PowerPoint Lectures to accompany

Physical Science, 6e

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Chapter 6Electricity

and magnetism

Electric charge

• Electron theory of charge– Ancient mystery: “Amber

effect”– J. J. Thompson:

identified negatively charged electrons

• Today: – Basic unit of matter =

atom– Atoms made up of

electrons and nuclei containing positively charged protons and neutral neutrons (See Ch. 8)

Electric charge and electrical forces

• Charges in matter– Inseparable property of

certain particles – Electrons: negative

electric charge– Protons: positive electric

charge

• Charge interaction– Electric force– “Like charges repel;

unlike charges attract”

• Ions: non-zero net charge from loss/gain of electrons

Electrostatic charge

• Stationary charge confined to an object

• Charging mechanisms– Friction– Contact with a charged

object– Polarization

(reorientation induced without changing net charge)

Electrical conductors and insulators

• Electrical conductors– Electrons are free to move throughout material– Added charge dissipates– Examples: metals, graphite (carbon)

• Electrical insulators– Electron motions restricted– Added charge tends to remain on object– Examples: Glass, wood, diamond (carbon)

• Semiconductors– Conduct/insulate depending on circumstances– Applications: Computer chips, solar cells, ...

Measuring electric charge

• Unit of charge = coulomb (C)– Equivalent to charge of

6.24x1018 electrons!

– Fundamental metric unit (along with m, kg and s)

• Electron charge– Fundamental charge

– Smallest seen in nature

– Quantity of charge and the number of electrons

Measuring electric forces

Coulomb’s law• Relationship giving

force between two charges

• Similar to Newton’s law of gravitation

• k versus G implies gravity weaker

• Force Fields.– The condition of space around an object is changed

by the presence of an electrical charge.– The electrical charge produces a force field, that is

called an electrical field since it is produced by electrical charge

– All electrical charges are surrounded by an electrical field just like all masses are surrounded by gravitational fields.

– A map of the electrical field can be made by bringing a positive test charge into an electrical field.• When brought near a negative charge the test

charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled.

• You can draw vector arrows to indicate the direction of the electrical field

• This is represented by drawing lines of force or electrical field lines

–These lines are closer together when the field is stronger and farther apart when it is weaker.

Force fields

How do forces act through space?

• Charges surrounded by electric fields (Vector fields/directional)

• Fields and charges inseparable

• Fields act on other charges– Direction of fields = motion

of positive test charge in the field

– Visualized with lines of force

• Same ideas apply to gravity and magnetism

• Electrical Potential.– An electrical charge has an electrical field that

surrounds it.– In order to move a second charge through this field

work must be done– Bringing a like charge particle into this field will

require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated.

• In both of these cases the electrical potential is changed.

Electric potential

• Scalar field associated with potential energy

• Units = volts (V) • Related to work involved

in positioning charges• Potential difference

important in producing forces and moving charges

• Analogous to moving masses in gravitational fields

– The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt• A volt is a measure of the potential difference

between two points• electric potential =work to create .

potential charge moved

• PD=W • Q• The voltage of an electrical charge is the energy

transfer per coulomb.– The energy transfer can be measured by the work

that is done to move the charge or by the work that the charge can do because of the position of the field.

Electric current

Earlier - electrostatics• Charges more or less fixed in placeNow - charge allowed to move• Electric current

– Flow of charge, not flow of current– Reason for charge flow - potential (voltage)

differences

• Electric circuits– Structures designed to localize and utilize currents

Electric circuits

Structure• Voltage source

– Energy input– Necessary for continuing

flow

• Circuit elements– Energy used up as heat,

light, work, …

• Current flow convention: from high potential to low potential through the external circuit

• Water/pump analogy

– Voltage is a measure of the potential difference between two places in a circuit.• Voltage is measured in joules/coloumb.

– The rate at which an electrical current (I) flows is the quantity (q) that moves through a cross section of a conductor in a give unit of time (t)

I=q/t• the units of current are coulombs/second.• A coulomb/second is an ampere (amp)

– In an electrical circuit the rate of current is directly proportional to the difference in electrical potential between two parts of the circuit IPD.

