external forces You accelerate – you move at a changing velocity
Acceleration depends on force and mass
Falling Balls
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Observations about Falling Balls
When you drop a ball, it begins at rest, but acquires downward speed
covers more and more distance each second
When you tossed a ball straight up, it rises to a certain height
comes momentarily to a stop
and then descends, much like a dropped ball
A thrown ball travels in an arc
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6 Questions about Falling Balls
1. Why does a dropped ball fall downward?
2. How differently do different balls fall?
3. How would a ball fall on the moon?
4. How does a falling ball move after it is dropped?
5. How can a ball move upward and still be falling?
6. How does a ball's horizontal motion affect its fall?
Question 1
Q: Why does a dropped ball fall downward?
A: Earth’s gravity exerts a downward force on the ball
That force is the ball’s weight
That weight points toward earth’s center
Its weight causes the falling ball to accelerate downward—toward earth’s center
Question 2
Q: How differently do different balls fall?
A: Not differently. They all fall together!
A ball’s weight is proportional to its mass:
Question 2
Q: How differently do different balls fall?
A: Not differently. They all fall together!
A ball’s weight is proportional to its mass:
That ratio is equivalent to an acceleration:
Acceleration Due to Gravity
That ratio is the acceleration of a falling object!
It is called the acceleration due to gravity
Near earth’s surface, all falling balls accelerate downward at9.8 meter/second2
Question 3
Q: How would a ball fall on the moon?
A: It would fall more slowly.
Moon’s acceleration due to gravity is
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Question 4
Q: How does a falling ball move after it is dropped?
A: It accelerates downward, covering more distance each second
A falling ball experiences only its weight Its acceleration is constant and downward
Its velocity increases in the downward direction
When dropped from rest, the ball’s velocity starts at zero and
increases in the downward direction
the ball’s altitude decreases atan ever faster rate
Question 5
Q: How can a ball move upward and still be falling?
A: It may be moving upward, but it is still accelerating downward!
A falling ball accelerates downward, but its
initial velocity can be anything, even upward!
When thrown upward, ball’s velocity starts upward but
increases downward
ball’s altitude increases at an ever slower rate until…
velocity is momentarily zero
and then ball falls downward…
Question 6
Q: How does a ball's horizontal motion affect its fall?
A: It doesn’t. The ball falls vertically, but coasts horizontally.
Ball’s acceleration ispurely vertical (downward)
It falls vertically
It coasts horizontally
Its path is a parabolic arc
Summary About Falling Balls
Without gravity, an isolated ball would coast
With gravity, an isolated ball experiences its weight,
accelerates downward,
and its velocity becomes increasingly downward
Whether going up or down, it’s still falling
It can coast horizontally while falling vertically
Ramps
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Observations About Ramps
It’s difficult to lift a heavy wagon straight up
It’s easer to push a heavy wagon up a ramp
How hard you must push depends on the ramp’s steepness
The gentler the slope of the ramp, the smaller the push you must exert on the wagon
the longer the distance you must push the wagon to raise it upward
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5 Questions about Ramps
1. Why doesn’t a wagon fall through a sidewalk?
2. Why does a sidewalk perfectly support a wagon?
3. How does a wagon move as you let it roll freely on a ramp?
4. Why is it harder to lift a wagon up than to lower a wagon down?
5. Why is it easier to pull a wagon up a ramp than to lift it up a ladder?
Question 1
Q: Why doesn’t a wagon fall through a sidewalk?
A: The sidewalk pushes up on it and supports it.
The sidewalk and the wagon cannot occupy the same space
The sidewalk exerts a support force on the wagon that prevents the wagon from penetrating the sidewalk’s surface
acts perpendicular to the sidewalk’s surface: it is directed straight up
balances wagon’s downward weight
Question 2
Q: Why does a sidewalk perfectly support a wagon?
A: The sidewalk and wagon negotiate by denting and undenting.
The wagon and sidewalk dent one another slightly the more they dent, the harder they push apart
the wagon accelerates up or down in response to net force
the wagon bounces up and down during this negotiation
The wagon comes to rest at equilibrium—zero net force
The Wagon and Sidewalk Negotiate
If the wagon and sidewalk dent one another too much, each one pushes the other away strongly
the wagon accelerates away from the sidewalk
If the wagon and sidewalk dent one another too little, each one pushes the other away weakly (or not at all)
the wagon accelerates toward the sidewalk
If the wagon and sidewalk dent one another just right, the wagon is in equilibrium (zero net force)
Newton’s Third Law
For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.
Misconception Alert
The forces two objects exert on one another must be equal and opposite, but each force of that Newton’s third law pair is exerted on a different object, so those forces do not cancel one another.
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Question 3
Q: How does a wagon move as you let it roll freely on a ramp?
A: The wagon accelerates downhill.
The wagon experiences two forces: its weight and a support force
The sum of those forces is the ramp force: a small downhill net force
weight
support force
ramp force (vector sum)
Pushing the wagon up the Ramp
To start the wagon moving uphill push wagon uphill more than the downhill ramp force
net force is uphill, so wagon accelerates uphill
To keep the wagon moving uphill push wagon uphill just enough to balance ramp force
wagon continues uphill at constant velocity
To stop the wagon moving uphill, push wagon uphill less than the downhill ramp force
net force is downhill, so wagon accelerates downhill
Question 4
Q: Why is it harder to lift a wagon up than to lower a wagon down?
A: You do work on the wagon when you lift it.The wagon does work on you when you lower it.
Energy – a conserved quantity it can’t be created or destroyed
it can be transformed or transferred between objects
is the capacity to do work
Work – mechanical means of transferring energy
work = force · distance
(where force and distance are in same direction)
Transfers of Energy
Energy has two principal forms Kinetic energy – energy of motion
Potential energy – energy stored in forces
Your work transfers energy from you to the wagon Your chemical potential energy decreases
wagon’s gravitational potential energy increases
Question 5
Q: Why is it easier to pull a wagon up a ramp than to lift it up a ladder?
A: On the ramp, you do work with a small force over a long distance. On the ladder, you do work with a large force over a small distance.
For a shallow ramp: work = Force · DistanceFor a steep ramp: work = Force · Distance
For a ladder: work = Force · Distance
Mechanical Advantage
Mechanical advantage is doing the same work, using a different balance of force and distance
A ramp provides mechanical advantage You lift wagon with less force but more distance
Your work is independent of the ramp’s steepness
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Summary about Ramps
Ramp reduces the force you must exert to lift the wagon
Ramp increases the distance you must push to lift the wagon
You do work pushing the wagon up the ramp
The ramp provides mechanical advantage It allows you to push less hard
but you must push for a longer distance
Your work is independent of ramp’s steepness
Seesaws
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Observations about Seesaws
A balanced seesaw rocks back and forth easily
Equal-weight children sitting on the seats balance a seesaw
Unequal-weight children don’t normally balance the seesaw
Moving the heavier child toward the pivot restores the balance
Moving both children closer to the pivot speeds up the motion
6 Questions about Seesaws
1. How does a balanced seesaw move?
2. Why does a seesaw need a pivot?
3. Why does a lone seesaw rider plummet to the ground?
4. Why do the riders' weights and positions affect the seesaw's motion?
5. Why do the riders' distances from the pivot affect the seesaw's responsiveness?
6. How do the seesaw's riders affect one another?
Question 1
Q: How does a balanced seesaw move?
A: It moves at a constant rotational speed about a fixed axis in space
…because the seesaw has rotational inertia!
Physical Quantities
1. Angular Position – an object’s orientation
2. Angular Velocity – change in angular position with time
3. Torque – a twist or spin
All three are vector quantities Angular Position is the angle and rotation axis, relative to a reference
Angular Velocity is angular speed and rotation axis, relative to a reference
Torque is amount and rotation axis of a twist or spin
Net torque is the vector sum of all torques on an object
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Newton’s First Law of Rotational Motion
A rigid object that’s not wobbling and that is free of outside torques rotates at constant angular velocity.
Question 2
Q: Why does a seesaw need a pivot?
A: To prevent translational motion (i.e., falling)
The pivot is placed at the seesaw’s natural pivot, its center of mass
The pivot allows rotational motion, but not translation motional Translational motion is a change in position
Rotational motion is a change in angular position
Question 3
Q: Why does a lone seesaw rider plummet to the ground?
A: That rider’s weight twists the seesaw and thereby alters its motion
The weight of a lone rider produces a torque on the seesaw
torque = lever arm · forceperp(where the lever arm is a vector from the pivot to the location of the force)
That torque causes the seesaw to experience angular acceleration
4. Angular Acceleration – change in angular velocity with time another vector quantity
the rate and rotation axis of the change in angular velocity
Newton’s Second Lawof Rotational Motion
An object’s angular acceleration is equal to the net torque exerted on it divided by its rotational mass. The angular acceleration is in the same direction as the torque.
5. Rotational Mass – measure of rotational inertia
Question 4
Q: Why do the riders' weights and positions affect the seesaw's motion?
A: To balance the seesaw, their torques on it must sum to zero.
Adding a second rider adds a second torque The two torques act in opposite directions
If net torque due to gravity is zero, seesaw is balanced
A rider’s torque is their weight times their lever arm A heavier rider should sit closer to the pivot
A lighter rider should sit farther from the pivot
To cause angular acceleration, a rider leans or touches the ground
Question 5
Q: Why do the riders' distances from the pivot affect the seesaw's responsiveness?
A: Moving the riders toward the pivot reduces the seesaw’s rotational mass more than it reduces the gravitational torque on the seesaw
Lever arm is a vector from the pivot to the rider Gravitation torque is proportional to lever arm
Rotational mass is proportional to lever arm2
Angular acceleration is proportional to 1/lever arm
Moving the riders toward the pivot increases angular acceleration
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Question 6
Q: How do the seesaw's riders affect one another?
