the four fundamental forces ● what are the four fundamental forces?
TRANSCRIPT
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The Four Fundamental Forces
● What are the four fundamental forces?
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The Four Fundamental Forces
● What are the four fundamental forces?– Gravitational, Electromagnetic, Strong and Weak Nuclear
● Gravity: Increases with masses, Inverse square law force, Always attractive
● Electromagnetic: Increases with charges, Inverse square law force, Opposites attract and likes repel
● Strong: Holds positively charged nucleus together, Extremely short range (10-15m)
● Weak: Involved in radioactive decay
– Strong force 100 times EM and Weak forces, 1039 times gravity
● Holy grail of physics is to unify these four forces!
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The Electrical Force
● Coulomb's Law
F = k
● Compare to Gravity F = G
– k = 9 Trillion N•m2/C2
– G = 6.67 x 10-11 N•m2/kg2
q1q
2
r2
m1m
2
r2
Like electric chargesrepel and opposites attract.
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Electric Fields
● Permeate all of space– Convention: represents
force felt by a positive test charge
Electric field lines begin at positivecharges and terminate at negativecharges (or at infinity).
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Conductors and Insulators● What feature of metals makes them good conductors
of heat?
● Does this also explain the electrical conductivity of metals?
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Conductors and Insulators● What feature of metals makes them good conductors
of heat?– Loosely bound outer electrons.
● Does this also explain the electrical conductivity of metals?– Yes. Electrons in a metal flow in an applied electric field.
● Insulators are materials in which the outer electrons are tightly bound– Poor conductors of both heat and electricity
● Cork, rubber, glass, etc.
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● Why does a balloon stick to the wall if you first rub it against your clothes?
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Static Electricity and Conservation of Charge
● Electrons in your clothes are loosely bound => dislodged– Charge is conserved
● Balloon – net negative charge
– But wall is electrically neutral, so why does balloon stick?
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Static Electricity and Conservation of Charge
● Electrons in your clothes are loosely bound => dislodged– Charge is conserved
● Balloon – net negative charge
– But wall is electrically neutral, so why does balloon stick?
● Balloon polarizes surface molecules in wall
– Opposites attracted, likes repelled– Net charge of wall is still zero
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Charging by Induction● A charged rod, brought
near two conducting spheres in contact, will cause charges to separate.– Separating the spheres
leaves each with an equal and opposite net charge
● Conservation of charge
– No contact with rod required
● Electric Field
– Process is called induction
A charge rod brought close to a conductor induces a charge, while the same rod brought near an insulator polarizes the surface atoms.
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Potential Energy vs. Electric Potential● A charged object has P.E.
because of its location in an electric field
electric potential =
– Dividing by charge => electric potential only property of electric field
– Voltage
1 volt = 1 Work is required to bring likecharges together. Twice the charge = twice the work.
electric potential energycharge
JouleCoulomb
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Electric Current● Charge flows from one end of
a conductor to the other as long as they are at different electric potentials– Current requires a potential
difference or voltage● Acts like an “electrical pump”
which keeps charge flowing– Batteries, generators, etc.
– Similar to heat flow from hot to cold ends of a conductor
● Ceases when temperatures equalize
Water will flow from a higherreservoir to a lower one. Once the water levels are equal, the flow stops. A pump can yield a continuous flow by maintaining a pressure difference.
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Circuits
● What is it that flows through an electrical circuit?
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Circuits
● Charged particles flow through an electric circuit– In a metal, the free
conduction electrons flow– In fluids, it is often the
positive ions that flow● Charge carriers flow through
a circuit due to an applied voltage across the circuit
Water flows through a pipe aslong as there is a difference inpressure between its ends. Onlythe water flows, not the pressure.
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Electrical Resistance● What factors determine the rate
that water flows through a pipe?
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Electrical Resistance● What factors determine the rate
that water flows through a pipe?– The pressure difference– The resistance of the pipe to flow
● Thicker pipe => less resistance● Longer pipe => more resistance
● Similar to electrical resistance– Current depends on voltage and on
properties of conductor● Thicker wire => less resistance● Longer wire => more resistance● Colder wire => less resistance● Copper less resistant than steel
More water flows through a thick hose than through a thinhose connected to the samewater source (same pressure).
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Ohm's Law
● Does the current in a circuit increase or decrease as– the applied voltage is increased?– the resistance of the circuit is increased?
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Ohm's Law
● Does the current in a circuit increase or decrease as– the applied voltage is increased? Increase– the resistance of the circuit is increased? Decrease
● The current in a circuit is directly proportional to the voltage across the circuit and inversely proportional to the resistance.
Current = Amperes = voltage
resistance
volts
ohms
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Direct Current and Alternating Current● Direct Current (DC)
– Charges flow in only one direction
– Terminals of a battery always have the same polarity
● Electrons repelled by negative terminal and attracted to positive terminal
● Alternating Current (AC)– Charges oscillate back and
forth (no net displacement)● Generators, Power in homes
– North America: 60-hertz
An AC generator changes the polarityof the voltage 60 times a second.
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Speed and Source of Electrons in a Circuit
● How fast do the electrons move through– a DC circuit?– an AC circuit?
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Speed and Source of Electrons in a Circuit● DC circuit => drift velocity of
about 1/100th cm/s● Electrons in an AC circuit
have a net drift velocity of zero!
● Electric field travels through the circuit at speed of light– Causes electrons all along wire
to “move in step”● Current not caused by collisions● Collisions are related to resistance
Solid line: Random motion ofan electron bouncing around in atomic lattice of a metal(~1/200th the speed of light)
Dashed line: Altered path in the presence of an electric field.
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Let There Be Light!● What makes the bulb give
off light?
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Let There Be Light!● What makes the bulb give
off light?– The resistance of the filament
(due to collisions) causes heating => filament glows
● Where do the electrons come from?
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Let There Be Light!● What makes the bulb give
off light?– The resistance of the filament
(due to collisions) causes heating => filament glows
● Where do the electrons come from?– They are already there in the
metal.● The electric company supplies
energy (as an electric field) not electrons!