A simple electric circuit carrying a current of 1.00 coulomb per second through a cross section of a

conductor has a current of 1.00 amp.

The nature of current

• Historically - nature of “electrical fluid” unknown

• Current thought to be a flow of positive charge

• Reality - more complicated, depending on material

• Opposite correct in metals, current = electron flow

Current mechanisms

Liquids and gases• Both positive and negative

charges move, in opposite directions

Metals• Delocalized electrons free to

move throughout metal• “Electron gas”• Drift velocity of electrons

slow• Electric field moves through

at nearly light speed

Electrons move very slowly in a direct

current circuit. With a drift velocity of 0.01

cm/s, more than 5 hr would be required for an electron to travel 200 cm from a car

battery to the brake light. It is the electric

field, not the electrons, that moves at near the

speed of light in an electric circuit.

– The current that occurs when there is a voltage depends on:• The number of free electrons per unit volume of

the conducting material.• The fundamental charge on each electron.• The drift velocity which depends on the electronic

structure of the conducting material and the temperature.

• The cross-sectional area of the conducting wire.

– It is the electron field, and not the electrons, which does the work.• It is the electric field that accelerates electrons

that are already in the conducting material.– It is important to understand that:

• An electric potential difference establishes, at nearly the speed of light, an electric field throughout a circuit.

• The field causes a net motion that constitutes a flow of charge.

• The average velocity of the electrons moving as a current is very slow, even thought he electric field that moves them travels with a speed close to the speed of light.

More current details

• Current = charge per unit time

• Units = ampere, amps (A)

• Direct current (DC)– Charges move in one

direction– Electronic devices,

batteries, solar cells

• Alternating current (AC)– Charge motion oscillatory– No net current flow

What is the nature of the electric current carried by these conducting lines? It is an electric field that

moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, an alternating current. As opposed to DC.

Electrical resistance

• Loss of electron current energy

• Two sources– Collisions with other

electrons in current– Collisions with other

charges in material

• Ohm’s law

– The relationship between voltage, current, an resistance is• V=IR• Ohms Law

– The magnitude of the electrical resistance of a conductor depends on four variables.• The length of the conductor.• The cross-sectional area of the conductor.• The material the conductor is made of.• The temperature of the conductor.

More on resistance

• Resistance factors– Type of material– Length– Cross sectional area – Temperature

• Superconductors– Negligible resistance

at very low temperatures

Electrical power and work

Three circuit elements contribute to work

• Voltage source• Electrical device• Conducting wires

– Maintain potential difference across device

– Input wire at higher potential than output wire

– Output wire = "ground" for AC circuits

– No potential difference, no current (bird on a wire)

Power in circuits

Electric bills

– The work done by a voltage source is equal to the work done by the electrical field in an electrical device.• W=Vq• The electrical potential is measured in

joules/coulomb and a quantity of charge is measured in coulombs, so the electrical work is measure in joules.

• A joule/second is a unit of power called the watt.• power = current (in amps) X potential (in volts)

–P=IV

This meter measures the amount of electric work done in the circuits, usually over a time period of a

month. The work is measured in kWhr

Magnetism

Earliest ideas• Associated with naturally occurring magnetic

materials (lodestone, magnetite)• Characterized by “poles” - “north seeking” and “south

seeking”• Other magnetic materials - iron, cobalt, nickel

(ferromagnetic)

Modern view• Associated with magnetic fields

• Field lines go from north to south poles

Magnetic poles and fields

• Magnetic fields and poles inseparable

• Poles always come in north/south pairs

• Field lines go from north pole to south pole

• Like magnetic poles repel; unlike poles attract

Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to

the poles, revealing their location.