A: They exert torques on one another and also exchange energy.
Each rider experiences two torques about pivot: A gravitational torque produced by that rider’s weight
A torque exert by the other rider
For a balanced seesaw, the torques on each rider sum to zero
The riders exert equal but opposite torques on one another
Newton’s third law of rotational motion
For every torque that one object exerts on a second, there is an equal but oppositely directed torque that the second object exerts on the first.
Summary about Seesaws
A balanced seesaw experiences zero net torque
A balanced seesaw has a constant angular velocity
A non-zero net torque causes angular acceleration of the seesaw
Heavier riders need smaller lever arms
Lighter riders need larger lever arms
Wheels
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Observations about Wheels
Friction makes wheel-less objects skid to a stop
Friction can waste energy and cause wear
Wheels mitigate the effects of friction
Wheels can also propel vehicles
6 Questions about Wheels
1. Why does a wagon need wheels?
2. Why is sliding a box across the floor usually hardest at the start?
3. How is energy wasted as a box skids to a stop?
4. How do wheels help a wagon coast?
5. How do powered wheels propel a bicycle or car forward
6. How is energy present in a wheel?
Question 1
Q: Why does a wagon need wheels?
A: Friction opposes a wheel-less wagon’s motion
Frictional forces oppose relative sliding motion of two surfaces
act along the surfaces to bring them to one velocity
come in Newton’s third law pairs
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Question 2
Q: Why is sliding a box across the floor usually hardest at the start?
A: Static friction is usually stronger than sliding friction.
Static friction opposes the start of sliding varies in amount from zero to a maximum value
Sliding friction opposes ongoing sliding has a constant value proportional to support force
Static friction’s maximum usually exceeds sliding friction
Question 3
Q: How is energy wasted as a box skids to a stop?
A: That energy becomes thermal energy.
Only sliding friction wastes energy The two surfaces travel different distances
The missing work becomes thermal energy
The surfaces also experience wear
The Many Forms of Energy
Kinetic: energy of motion
Potential: stored in forces between objects Gravitational
Magnetic
Electrochemical
Nuclear
Thermal energy: disorder into tiny fragments Reassembling thermal energy is statistical impossible
Elastic
Electric
Chemical
Question 4
Q: How do wheels help a wagon coast?
A: Wheels can eliminate sliding friction.
Wheels & roller bearings eliminate sliding friction rollers eliminate sliding friction, but don’t recycle
simple wheels have sliding friction at their hub/axle
combining roller bearings with wheels is ideal
Question 5
Q: How do powered wheels propel a bicycle or car forward?
A: They use static friction to obtain a forward force from the ground.
As you or an engine exert torque on a powered wheel static friction from the ground produces an opposing torque
The two torques partially cancel, reducing the wheel’s angular acceleration
The ground’s static frictional force pushes the vehicle forward
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Practical Wheels
Free wheels are turned by the vehicle’s motion
Powered wheels propel the vehicle as they turn.
Question 6
Q: How is energy present in a wheel?
A: Kinetic energy, both translation and rotational.
For a translating wheel:
For a rotating wheel:
The wheel of a moving vehicle has both forms of kinetic energy!
Summary about Wheels
Sliding friction wastes energy Wheels eliminate sliding friction
A vehicle with wheels coasts well
Free wheels are turned by static friction
Powered wheels use static friction to propel car
Bumper Cars
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Observations about Bumper Cars
Moving cars tend to stay moving
Changing a car’s motion takes time
Impacts alter velocities and angular velocities
Cars often appear to exchange their motions
The fullest cars are the hardest to redirect
The least-full cars get slammed during collisions
5 Questions about Bumper Cars
1. Does a moving bumper car carry a force?
2. How is momentum transferred from one bumper car to another?
3. Does a spinning bumper car carry a torque?
4. How is angular momentum transferred from one bumper car to another?
5. How does a bumper car move on an uneven floor?
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Question 1
Q: Does a moving bumper car carry a force?
A: No, the bumper car carries momentum.
Momentum is a conserved vector quantity cannot be created or destroyed, but it can be transferred
has a direction (unlike energy)
has no potential form and cannot be hidden (unlike energy)
combines bumper car’s inertia and velocity
momentum = mass · velocity
Question 2
Q: How is momentum transferred from one bumper car to another?
A: The bumper cars push on one another for a period of time.
Bumper cars exchange momentum via impulses
impulse = force · time
When car1 gives an impulse to car2, car2 gives an equal but oppositely directed impulse to car1.
Individual momenta change as the result of an impulse
The total momentum doesn’t change
Car with least mass changes velocity most, so the littlest riders get pounded
Question 3
Q: Does a spinning bumper car carry a torque?
A: No, the bumper car carries angular momentum
Angular momentum is a conserved vector quantity cannot be created or destroyed, but it can be transferred
has a direction (unlike energy)
has no potential form and cannot be hidden (unlike energy)
combines bumper car’s rotational inertia and velocity
angular momentum = rotational mass· angular velocity
Question 4
Q: How is angular momentum transferred from one bumper car to another?
A: The bumper cars twist one another for a period of time.
Bumper cars exchange angular momentum via angular impulsesangular impulse = torque · time
When car1 gives an angular impulse to car2, car2 gives an equal but oppositely directed angular impulse to car1.
Individual angular momenta change as the result of an angular impulse
The total angular momentum doesn’t change
Car with least rotational mass changes angular velocity most.
Rotational Mass can Change
Mass can’t change, so the only way an object’s velocity can change is if its momentum changes
Rotational mass can change, so an object that changes shape can change its angular velocity without changing its angular momentum
Question 5
Q: How does a bumper car move on an uneven floor?
A: It accelerates in the direction that reduces its potential energy as quickly as possible
Forces and potential energies are related! A bumper car accelerates in the direction that reduces its total potential
energy as quickly as possible
The bumper car accelerates opposite to the potential energy gradient
On an uneven floor, that is down the steepest slope
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Summary about Bumper Cars
During collisions, bumper cars exchange momentum via impulses
angular momentum via angular impulses
Collisions have less effect on cars with large masses
cars with large rotational masses
Static Electricity
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Observations aboutStatic Electricity
Objects can accumulate static electricity
Clothes in the dryer often develop static charge
Objects with static charge may cling or repel
Static electricity can cause shocks
Static electricity can make your hair stand up
6 Questions aboutStatic Electricity
1. Why do some clothes cling while others repel?
2. Why do clothes normally neither cling nor repel?
3. Why does distance weaken static effects?
4. Why do clingy clothes stick to uncharged walls?
5. Why do clingy clothes crackle as they separate?
6. Why do some things lose their charge quickly?
Question 1
Q: Why do some clothes cling while others repel?
A: They carry electric charges that attract or repel.
Electric charges comes in two types: Charges of the same type (“like charges”) repel
Charges of different types (“opposite charges”) attract
Franklin named the types “positive” and “negative”
Charge is actually a single conserved quantity
Question 2
Q: Why do clothes normally neither cling nor repel?
A: Clothes normally have zero net charge.
Electric charge is intrinsic to some subatomic particles is quantized in multiples of the fundamental charge is +1 f.c. for a proton, –1 f.c. for an electron
Equal protons and electrons → zero net charge Objects tend to have zero net charge → neutral
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Charge Transfers
Contact can transfer electrons between objects Surfaces have different chemical affinities for electrons
One surface can steal electrons for another surface
Rubbing different objects together ensures excellent contact between their surfaces
significant charge transfer from one to the other.
A dryer charges clothes via these effects
Question 3
Q: Why does distance weaken static effects?
A: Forces between charges decrease as 1/distance2.
These electrostatic forces obey Coulomb’s law:
Electric charge is measured in coulombs
One fundamental charge is 1.6 10-19 coulombs
Question 4
Q: Why do clingy clothes stick to uncharged walls?
A: The charged clothes polarize the wall.
When a negatively charged sock nears the wall, the wall’s positive charges shift toward the sock,
the wall’s negative charges shift away from it,
and the wall becomes electrically polarized.
Opposite charges are nearest → attraction dominates
Question 5
Q: Why do clingy clothes crackle as they separate?
A: Separating opposite charges boosts voltages.
Charge has electrostatic potential energy (EPE)
Voltage measures the EPE per unit of charge Work raises the voltage of positive charge
Work lowers the voltage of negative charge
Voltage is measured in volts (joules/coulomb)
Question 6
Q: Why do some things lose their charge quickly?
A: Charge can escape through electric conductors.
Insulators have no mobile electric charges
Conductors have mobile electric charges, usually electrons (metals), but can be ions (salt water)
that will accelerate (and flow) toward lowest EPE
Conductors allow charges to cancel or escape
Summary aboutStatic Electricity
All objects contain countless charges
Objects can transfer charge during contact
Clothes often develop net charges during drying
Oppositely charged clothes cling to one another
and spark as separation raises their voltages.
Conductivity tends to let objects neutralize.
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Xerographic Copiers
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Observations About Copiers
Copiers consume colored powder or “toner”
After jams, you can wipe off the powder images
Copies are often warm after being made
Copies are sometimes clingy with static electricity
3 Questions aboutXerographic Copiers
1. How can light arrange colored powder on paper?
2. How does a copier spray charge onto a surface?
3. How does a copier make its copies permanent?
Question 1
Q: How can light arrange colored powder on paper?
A: That light can control static electricity.
In a xerographic copier or printer, charge sprayed onto insulating layer
opposite charge flows onto layer’s back
layer acts as a charged capacitor
light selectively erases charge
remaining charge attracts toner particles
toner particles are bonded to paper
Question 2
Q: How does a copier spray charge onto a surface?