Sources of magnetic fields

• Microscopic fields– Intrinsic spins of subatomic particles (electrons,

protons, …)– Orbital motions of electrons in atoms

• Macroscopic fields– Permanent magnets– Earth’s magnetic field– Electric currents– Electromagnets

Permanent magnets

• Ferromagnetic materials

• Atomic magnetic moment– Electron/proton intrinsic

moments

– Electron orbital motion

• Clusters of atomic moments align in domains

• Not magnetized - domains randomly oriented

• Magnetized - domains aligned

Earth’s magnetic field

• Originates deep beneath the surface from currents in molten core

• Magnetic “north” pole = south pole of Earth’s magnetic field

• Magnetic declination = offset • Direction of field periodically

reverses– Deposits of magnetized

material– Last reversal - 780,000 yrs.

ago

• The Source of Magnetic Fields.

Since electrons are charges in motion, they produce magnetic fields as well as an electric field. • magnetism is a secondary property of electricity• the strength of the magnetic field increases with

the velocity of the moving charge. The magnetic

field does not exist if the charge is not moving• A magnetic field is a property of the space

around a

moving charge.

– Earth's Magnetic Field.• The Earth’s magnetic field is thought to originate

with moving charges.• The core is probably composed of iron and

nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.

• How the electric currents are generated is not yet

understood• There seems to be a relationship between rate

of

rotation and strength of planet’s magnetic field.

Electric currents and magnetism

• Moving charges (currents) produce magnetic fields

• Shape of field determined by geometry of current– Straight wire– Current loops– Solenoid

Electromagnets

• Structure– Ferromagnetic core– Current carrying wire wrapped around core

• Field enhanced by the combination• Can be turned on/off• Widely used electromagnetic device

Electric meters

• Instrument for measuring current (ammeter)

• Uses magnetic field produced by the current

• Magnetic field and, hence, deflection proportional to current

• Modified, can measure – Potential (voltmeter)– Resistance (ohmmeter)

Electromagnetic switches

Relays• Use low voltage control

currents to switch larger, high voltage currents on/off

• Mercury switch/thermostat

Solenoid switches• Moveable spring-loaded iron

core responds to solenoid field

• Water valves, auto starters, VCR switches, activation of bells and buzzers

Telephones and loudspeakersCoupling acoustic waves to electric currents

Telephone• Sound vibrates carbon

granules changing resistance

• Changing resistance varies current

Speaker • Varying current changes

field of electromagnet, moving permanent magnet

• Moving magnet vibrates spring attached to paper cone producing sound

Electric motors

• Convert electrical energy to mechanical energy

• Two working parts– Field magnet - stationary– Armature - moveable

electromagnet• Armature rotates by

interactions with field magnet – Commutator and brushes

reverse current to maintain rotation

Electromagnetic induction

Causes:• Relative motion between

magnetic fields and conductors

• Changing magnetic fields near conductors

Effect:• Induced voltages and currents

Induced voltage depends on• Number of loops

• Strength of magnetic field

• Rate of magnetic field change

Generators

Structure• Axle with main wire loops in

a magnetic field• Axle turned mechanically to

produce electrical energyAC generator• “Alternating current”• Sign of current and voltage

alternateDC generator• “Direct current”• Sign of current and voltage

constant

• Transformers.– A transformer has two basic parts.

• A primary coil, which is connected to a source of alternating current

• A secondary coil, which is close by.– A growing and collapsing magnetic field in the

primary coil induces a voltage in the secondary coil.

– A step up or step down transformer steps up or steps down the voltage of an alternating current according to the ratio of wire loops in the primary and secondary coils.• The power input on the primary coil equals the

power output on the secondary coil.• Energy losses in transmission are reduced by

stepping up the voltage.

Transformers

• Problems in power transmission– High currents - large

resistive losses

– High voltages - dangerous potential differences

• Solution: transformers boost/lower AC currents and voltages

• Basic relationships– Power in = power out

– Number of coils to voltage

Energy losses in transmission are reduced by increasing the voltage, so the voltage of generated power is stepped up at the power plant. (A) These transformers, for example, might step up the voltage from tens to hundreds of thousands of volts. After a step-down transformer reduces the voltage at a substation, still another transformer (B) reduces the voltage to 120 for transmission to three or four houses