A: It uses a corona discharge to charge the air
A fine wire having a large voltage ( or ) is covered with tightly packed “like” charges ( or )
repulsive forces are intense and push charges into air
this flow of charge into the air is a corona discharge
That discharge is caused by a strong electric field
Electric Field
Two views of electrostatic forces: Charge1 pushes on Charge2
Charge1 creates electric field that pushes Charge2
Electric field isn’t a fiction; it actually exists! a structure in space and time that pushes on charge
a vector field: a vector at each point in space and time
observed using a test charge at each point
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Voltage Gradient
A + test charge accelerates along electric field
path that reduces its total potential energy quickest
Voltage is electrostatic potential energy per charge decreasing voltage is decreasing potential energy
path of quickest potential decrease is voltage gradient
voltage gradient is essentially a slope of voltage
A voltage gradient is an electric field
Metals, Fields, & Corona Discharges
Inside a metal, charge can move At equilibrium: voltage is uniform, electric field is zero
Charge resides only on the metal’s surface
Outside a metal, charge cannot move At equilibrium: both voltage and electric field can vary
Near a thin wire or sharp point at large voltage, voltage varies rapidly with distance, so big electric field
charge is pushed into the air: a corona discharge
Question 3
Q: How does a copier make its copies permanent?
A: It fuses or melts the powder onto the paper.
Summary aboutXerographic Copiers
It sprays charge from a corona discharge
That charge precoats a special insulating surface
It projects a light onto surface
The charge escapes from illuminated regions
The remaining charge attracts toner particles
Those particles are fused to the paper as a copy
Flashlights
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Observations about Flashlights
They emit light when you switch them on
Brighter flashlights often have more batteries
Flashlights grow dimmer as their batteries age
Sometimes hitting a flashlight brightens it
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6 Questions about Flashlights
1. Why does a flashlight need batteries and a bulb?
2. How does power flow from batteries to bulb?
3. How does a flashlight’s switch turn it on or off?
4. How can a battery be recharged?
5. Why does a short-circuited flashlight get hot?
6. How do lightbulbs differ?
Question 1
Q: Why does a flashlight need batteries and a bulb?
A: Batteries power the bulb, which then emits light.
Batteries: chemical energy → electrostatic energy Bulb: electrostatic energy → light energy Since this energy transfer is ongoing, think power
Power is energy per unit of time The SI unit of power: 1 watt is 1 joule/second
Battery
Battery is chemically powered pump for charge pumps charge from its end to end develops a voltage rise from end to end
typically 1.5 volts for alkaline AAA, AA, C, and D cells typically 3 volts for lithium cells
does work pushing charge to higher voltage turns chemical potential into electrostatic potential
In a typical flashlight with two alkaline-cells total voltage rise in the battery chain is 3.0 V total electric power provided by batteries is 6 watts
Lightbulb Filament
Lightbulb filament lets charges flow through it carries charge from its entry end to its exit end develops a voltage drop from entry end to exit end receives energy from charge flowing to lower voltage turns electrostatic potential into thermal energy
In a typical flashlight with two alkaline-cells total voltage drop in lightbulb filament is 3.0 V total electric power consumed by lightbulb is 6 watts
Question 2
Q: How does power flow from batteries to bulb?
A: Power is carried by a current of charge in wires.
Current measures the rate of charge motion, electric charge crossing a boundary per unit of time,
The SI unit of current: 1 ampere is 1 coulomb/second
Batteries provide power to electric currents
Lightbulbs consume power from electric currents
The Direction of Current
Current is defined as the flow of positive charge but negative charges (electrons) carry most currents.
It’s difficult to distinguish between: Negative charges flowing to the right
Positive charges flowing to the left.
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Electric Current in a Flashlight
Current in the flashlight pumped from low voltage to high voltage by batteries
power provided = current · voltage rise
flows through a wire to the lightbulb
flows from high voltage to low voltage in lightbulbpower consumed = current · voltage drop
flows through a wire to the batteries
repeat… the current is traveling around a circuit
The current’s job is to deliver power, not charge!
About Wires and Filaments
Metals are imperfect conductors electric currents can’t coast through them electric fields are needed to keep currents moving
For a current to flow through a metal, that metal must have an electric field in it caused by a voltage drop and the associate gradient
Currents waste power in metal wires & filaments Wires waste as little power as possible Filaments waste much power and become very hot
Question 3
Q: How does a flashlight’s switch turn it on or off?
A: It opens or closes the circuit.
Steady current requires a “closed” circuit so charge loops and doesn’t accumulate anywhere
A flashlight’s electric circuit is closed (complete) when you turn the switch on
open (incomplete) when you turn the switch off
Question 4
Q: How can a battery be recharged?
A: Push current through it backward.
Battery provides power when current flows forward from end to end
current experiences a voltage rise
Battery receives power (and recharges) when current flows backward from end to end
current experiences a voltage drop
Effects of Current Direction
Batteries typically establish the current direction
Current direction doesn’t affect wires, heating elements, or lightbulb filaments,
Current direction is critically important to electronic components such as transistors and LEDs
and some electromagnetic devices such as motors.
Question 5
Q: Why does a short-circuited flashlight get hot?
A: Current bypasses the bulb and heats the wires.
If a conducting path bridges the filament, current bypasses the filament → circuit is “short”
there is no appropriate destination for the energy
energy loss and heating occurs in the wires
Short circuits can cause fires!
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Question 6
Q: How do lightbulbs differ?
A: They have different electrical resistances.
Current undergoes a voltage drop in a conductor
That voltage drop is proportional to the current:
voltage drop = resistance · currentwhere resistance is a characteristic of the conductor.
This relationship is known as Ohm’s law.
Resistance and Filaments
Batteries determine the filament’s voltage drop
The smaller a filament’s resistance, the more current it carries
the more electric power it consumes
A lightbulb filament is chosen to have enough resistance to limit power consumption
enough surface area to dissipate the thermal power
Summary about Flashlights
Current carries power from batteries to bulb
The switch controls the flashlight’s circuit
Current flows only when the circuit is closed
The batteries raise the current’s voltage
The lightbulb lower the current’s voltage
Household Magnets
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Observations aboutHousehold Magnets
They attract or repel, depending on orientation
Magnets stick only to certain metals
Magnets affect compasses
The earth is magnetic
Some magnets require electricity
5 Questions aboutHousehold Magnets
1. Why do any two magnets attract and repel?
2. Why must magnets be close to attract or repel?
3. Why do magnets stick only to some metals?
4. Why does a magnetic compass point north?
5. Why do some magnets require electricity?
21
Question 1
Q: Why do any two magnets attract and repel?
A: Each magnet has both north and south poles
Like magnetic poles repel, opposite poles attract
Magnetic pole is a conserved quantity North pole is a “positive” amount of pole.
South pole is a “negative” amount of pole.
The net pole on any object is always exactly zero!
Magnets
Unlike charges, free poles are never observed
A magnet always has equal north and south poles
It has magnetic polarization, but zero net pole A typical bar or button magnet is a magnetic dipole
A dipole has one north pole and one south pole
A fragment of a magnet has a net pole of zero it retains its original magnetic polarization
it is typically a magnetic dipole
Question 2
Q: Why must magnets be close to attract or repel?
A: Forces are weakened by distance and cancellation
The magnetostatic forces between poles are proportional to the amount of each pole
proportional to 1/distance2
Forces between Magnets
Each magnet has both north and south poles
Magnets simultaneously attract and repel
The net forces and net torques on magnets depend on distance and orientation
are typically dominated by the nearest poles
increase precipitously with decreasing distance
Question 3
Q: Why do magnets stick only to some metals?
A: Only a few metals are intrinsically magnetic.
Electrons have intrinsic magnetic dipoles In atoms, much of that magnetism remains active
In solids, that magnetism is usually cancelled perfectly
In a few solids, the cancellation is incomplete Iron and most steels are ferromagnetic materials
Refrigerators and Magnets
Ferromagnetic materials have magnetic domains Those domains ordinarily cancel on another
A magnet can alter those domains → magnetization
A magnet can magnetize a steel refrigerator it causes some domains to grow and others to shrink
the steel develops a net magnetic polarization
it attracts the magnetic pole that magnetized it
Magnets thus stick to steel refrigerators
22
Soft & Hard Magnetic Materials
Soft magnetic materials have domains the grow or shrink easily,
so they are easy to magnetize or demagnetize.
They quickly forget their previous magnetizations.
Hard magnetic materials have domains that don’t grow or shrink easily,
so they are hard to magnetize or demagnetize.
They can be magnetized permanently.
Question 4
Q: Why does a magnetic compass point north?
A: Earth’s magnetic field twists it northward.
The earth produces a magnetic field that pushes north poles northward, south poles southward
exerts torques on magnetic dipoles, such as compasses
A compass immersed in earth’s magnetic field aligns it so that its north pole points northward.
Magnetic Fields
A magnetic field is a structure in space and time that pushes on pole
a vector field: a vector at each point in space and time
observed using a north test pole at each point
Question 5
Q: Why do some magnets require electricity?
A: Electric currents are magnetic!
A current-carrying wire produces a magnetic field
A current-carrying coil mimics a bar magnet
An electromagnet typically uses an electric current to produce a magnetic field
to magnetize a ferromagnetic material
Electromagnetism (Version 1)
Magnetic fields are produced by magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields [for later…].
Electric fields are produced by electric charges and subatomic particles,
moving magnetic poles [for later…],
and changing magnetic fields [for later…].
Summary aboutHousehold Magnets
They all have equal north and south poles
They polarize soft magnetic materials and stick
They are surrounded by magnetic fields
Can be made magnetic by electric currents
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Electric Power Distribution
Turn off all electronic devices
Observations aboutElectric Power Distribution
Household electricity is alternating current (AC)
Household voltages are typically 120V or 240V
Power is distributed at much higher voltages
Power transformers are common around us
Power substations are there, but harder to find
4 Questions aboutElectric Power Distribution
1. Why isn’t power transmitted via large currents?
2. Why isn’t power delivered via high voltages?
3. What is “alternating current” and why use it?
4. How does a transformer transfer power?
Question 1
Q: Why isn’t power transmitted via large currents?
A: Too much power would be wasted in the wires.
Current-carrying wires consume and waste power power wasted = current · voltage drop in wire
voltage drop in wire = resistance · current (Ohm’s law)
power wasted = resistance · current2.
Large currents waste large amounts of power
Question 2
Q: Why isn’t power delivered via high voltages?
A: High voltage power is dangerous.
High voltages can produce large voltage gradients
Current may flow through unintended paths a spark hazard,
a fire hazard,
and a shock hazard.
The Voltage Hierarchy
Electric power delivered to a consumer ispower delivered = current · voltage drop
Large currents are too wasteful for transmission
High voltages are too dangerous for delivery
So electric power distribution uses a hierarchy: high-voltage transmission circuits in the countryside
medium-voltage circuits in cities
low-voltage delivery circuits in neighborhoods
Transformers transfer power between circuits!
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Question 3
Q: What is “alternating current” and why use it?
A: Fluctuating current → so transformers will work
In alternating current, the voltages of the power delivery wires alternate
and the resulting currents normally alternate, too.
AC and Transformers
Alternating voltage in the US completes 60 cycles per second,
so voltage and current reverse every 1/120 second.
AC complicates the design of electronic devices
AC permits the easy use of transformers, which can move power between circuits:
from a low-voltage circuit to a high-voltage circuit
from a high-voltage circuit to a low-voltage circuit
Question 4
Q: How does a transformer transfer power?
A: Its changing magnetic fields induce currents.
The primary coil carries an alternating current that produces an alternating magnetic field
that produces an electric field
that pushes current through the secondary coil
Power moves from primary coil to secondary coil
Electromagnetism (Version 2)
Magnetic fields are produced by magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields [more later…].
Electric fields are produced by electric charges and subatomic particles,
moving magnetic poles,
and changing magnetic fields.
Electromagnetic Induction
Moving poles or changing magnetic fields produce electric fields, which propel currents through conductors, which produce magnetic fields.
Changing magnetic effects induce currents Induced currents produce magnetic fields
Lenz’s Law
When a changing magnetic field induces a current in a conductor, the magnetic field from that current opposes the change that induced it
25
Transformers
Alternating current in one circuit can induce an alternating current in a second circuit
A transformer uses induction
to transfer powerbetween its circuits
but doesn’ttransfer any chargesbetween its circuits
Current and Voltage
A transformer must obey energy conservation
Power arriving in its primary circuit must equal power leaving in its secondary circuit
Since power is the product of voltage · current, a transformer can exchanging voltage for current
or current for voltage!
Step-Down Transformer
A step-down transformer has relatively few turns in its secondary coil
so charge is pushed a shorter distance
and experiences a smaller voltage rise
A larger currentat smaller voltageflows in thesecondary circuit
Step-Up Transformer
A step-up transformer has relatively many turns in its secondary coil
so charge is pushed a longer distance
and experiences a larger voltage rise
A smaller currentat larger voltageflows in thesecondary circuit
Power Distribution System
A step-up transformer increases the voltagefor efficient long-distance transmission
A step-down transformer decreases the voltagefor safe delivery to communities and homes
Inductor
Electric and magnetic fields both contain energy
Electromagnet has magnetic energy Stores energy as current increases
Releases energy as current decreases
Exhibits Lenz’s law Current change induces opposing current
Opposes any changes in current
Known as an inductor
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Summary aboutElectric Power Distribution
Electric power is transmitted at high voltages
Electric power is delivered at low voltages
Transformers transfer power between circuits
Transformers require AC power to operate
The power distribution system is AC
Radio
Turn off all electronic devices
Observations about Radio
It can transmit sound long distances wirelessly
It involve antennas
It apparently involves electricity and magnetism
Its reception depends on antenna positioning
Its reception weakens with distance
There are two styles of radio: AM and FM
3 Questions about Radio
1. How can a radio wave exist?
2. How is a radio wave emitted and received?
3. How can a radio wave represent sound?
Question 1
Q: How can a radio wave exist?
A: Electric and magnetic fields create one another.
Radio waves are electromagnetic waves: structures made only of electric and magnetic fields
they are emitted or received by charge or pole
they are self-sustaining while traveling, even in vacuum
their electric and magnetic fields recreate one another
Electromagnetism (Version 3)
Magnetic fields are produced by magnetic poles and subatomic particles,
moving electric charges,
and changing electric fields.
Electric fields are produced by electric charges and subatomic particles,
moving magnetic poles,
and changing magnetic fields.
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Structure of a Radio Wave
Electric field is perpendicular to magnetic field
Changing electric fieldcreates magnetic field
Changing magnetic fieldcreates electric field
Polarization of the waveis associated with thewave’s electric field
Question 2
Q: How is a radio wave emitted and received?
A: Accelerating charge ↔ electromagnetic wave
Accelerating charge causes electromagnetic wave It makes an electric field that changes with time
It makes a magnetic field that changes with time
and the two fields can form an electromagnetic wave
Electromagnetic wave causes accelerating charge Its electric field pushes on the charge
A Tank Circuit
For bigger wave, slosh charge in a tank
Tank circuit is a harmonic oscillator It consists of a capacitor and an inductor
Charge cycles through the circuit
Tank’s energy alternates between magnetic field in its inductor
electric field in its capacitor
Tank circuit can accumulate energy
Frequency set by capacitor & inductor
An Antenna is a Tank Circuit
An antenna is a straightened tank circuit!
Antenna’s frequency is set by its length
Resonant when it is ½ radio wavelength long
A conducting surface can act as half the antenna
Above a conductingsurface, antenna isresonant when it is¼ wavelength long
Emitting and Receiving Waves
A transmitter uses a tank circuit to slosh charge up and down its antenna,which acts as a second tank.
A receiver uses a tank circuit todetect charge sloshing on itstank-circuit antenna.
Transmitter antenna chargeaffects receiver antenna charge
Antenna orientations matter!
Question 3
Q: How can a radio wave represent sound?
A: Vary the wave to send sound information.
AM or “Amplitude modulation” Fluctuating amplitude conveys sound information
FM or “Frequency modulation” Fluctuating frequency conveys sound information
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AM Modulation
Information can be encoded as a fluctuating amplitude of the radio wave
The air pressure variationsthat are sound cause changesin the amount of chargemoving on the antenna andthus the intensity of the wave
The receiver detects thesechanges in radio wave intensity.
FM Modulation
Information can be encoded as a fluctuating frequency of the radio wave
The air pressure variationsthat are sound cause slightshifts in the frequency ofcharge motion on the antennaand the frequency of the wave
The receiver detects thesechanges in radio wave frequency.
Summary about Radio
Accelerating charges cause electromagnetic waves
Electromagnetic waves cause accelerating charges
Those waves are only electric and magnetic fields Accelerating charge on a transmitting antenna
produces a radio wave
that causes charge to accelerate on a receiving antenna
Radio waves can represent sound information
Microwave Ovens
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Observations About Microwaves
Microwave ovens cook food from inside out
They often cook foods unevenly
They don’t defrost foods well
You shouldn’t put metal inside them?!
Do they make food radioactive or toxic?
4 Questions aboutMicrowave Ovens
1. Why do microwaves cook food?
2. How does metal respond to microwaves?
3. Why do microwave ovens tend to cook unevenly
4. How does the oven create its microwaves?
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Question 1
Q: Why do microwaves cook food?
A: Water in the food responds to their electric fields.
Microwaves are a class of electromagnetic waves Long-wavelength EM waves: Radio & Microwave
Medium-wavelength: IR, Visible, UV light
Short-wavelength: X-rays & Gamma-rays
Microwaves have rapidly fluctuating electric fields.
Water Molecules
Water molecules are unusually polar
An electric field tends to orient water molecules
A fluctuating electric field causes water molecules to fluctuate in orientation
Microwave Heating
Microwaves have alternating electric fields
Water molecules orient back and forth
Liquid water heats due to molecular “friction”
Ice doesn’t heat due to orientational stiffness
Steam doesn’t heat due to lack of “friction”
Food’s liquid water content heats the food
Question 2
Q: How does metal respond to microwaves?
A: Currents flow back and forth in the metal.
Non-conductors polarize in the microwaves
Conductors carry currents in the microwaves Good, thick conductors reflect microwaves
Poor or thin conductors experience resistive heating
Sharp conductors initiate discharges in the air
Question 3
Q: Why do microwave ovens tend to cook unevenly?
A: Interference produces nonuniform electric fields.
Interference is when the fields add or cancel Adding fields are constructive interference
Canceling fields are destructive interference
Reflections lead to interference in a microwave
Most ovens “stir” the waves or move the food
Question 4
Q: How does the oven create its microwaves?
A: A magnetron tube radiates microwaves.
Magnetrons are vacuum tubes invented in WWII
Electrons travel through empty space obtaining power from a strong electric field
bent by a strong magnetic field and the Lorentz force
delivering power to an electromagnetic field
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Generating Microwaves
Magnetron tube has tank circuits in it
Streams of electrons amplify tank oscillations
A loop of wire extracts energy from the tanks
A short ¼-wave antenna emits the microwaves
Summary aboutMicrowave Ovens
They cook food because of its water content
Polar water molecules heat in microwave fields
Thin or sharp metals overheat or spark
The microwaves are produced by a magnetrons
Sunlight
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Observations about Sunlight
Sunlight appears whiter than most light
Sunlight makes the sky appear blue
Sunlight becomes redder at sunrise and sunset
It reflects from many surfaces, even nonmetals
It bends and separates into colors in materials
5 Questions about Sunlight
1. Why does sunlight appear white?
2. Why does the sky appear blue?
3. How does a rainbow break sunlight into colors?
4. Why are soap bubbles and oil films so colorful?
5. Why do polarizing sunglasses reduce glare?
Question 1
Q: Why does sunlight appear white?
A: We perceive 5800 K thermal light as “white”
Light is a class of electromagnetic waves Single-frequency light has a rainbow color
A thermal mixture of rainbow colors can look white
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Spectrum of Sunlight
Sunlight is thermal radiation—heat from the sun Charges in the sun’s hot photosphere jitter thermally
Accelerating charges emits electromagnetic waves
The sun emits a black-body spectrum at 5800 K
We perceive thermal light at 5800 K as “white”
Question 2
Q: Why does the sky appear blue?
A: Air particles Rayleigh-scatter bluish light best
Rayleigh scattering occurs when passing sunlight polarizes tiny particles in the air,
this alternating polarization reemits light waves, so
air particles scatter light—they absorb and reemit it.
Rayleigh Scattering
Air particles are so small that they are much less ½ the wavelength of light
poor antennas for light
scatter long-wavelengths (reds) particularly poorly
scatter short-wavelengths (violets) somewhat better
Rayleigh scattered sunlight appears bluish
Unscattered sunlight (solar disk) appears reddish
Effect is strongest at sunrise and sunset
Question 3
Q: How does a rainbow break sunlight into colors?
A: Rainbow colors take different paths in raindrops
Sunlight slows while it passes through matter Light waves electrically polarize the matter
That polarization delays and slows the light wave
Index of refraction = reduction factor for light’s speed
Index of refraction varies slightly with color
Light at Interfaces
When light changes speed at an interface, relationship between electric & magnetic fields changes
it refracts—its path bends as it cross the interface it bends toward the perpendicular if it slows down
it bends away from the perpendicular if it speeds up
it reflects—part of it bounces off the interface the reflection is almost perfect for metal surfaces
the reflection is partial for insulator surfaces
Light and Dispersion
The rainbow colors of light in sunlight have different frequencies
polarize material slightly differently
and therefore travel at slightly different speeds
Index of refraction depends slightly on color
Violet light usually travels slower than red light violet light usually refracts more than red
violet light usually reflects more than red
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Rainbows
Occur when sunlight encounters water droplets and undergoes refraction, reflection, and dispersion.
Question 4
Q: Why are soap bubbles and oil films so colorful?
A: They display color-dependent interference effects
Light waves following different paths can interfere
The two partial reflections froma soap or oil film can interfere
Different colors of light caninterfere differently
Question 5
Q: Why do polarizing sunglasses reduce glare?
A: Glare is mostly horizontally polarized light
Sunlight is a uniform mix of polarizations
When sunlight partially reflects at a shallow angle its different polarizations reflect differently
it becomes polarized—it is no longer a uniform mix
Polarizing sunglasses block horizontal polarization
Reflection of Polarized Light
Polarization affects angled reflections
When light’s electric field is parallel to a surface there is a large fluctuating surface polarization
and thus a strong reflection.
When electric field is perpendicular to a surface there is a small fluctuating surface polarization
and thus a weak reflection.
Glare is mostly polarized parallel to the surface
Polarization and Sunlight
Polarizing sunglasses block horizontally polarized light
and thus block glare from horizontal surfaces.
Rayleigh scattering has polarizing effects, so much of the blue sky is polarized light, too.
Polarizing sunglasses darken much of the sky
Summary about Sunlight
Sunlight is thermal light at about 5800 K
It undergoes Rayleigh scattering in the air
It bends and reflects from raindrops
It interferes colorfully in soap and oil films
It reflects in a polarizing fashion from surfaces
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Discharge Lamps
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Observations aboutDischarge Lamps
They often take moment to turn on
They come in a variety of colors, including white
They are often whiter than incandescent bulbs
They last longer than incandescent bulbs
They sometimes hum loudly
They flicker before they fail completely
5 Questions aboutDischarge Lamps
1. Why phase out incandescent lightbulbs?
2. How can colored lights mix so we see white?
3. Why does a neon lamp produce red light?
4. How can white light be produced without heat?
5. How do gas discharge lamps produce light?
Question 1
Q: Why phase out incandescent lightbulbs?
A: Because they waste too much electric power.
Incandescent lightbulb is a thermal light source with a relatively low filament temperature of 2700 K
It emits mostly invisible infrared light
Less than 10% of its thermal power is visible light
Non-thermal light sources can be more efficient
Question 2
Q: How can colored lights mix so we see white?
A: Primary colors of light trick our vision.
We have three groups of light-sensing cone cells Their peak responses are to red, green, and blue light
Those are therefore the primary colors of light
Mixtures of primary colorscan make us see any color
Question 3
Q: Why does a neon lamp emit red light?
A: Neon’s quantum structure dictates light emission
Electrons obey the rules of quantum physics In matter, electrons exist as quantum standing waves
three-dimensional patterns of nodes and antinodes
each wave “cycles” in place—it does not change with time
In atoms, those standing waves are called orbitals
In solids, those standing waves are called levels
Quantum structure dictates atom’s light emission
34
Quantum Physics
Classical physics (pre-1900) thought that everything in nature is a particle or a wave
electrons, atoms, and billiard balls are particles
light and sound are waves
Modern physics (post-1900) recognizes that everything in nature is both particle and wave
things are most wave-like when they are left alone
things are most particle-like when they interact
Electrons in Matter
Electrons exist in matter as quantum standing waves
have energies that are set by their specific waves
obey the Pauli exclusion principle:
No two indistinguishable Fermi particlesever occupy the same quantum wave
have two distinguishable states: spin-up or spin-down
can occupy each wave in 1’s or 2’s, but no more than 2
tend to occupy lowest energy waves, 2 per wave
Electrons in a Neon Atom
Electrons in any atom tend to settle into the lowest energy orbitals
cannot be more than 2 to an orbital
Lowest energy arrangement is atom’s ground state
Higher energy arrangements are atom’s excited states
In a neon atom, the nucleus has 10 protons
electrical neutrality requires it to have 10 electrons
ground state has electrons in 5 lowest-energy orbitals
Neon Discharge Lamps
Neon lamp metal electrodes inject free charges into dilute neon
plasma forms—a vapor of charged particles
electric field causes current to flow in the plasma
current is mostly electrons streaming toward positive
electrons often collide violently with neon atoms
Neon Lamps and Excited States
Collisions in the plasma occasionally ionize neon atoms, sustaining the plasma
cause electronic excitations of the neon atoms
In a neon atom, electrons normally occupy ground state orbitals
collisions can shift electrons to higher energy orbitals
light emission can return them to lower energy orbitals
Atoms interact with light via radiative transitions
Radiative transition that emits light is fluorescence
Light from Atoms
The quantum physics of light: Light travels as a wave (diffuse rippling fields)
Light is emitted or absorbed as a particle (a photon).
A photon carries a specific amount of energyPhoton energy = Planck constant · frequency
An atom’s orbitals differ by specific energies Orbital energy differences set the photon energies
Excited atom emits a specific spectrum of photons
35
Atomic Fluorescence
Photon energy is the difference in orbital energies Small energy differences infrared (IR) photons
Moderate energy differences red photons
Big energy differences blue photons
Even bigger energy differences ultraviolet (UV) photons Each atom has its own fluorescence spectrum
Neon’s fluorescence spectrum is dominated by red light
Question 4
Q: How can white light be produced without heat?
A: Synthesize the proper mixture of primary colors.
Fluorescent tubes use a discharge in mercury gas to produce UV light
UV light causes phosphors on the tube wall to glow
phosphors synthesize white light from primary colors
Phosphors
A mercury discharge emits mostly UV light A phosphor can convert UV light to visible
Absorb a UV photon, emit visible photon. Missing energy usually becomes thermal energy.
Fluorescent lamps use white phosphors They imitate thermal whites at 2700 K, 5800 K, etc.
Specialty lamps use colored phosphors Blue, green, yellow, orange, red, violet, etc.
Starting Fluorescent Lamps
Starting a discharge requires electrons in the gas
Those electrons can be injected into the gas by heated filaments with special coatings
or by high voltages
Once discharge starts, it can sustain the plasma
Starting the discharge damages the electrodes Atoms are sputtered off the electrodes
Damage limits the number of times a lamp can start
Stabilizing Fluorescent Lamps
Gas discharges are electrically unstable Gas is initially insulating
Once discharge is started, gas become a conductor
The more current it carries, the better it conducts
Current tends to skyrocket out of control
Stabilizing discharge requires ballast Inductor ballast (old, 60 Hz, tend to hum)
Electronic ballast (new, high-frequency, silent)
Question 5
Q: How do gas discharge lamps produce light?
A: The discharge emits atomic fluorescence light
36
Low-Pressure Discharge Lamps
Mercury gas has its resonance line in the UV Low-pressure mercury lamps emit mostly UV light
Some gases have resonance lines in the visible
Low-pressure sodium vapor discharge lamps emit sodium’s yellow-orange resonance light,
so they are highly energy efficient
but extremely monochromatic and hard on the eyes.
High Pressure Effects
High pressures broaden each spectral line Collisions occur during photon emissions,
so frequency and wavelength become smeared out.
Collision energy shifts the photon energy
Radiation trapping occurs at high atom densities Atoms emit resonance radiation very efficiently
Atoms also absorb resonance radiation efficiently
Resonance radiation photons are trapped in the gas
Energy must escape discharge via other transitions
High-Pressure Discharge Lamps
At higher pressures, new spectral lines appear High-pressure sodium vapor discharge lamps
emit a richer spectrum of yellow-orange colors, are still quite energy efficient, but are less monochromatic and easier on the eyes.
High-pressure mercury discharge lamps emit a rich, bluish-white spectrum, with good energy efficiency. Adding metal-halides adds red to improve whiteness.
Summary aboutDischarge Lamps
Thermal light sources are energy inefficient
Discharge lamps produce more light, less heat
They carefully assemble their visible spectra
They use atomic fluorescence to create light
Some include phosphors to alter colors
LEDs and Lasers
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Observations about LEDs and Lasers
LEDs and Lasers often have pure colors
LEDs can operate for years without failing
Lasers produce narrow beams of intense light
Lasers are dangerous to eyes
Reflected laser light has a funny speckled look
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6 Questions aboutLEDs and Lasers
1. Why can’t electrons move through insulators?
2. How does charge move in a semiconductor?
3. Why does a diode carry current only one way?
4. How does an LED produce its light?
5. How does laser light differ from regular light?
6. How does a laser produce coherent light?
Question 1
Q: Why can’t electrons move through insulators?
A: Electrons can’t easily change levels in insulators.
Electrons obey the rules of quantum physics In matter, electrons exist as quantum standing waves
three-dimensional patterns of nodes and antinodes
each wave “cycles” in place—it does not change with time
In solids, those standing waves are called levels
To move, electrons must be able to switch levels
In an insulator, electrons can’t easily change levels
Electrons in Solids
Electrons settle into a solid’s lowest-energy levels
Levels clump into energy bands, leaving gaps
Fermi level: between last filled and first unfilled
Metals
In a metal, the Fermi level has empty levels just above it in energy
electrons near the Fermi level can change levels easily
electrons can move in response to electric fields
The electrons are like patrons in a partly filled theatre
Current can flow through a metal
Insulators
In an insulator, the Fermi level has no empty levels nearby
electrons can’t move in response to electric fields
The electrons are like patrons in a completely filled theatre
Current can’t flow through an insulator
Question 2
Q: How does charge move in a semiconductor?
A: A semiconductor is an insulator with a small gap
Pure semiconductors are “poor insulators” empty conduction band is just above full valence band
The electrons are like patrons in a theater with a full ground floor but a low empty balcony
Light or heat can allow current in pure semiconductor
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Doped Semiconductors
Dopant atoms can add or remove electrons
A p-type semiconductor has fewer electrons than normal
has some empty valence levels
An n-type semiconductor has more electrons than normal
has some filled conduction levels
Question 3
Q: Why does a diode carry current only one way?
A: Its p-n junction is asymmetric and special
When p-type and n-type semiconductors touch they have different affinities for electrons
electrons migrate from the n-type to p-type
leaving a depletion region with no mobile charges
That junction can conduct current only one way
pn-Junction (before contact)
Before p-type semiconductor meets n-type, each material can conduct electricity
each material is electrically neutral throughout
pn-Junction (after contact)
After p-type semiconductor meets n-type, electrons migrate from the n-type to the p-type
an insulating depletion region appears at junction
depletion region develops an electric field
Forward in the Diode
When electrons enter n-type and exit p-type, depletion region’s electric field is cancelled away
depletion region shrinks
diode can conduct current
Reverse in the Diode
When electrons enter p-type and exit n-type, depletion region’s electric field is reinforced
depletion region grows
diode cannot conduct current
39
Question 4
Q: How does an LED produce its light?
A: Electrons emit light while changing bands
LEDs are Light-Emitting Diodes LED is a diode and has a pn-junction
Electrons cross junction in the conduction band
Electrons dropping into the valence band emit light Electron briefly orbits the empty valence level
Electron drops into valence level via a radiative transition
The larger the band gap, the bluer the light
Question 5
Q: How does laser light differ from regular light?
A: Laser light is a single electromagnetic wave.
Most light sources produce photons randomly Each photon usually has its own wave
Laser light involves duplicate photons Laser amplification duplicates an initial photon
Each photon becomes part of a single giant wave
Spontaneous Emission
Excited atoms normally emit light spontaneously
These photons are uncorrelated and independent
Each photons has its own wave mode
These independent waves are incoherent light
Stimulated Emission
Excited atoms can be stimulated into duplicating passing light
These photons are correlated and identical
The photons all have the same wave mode
This single, giant wave iscoherent light
Question 6
Q: How does a laser produce coherent light?
A: A laser amplifier duplicates an initial photon.
Excited atom-like systems can act as laser medium Duplicate photons they’re capable of emitting
Duplication is perfect: the photons are true clones
Laser Oscillation
A laser medium can amplify its own light A laser medium in a resonator acts as an oscillator
It duplicates its one of its own spontaneous photons
Duplicated photons leak from semitransparent mirror
The photons from this oscillator are identical
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Summary about Lasers and LEDs
Lasers produce coherent light by amplification
Coherent light contains many identical photons
Laser amplifiers and oscillators are common
LEDs are incoherent, light-emitting diodesCameras
Turn off all electronic devices
Observations about Cameras
They record a scene on an image sensor
Good cameras need focusing, cheap ones don’t
Many cameras have zoom lenses
Some cameras have bigger lenses than others
They have ratings like focal length and f-number
5 Questions about Cameras
1. Why does a camera need a lens?
2. Why do most camera lenses need focusing?
3. Why are lenses telephoto or wide-angle?
4. Why do fancy lens’s have internal apertures?
5. Why is a good camera lens so complex inside?
Question 1
Q: Why does a camera need a lens?
A: Lens bends rays from one point to one point.
An illuminated object reflects or scatters light The object’s light produces diffuse illumination
A converging lens bends light rays via refraction Light rays spreading from a point converge to a point
Real Images
An image forms in space on far side of the lens The image is a pattern of light in space that exactly resembles the object,
except for size and orientation
The image is “real” – you can put your hand in it
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Question 2
Q: Why do most camera lenses need focusing?
A: So that the real image forms on the image sensor.
The sensor records the pattern of light it receives
When focused, the real image forms on the sensor
Focusing
Distant object’s light diverges slowly Real image forms near to the lens
Nearby object’s light diverges quickly Real image forms far from the lens
Lens can only focus perfectlyon one object at a time
Lens Diameter and Focusing
Larger lens gathers more light so the image is brighter
but focus is more critical
and there is less depth of focus.
Smaller lens gathers less light so the image is dimmer
but focus is less critical
and there is more depth of focus.
Question 3
Q: Why are lenses telephoto or wide-angle?
A: Lens’ focal length (FL) determines image size
Focal length measures lens’s converging strength Long FL: long image distance, large dim image
Short FL: short image distance, small bright image
Question 4
Q: Why do fancy lenses have internal apertures?
A: To vary image brightness and depth of focus
f-number is focal length divided by lens diameter Small f-number: bright image, small depth of focus
Large f-number: dim image, large depth of focus
Sophisticated lenses have adjustable f-numbers
Question 5
Q: Why is a good camera lens so complex inside?
A: To allow zooming and to correct image flaws.
Zooming: adjustable focal length using complex lenses
Dispersion different colors focus differently
Reflections fog in photographic images
Spherical aberration imperfect focus of spherical lens
Poor focusing off axis coma distortions
Spherical focus projected on flat film astigmatism
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Zoom Lenses
A zoom lens typically forms three images overall Its first lens group produces a real image
Its second lens groupprojects a resized image
Its third lens groupprojects an image ontothe image sensor
Summary about Cameras
They use converging lenses to form real images
Lens focal length sets image size
Lens f-number sets image brightness
The image sensor records the pattern of light
Optical Recording and Communications
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Observations about Optical Recording and Communications
Optical disks can store lots of audio or video
That audio or video is of the highest quality
Optical disks continue to play perfectly for years
Playback of optical disks involves lasers
Lasers and fibers are used in communication
5 Questions about Optical Recording and Communication
1. How is information represented digitally?
2. How is information recorded on an optical disk?
3. How is information read from an optical disk?
4. How can light carry information long distances?
5. Why does light follow an optical fiber’s bends?
Question 1
Q: How is information represented digitally?
A: As a sequence of digital symbols
Audio or video info is a sequence of numbers Each number is represented as digital symbols
Offers good noise-immunity Permits error correction to further reduce noise
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Digital Audio
The air pressure in sound is measured thousands of times per second
Each measurement is represented digitally using about 16 binary digits, each a 0 or a 1.
Question 2
Q: How is information recorded on an optical disk?
A: As reflective symbols in a spiral track.
Symbols are lengths/spacing of reflective regions
Symbols are as small as can be detected optically Laser wavelength determines symbol density
Question 3
Q: How is information read from an optical disk?
A: A focused laser beam reflects from the disk.
Diode laser light is focused on the shiny layer
Reflected light is detected by photodiodes
Playback Issues
Laser light must focus exactly on the symbols
Light wave forms a “waist”—its minimum width Wave limits on focusing are known as diffraction
Waist can’t be much smaller than a wavelength
Symbols can’t be much smaller than a wavelength
Question 4
Q: How can light carry information long distances?
A: Changes in that light can represent information.
Analog representations such as AM modulation often used for remote process monitoring.
Digital representations such as pulse codes pulses with discrete amplitudes used as symbols
provides noise-immunity, error correction, compression, and channel-sharing.
Question 5
Q: Why does light follow an optical fiber’s bends?
A: The light is trapped by total internal reflection.
As light enters a material with alower index of refraction, it bendsaway from the perpendicular
If bend exceeds 90°, light reflectsperfectly: total internal reflection.
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Optical Fibers
High-index glass core in a low-index glass sheath Light in core encounters interface at shallow angle
Light experiences total internal reflection
Light is trapped; it bounces endless through the core
Light travels for many kilometers in ultrapure fibers
Pulse spreading can limit information bandwidth Minimize light path difference: single mode fibers
Minimize dispersion: operate at dispersion minimum
Optical Fiber Types
Summary about Optical Recording and Communication
Optical disks store information as pits and flats
Focused laser light reads that information
Digital representations allow perfect playback
Optical fibers carry information as lightAudio Players
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Observations about Audio Players
They are part computer, part sound system.
They require electric power, typically batteries.
They reproduce sound nearly perfectly.
They are sensitive to static charge.
4 Questions about Audio Players
1. How does an audio player “store” sound?
2. How does it move sound information around?
3. How does the audio player’s computer work?
4. How does the audio player’s amplifier work?
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Question 1
Q: How does an audio player “store” sound?
A: It represents that sound as digital information
It uses representations of sound information, sequences of air pressure measurements
that contain everything needed to recreate the sound.
Recording and recreating are done in analog form
Storing and retrieving are done in digital form
Analog Representation
One physical quantity represents one number
Any continuous physical quantity can be used: the voltage on a wire,
the current in a circuit,
the strength of a permanent magnet.
This direct representation is sensitive to noise
Analog representations are “imperfect.”
Digital Representation
A group of “symbols” represents a number
A symbol can be any discrete physical quantity: a positive or negative charge on a capacitor
an integer value of voltage on a wire
a north or south magnetic pole on a magnet
This indirect representation is insensitive to noise
Digital representations can be “perfect.”
Question 2
Q: How does it move sound information around?
A: It uses MOSFET electronic switches.
A MOSFET Transistor consists of two back-to-back pn-junctions
with a nearby “gate” surface that can store charge.
Gate charge controls current flow in MOSFET
MOSFET Transistor Off
A typical MOSFET Transistor normally has a vast depletion region in its “channel”
normally can’t conduct electric current
MOSFET Transistor On
Charging that MOSFET’s gate alters the filling of electron levels in the channel
so the depletion region vanishes
and the device can conduct electric current.
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Question 3
Q: How does the audio player’s computer work?
A: It uses MOSFETs to form logic elements.
Computers perform logical operations with bits A bit is a base-two digit
It can hold one of only two symbols: 0 or 1.
Bit-wise representation of numbers is called binary
MOSFET logic elements manipulate bits
Logic Elements
Inversion (the NOT logic element) One input bit, one output bit
Output bit is inverse of input bit
Not-And (the NAND logic element) Two input bits, one output bit
Output bit is inverse “and” of input bits
Any function and thus any computercan be built from these two logic elements
CMOS Logic Elements
Bits are represented by charge (1 is +, 0 is zero)
Uses complementary MOSFETs n- and p-channel MOSFETs are paired
CMOS Inverter has 2 MOSFETS
CMOS NAND has 4 MOSFETS
Question 4
Q: How does the audio player’s amplifier work?
A: It uses MOSFETs as analog amplifiers.
MOSFET lets a tiny charge control a big current
Amplifier has three circuits: Input current represents sound
Output current is amplified version
Power current provides power
Summary about Audio Players
Represent sound in digital and analog forms
Use MOSFETs to work with sound information
Digital computer comprised of CMOS logic
Analog amplifier based on MOSFETs.Nuclear Weapons
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Observations aboutNuclear Weapons
They release enormous amounts of energy
They produce incredible temperatures
They produce radioactive fallout
They are relatively difficult to make
They use chain reactions
4 Questions aboutNuclear Weapons
1. Where is nuclear energy stored in atoms?
2. Why are some atomic nuclei unstable?
3. How does a nuclear chain reaction work?
4. Why do nuclear explosions produce fallout?
Question 1
Q: Where is nuclear energy stored in atoms?
A: In each atom’s central nugget, its nucleus.
Each atom has an ultra-dense core or nucleus Extremely small (1/100,000th of atom’s diameter)
Contains about 99.95% of the atom’s mass
Contains most of the atom’s potential energy Evidence is related to: E=mc2
Structure of Nucleus
Nucleus contains two kinds of nucleons Protons are positively charged
Neutrons are electrically neutral
Two forces are active in a nucleus Electrostatic repulsion between protons
Nuclear force attraction between touching nucleons
At short distances, the nuclear force dominates
At long distances, the electric force dominates
Question 2
Q: Why are some atomic nuclei unstable?
A: They can release potential energy by breaking
In a stable nucleus, all objects remain in place For classical stability: stable equilibrium is required
For quantum stability: potential energy minimum
Objects quantum-tunnel out of unstable nuclei
Quantum Tunneling
Each object in a nucleus is in equilibrium
Classical object must climb potential hill to escape Without extra energy, object is trapped
Classical nucleus would be stable
Quantum object can escape without climbing hill Heisenberg uncertainty principle makes
position of wave-particle uncertain
Particle-wave can “tunnel” through potential hill
Objects can tunnel out of unstable nuclei
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Radioactivity
Large nuclei have two possible problems: Too many protons: too much electrostatic potential
Too many neutrons: isolated neutrons are unstable
Balance between protons and neutrons is tricky
Large nuclei tend to fall apart spontaneously Known as radioactive decay
Such decay is statistical, with a characteristic half-life
May include a splitting process called fission
Question 3
Q: How does a nuclear chain reaction work?
A: Decaying nuclei can shatter one another.
Radioactive decay releases tiny projectiles
Projectiles can collide with and agitate other nuclei
Agitated nuclei can decay, releasing new projectiles
and this can repeat, over and over again
Induced Fission
A large nucleus may break when struck Collision knocks its nucleons
out of equilibrium
Collision-altered nucleusmay undergo induced fission
Since a neutron isn’t repelledby nucleus, it makes an idealprojectile for inducing fission
Chain Reaction
Since neutrons can induce fission and induced fission releases neutrons,
this cycle can repeat,a chain reaction!
Each fission releases energy Many fissions release
prodigious amounts of energy
Sudden energy releaseproduces immense explosion
Requirement for a Bomb
A fission bomb requires 4 things: An initial neutron source
a fissionable material (undergoes induced fission)
each fission must release additional neutrons
material must use fissions efficiently (critical mass)
235U and 239Pu are both fissionable materials, but 235U is rare and must be separated from 238U
and 239Pu is made by exposing 238U to neutrons.
The Gadget & Fat Man
Each of these fission bombs started as a 239Pu sphere below critical mass (6 kg)
It was crushed explosively to supercritical mass
and promptly underwentan explosive chain reaction.
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Little Boy
This bomb started as a 235U hollow sphere below critical mass (60 kg)
A cannon fired a 235U plug through that sphere so that it exceeded critical mass
and it promptlyunderwent anexplosive chainreaction.
Question 4
Q: Why do nuclear explosions produce fallout?
A: Many fission-fragment nuclei are radioactive.
Large nuclei are neutron-rich High proportion of neutrons dilutes proton repulsion
Fragments of large nuclei are still neutron-rich They have too many neutrons for their size
They tend to be radioactive
Radioactive Fallout
Fission-fragment nuclei gather electrons Electrons accumulate until neutral atoms form
Atoms are chemically identical to ordinary atoms
But atoms have unstable nuclei—too many neutrons
We incorporate those atoms into our bodies Iodine-131 (8-day half-life)
Cesium-137 (30-year half-life)
Strontium-90 (29-year half-life)
Radioactive atoms decay, damaging molecules
Summary aboutNuclear Weapons
Nuclear energy is stored in atomic nuclei
Nuclear fission released electrostatic potential
Each fission releases an astonishing energy
Induced fission permits a chain reaction
Fission bombs explode via a chain reaction
Fission fragment nuclei form radioactive atoms
Nuclear Reactors
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Observations aboutNuclear Reactors
They provide enormous amounts of energy
They consume nuclear fuel
They produce radioactive waste
Their nuclear fuel is sometimes reprocessed
They have safety issues
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5 Questions aboutNuclear Reactors
1. How can a nuclear reactor use natural uranium?
2. How do thermal fission reactors work?
3. How can a fission chain reaction be controlled?
4. How is energy extracted from a nuclear reactor?
5. What are the safety issues with nuclear reactors?
Question 1
Q: How can a nuclear reactor use natural uranium?
A: Slow the fission neutrons to thermal speeds.
Slow-moving (thermal) neutrons can fission the fissionable portion of uranium
are ignored by the nonfissionable portion of uranium
Slow-moving neutrons can fission natural uranium
A chain reaction is possible in natural uranium
Uranium Isotopes
Uranium-235 (235U - 92 protons, 143 neutrons) is radioactive – fissions and emits neutrons
fissionable – breaks when hit by neutrons
a rare fraction of natural uranium (0.72%)
Uranium-238 (238U - 92 protons, 146 neutrons) is radioactive – emits helium nuclei, some fissions
nonfissionable – absorbs fast neutrons without fission
a common fraction of natural uranium (99.27%)
Natural Uranium
Fissioning uranium nuclei emit fast neutrons Fast neutrons can fission 235U,
Fast neutrons are strongly absorbed by 238U.
Natural uranium contains mostly 238U, with some 235U
chain reactions won’t work in natural uranium
Thermal (Slow) Neutrons
Slow neutrons affect uranium isotopes differently Slow neutrons can still fission 235U
Slow neutrons don’t interact with 238U
A 235U chain reaction can occur in natural uranium if its neutrons are slowed by a moderator
Moderator nuclei small nuclei that don’t absorb colliding neutrons
extract energy and momentum from the neutrons
slow neutrons to thermal speeds
Question 2
Q: How do thermal fission reactors work?
A: Using moderators, they “burn” ordinary uranium.
Reactor core is natural or slightly enriched uranium
Moderator is interspersed throughout core
Moderator slows neutrons to thermal speeds
Nuclear chain reaction occurs only among 235U
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Choosing Moderators
Hydrogen nuclei (protons) Near-perfect mass match with neutrons
Outstanding energy and momentum transfer
Slight possibility of absorbing neutrons
Deuterium nuclei (heavy hydrogen isotope) Excellent mass match with neutrons
Excellent energy and momentum transfer
No absorption of neutrons
More about Moderators
Carbon Adequate mass match with neutron
Adequate energy and momentum transfer
Little absorption of neutrons
Choosing a moderator Deuterium is best, but it’s rare (used as heavy water)
Hydrogen is next best (used as ordinary water)
Carbon is acceptable and a convenient solid
Question 3
Q: How can a fission chain reaction be controlled?
A: Use feedback to operate right at critical mass.
Critical mass is governed by size and structure of reactor core
type of nuclear fuel (extent of 235U enrichment)
location and quality of moderator
positions of neutron-absorbing control rods
accumulation of fission fragment nuclei
Controlling Chain Reactions (Part 1)
Critical mass governs chain reaction rate Below it, fission rate diminish with each generation
Above it, fission rate increases with each generation
Generation rate of prompt neutrons is very short
Controlling prompt-neutron fission is difficult!
Delayed neutrons make reaction controllable Some fissions produce short-lived radioactive nuclei
These radioactive nuclei emit neutrons after a while
Delayed neutrons contribute to the chain reactions
Controlling Chain Reactions (Part 2)
There are two different critical masses Prompt critical: prompt neutrons sustain chain reaction
Delayed critical: prompt and delayed neutrons required
Reactors operate Below prompt critical mass
Above delayed critical mass
Control rods can adjust above or below criticality
Question 4
Q: How is energy extracted from a nuclear reactor?
A: It becomes heat and operates heat engines.
A nuclear reactor releases thermal power Fissions release thermal energy into the reactor core
Thermal energy is extracted by a coolant
Coolant is used to power a heat engine (e.g., steam)
Heat engine is used to generate electrical power
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Question 5
Q: What are the safety issues with nuclear reactors?
A: Preventing the release of radioactive nuclei.
Fission reactors produce radioactive nuclei (from fissions)
plutonium (from neutron capture by 238U)
Containing those nuclei forever is a challenge
Long Term Safety Issues
Radioactive nuclei are a radiation hazard They cannot be made safe, they can only be stored
They must be sequestered securely for eons
Plutonium is a nuclear proliferation hazard It can be used as a nuclear fuel in reactors
It can also be used in weapons
Short Term Safety Issues
To avoid catastrophes, chain reactions must never get out of control
reactors and spent fuel must never overheat
Reactors must be designed so that they are stable—chain reaction stops if overheating occurs
redundant—no single failure can cause a disaster
easy to quench—chain reaction stops immediately
Nuclear Accidents (Part 1)
Windscale Pile 1 (Britain) Carbon moderator burned while being annealed
Three Mile Island (US) Cooling pump failed and core overheated (while off)
Chernobyl Reactor 4 (USSR) Coolant boiled in overmoderated graphite reactor
Exceeded prompt critical and partially vaporized
Tokia-mura Accident (Japan) Critical mass was reached while processing uranium
Nuclear Accidents (Part 2)
Fukushima Daiichi (Japan) Earthquake caused reactors to be shut down
Tsunami disabled cooling pumps
Cooling water in reactors and spent fuel storage boiled
Reactor cores and spent fuel overheated and melted
Explosions damaged containment structures
Radioactive nuclei leaked into air and water
Summary about Nuclear Reactors
They slow fission neutrons using a moderator
They can use natural or slightly enriched uranium
They control their chain reactions carefully
They produce radioactive waste and plutonium
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Medical Imaging and Radiation
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Observations AboutMedical Imaging and Radiation
They do their jobs right through your skin
Imaging involves radiation of various sorts
Some imaging radiation is itself hazardous
Radiation can make you well, sick, or neither
Some radiation involves radioactivity
Some radiation involves accelerators
5 Questions about Medical Imaging and Radiation
1. How are X-rays produced?
2. Why do X-rays image bones rather than tissue?
3. How does CT scanning create a 3D image?
4. How do gamma-rays kill cancerous tissue?
5. Why does MRI image tissue, not bone?
Question 1
Q: How are X-rays produced?
A: By accelerating electrons or atomic fluorescence
X-rays are high-frequency electromagnetic waves An X-ray photon carries a large amount of energy
X-rays are produced by very energetic events Extremely rapid accelerations of electrons
Extremely energetic radiative transitions in atoms
Bremsstrahlung X-rays
When a fast-moving electron whips around a massive nucleus electron accelerates extremely rapidly
may emit much of its energy as an X-ray photon
Characteristic X-rays
When a fast-moving electron hits a massive atom it may knock an electron out of a tightly bound orbital
and leave the atom (now an ion) ina highly excited state.
Atom then undergoes a radiativetransition to a less excited state
and emits an X-ray photon.
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Producing X-rays
X-rays are produced with high-energy electrons Accelerate electrons to 10kV - 100kV
Let electrons hit heavy atoms
Some X-rays emitted via bremsstrahlung and some as characteristic X-rays
X-ray tube filters awaythe lowest energy photons,because they’re uselessand cause skin burns.
Question 2
Q: Why do X-rays image bones rather than tissue?
A: The larger atoms in bone can absorb X-rays.
X-rays interact with atoms in two principal ways Rayleigh scattering—absorption and reemission
Photoelectric effect—absorption and electron ejection
All atoms participate in Rayleigh scattering
Large atoms participate in the photoelectric effect
X-rays Rayleigh Scattering
Rayleigh scattering is what makes the sky blue An atom absorbs a photon and then reemits it
This “scattered” photon travels in a new direction
All atoms participate in X-ray Rayleigh scattering
X-ray Photoelectric Effect
Photoelectric effect is a radiative transition X-ray photon shifts electron from orbital to free wave
Free electron carries photon’s residual energy
Effect most likely for small free-electron energies so effect most likely for atoms with many-electrons
X-ray Imaging
An atom that blocks X-rays casts a shadow Many-electron atoms produce strong shadows
Few-electron atoms cast essentially no shadows
X-ray imaging observes shadows of large atoms
Unfortunately, all atoms Rayleigh scatter X-rays Rayleigh scattering causes a distracting haze
Haze is filtered away by collimating structures
Question 3
Q: How does CT scanning create a 3D image?
A: Many X-ray images are analyzed by computer
X-rays are taken from different angles Shadows overlap differently at each angle
Computer analyzes these X-ray images Reconstructs 3D arrangement of parts
Displays cross sections of the body
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Question 4
Q: How do gamma-rays kill cancerous tissue?
A: These high-energy photons destroy molecules.
Gamma-rays are extremely high energy photons cause widespread damage to molecules and tissues
Gamma-rays are produce by high-energy events radiative transitions within atomic nuclei
high-energy bremsstrahlung (particle accelerators)
Producing Gamma Rays
Radioactive decay can produce gamma rays Decay may leave nucleus in an excited state
Radiative transition in nucleus emits gamma photon
Decaying cobalt-60 (Co60) emits two gamma photons
Electron accelerators can produce gamma rays Electrons are accelerated to extreme energies
Near heavy nuclei, emit Bremsstrahlung gamma rays
Gamma-rays and Matter
Gamma rays interact with individual charges
Compton scattering: photon hits electron They exchange energy & momentum
Both particles leave the atom
Atom, molecule, tissue are damaged
Pair production: photon → electron & positron Positron is anti-matter version of electron
Positron and another electron soon annihilate
Resulting gammas damage atoms, molecules, tissue
Radiation Therapy
Gamma rays are highly penetrating in tissue Little Rayleigh scattering and photoelectric effect
Much Compton scattering and pair production
Each gamma ray event damages many molecules Gamma rays can cause enough damage to kill cells
Approaching tumors from many angles minimizes collateral damage to healthy tissue
Question 5
Q: Why does MRI image tissue, not bone?
A: MRI images hydrogen nuclei, common in tissue.
Magnetic Resonance Imaging images hydrogen Hydrogen nuclei are protons Protons are magnetic—they are tiny dipole magnets Protons tend to orient in an external magnetic field Radio waves can influence that orientation
MRI is based on proton-orienting effects
Nuclear Magnetic Resonance
MRI grew out of Nuclear Magnetic Resonance Nuclei in a magnetic field interact with radio waves Yields information about atoms and their environments
Why NMR works: many nuclei are magnetic dipoles In magnetic field, dipole’s energy depends on orientation Nuclei have quantized orientations (quantum physics) In magnetic field, nuclei have quantized energies Nuclear orientations can change via radiative transitions
NMR studies atoms via those radiative transitions
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Magnetic Resonance Imaging
NMR typically studies hydrogen nuclei (protons) Protons have two quantized orientations H-NMR flips protons between their two orientations
Magnetic resonance imaging is based on H-NMR Person is placed in a carefully designed magnetic field That magnetic field varies with location and time Protons are probed with sophisticated radio waves MRI machine senses where and when protons respond MRI machine assembles image a person’s hydrogen
MRI images hydrogen and its tissue environment.
Summary about Medical Imaging and Radiation
X-rays and gamma-rays are high energy photons
X-rays scatter from heavy atoms, for imaging.
Gamma-rays disrupt cells, for therapy.
MRI detects and locates hydrogen nuclei.