09. Magnetism & Electromagnetic Induction

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Conceptual

PhysicalScience5th Edition

Chapter 9:

MAGNETISM ANDELECTROMAGNETIC

This lecture will help you

understand:

• Magnetic Poles

• Magnetic Fields

• Magnetic Domains

• Electric Currents and Magnetic Fields• Magnetic Forces on Moving Charges

• Electromagnetic Induction

• Generators and Alternating Current

• Power Production• The Transformer —Boosting or Lowering Voltage

• Field Induction

Magnetic Poles

Magnetic poles are in all magnets:• you can’t have one pole without the other

• no single pole known to exist

Example:

– simple bar magnet—poles atthe two ends

– horseshoe magnet: bent

U shape—poles at ends

Magnetic Poles

Magnetic force

• force of attraction or repulsion between a

pair of magnets depends on which end of

the magnet is held near the other

• behavior similar to electrical forces

• strength of interaction depends on thedistance between the two magnets

Magnetic Poles

Magnetic poles

• give rise to magnetic force

• two types interacting with each other

– north pole (north-seeking pole) – south pole (south-seeking pole)

Rule for magnetic forces between magnetic poles:

• Like poles repel; opposite poles attract

A weak and strong magnet repel each other. The greater repelling force is by the

A. stronger magnet.

B. weaker magnet. C. Both the same.

D. None of the above.

Magnets

CHECK YOUR NEIGHBOR

Magnets

CHECK YOUR ANSWER

A weak and strong magnet repel each other. The greater repelling force is by the

A. stronger magnet.

B. weaker magnet. C. Both the same.

Explanation:

Remember Newton’s third law!

Magnetic Fields

Magnetic fields:• occupy the space around a magnet

• produced by moving electric charges

Field shape revealed by magnetic field lines that spread from one pole,curve around magnet, and return to other pole

Lines closer together field strength is greater

Magnetic Fields

Magnetic fields

• produced by two kinds of electron motion – electron spin

• main contributor to magnetism• pair of electrons spinning in same direction creates a

stronger magnet

• pair of electrons spinning in oppositedirection cancels magnetic field of theother

– electron revolution

Magnetic Domains

Magnetic domains

• clustered regions of alignedatoms

• oriented in random fashion —

magnetic fields produced byeach can cancel the fields of other.

• When oriented in onedirection, then the substancecontaining them is a magnet

• Magnet strength depends onnumber of magnetic domains thatare aligned.

Magnetic Domains

Electric Currents and Magnetic

FieldsConnection between electricity and magnetism

Magnetic field forms a pattern of concentric circles arounda current-carrying wire

• when current reverses direction,

the direction of the field lines

reverse

Fields

FieldsMagnetic field intensity• increases as the number of loops increase in a current-

carrying coil

El t i C t d M ti Fi ld

Electric Currents and Magnetic Fields

CHECK YOUR NEIGHBOR

An electromagnet can be made stronger by

A. increasing the number of turns of wire.

B. increasing the current in the coil.

C. Both A and B.D. None of the above.

El t i C t d M ti Fi ld

Electric Currents and Magnetic Fields

CHECK YOUR ANSWER

An electromagnet can be made stronger by

A. increasing the number of turns of wire.

B. increasing the current in the coil.

C. Both A and B.

Magnetic Forces on Moving Charges

Charged particles moving in a magnetic fieldexperience a deflecting force—greatest when

moving at right angles to magnetic field lines.

Magnetic Forces Current-Carry Wires

Magnetic Force and Levitation

• When an upward

magnetic force is greater

than gravity, then an

object can levitate.

• A magnetically levitated

vehicle is shown in the

figure to the right – a

magplane.

• No friction, no vibrations

Magnetic Force in Space

• Earth’s magnetic field

deflects many

charged particles that

make up cosmicradiation.

Magnetic Force

The magnetic force on a moving charged particle can

change the particle’s

A. speed.

B. direction.

C. Both A and B.

D. Neither A nor B.

Magnetic Force

CHECK YOUR NEIGHBOR

Magnetic Force

CHECK YOUR ANSWER

The magnetic force on a moving charged particle can

change the particle’s

A. speed.

B. direction.

C. both A and B.

D. neither A nor B.

Explanation:

Only an electric force can change the speed of a charged particle. Since

the magnetic force acts at right angles to velocity, it can only change the

direction of a moving charged particle.

Magnetic Force on Moving

ChargesElectric meters

• detect electric current

Examples:

• magnetic compass

• compass in a coil of wires

ChargesGalvanometer

• current-indicating device named after Luigi Galvani

• called ammeter when calibrated to measure current (amperes)

• called voltmeter when calibrated to measure electric potential (volts)

ChargesElectric motor • different from galvanometer in

that each time the coil makes ahalf rotation, the direction of the

current changes in cyclic fashionto produce continuous rotation

Motor and Generator

CHECK YOUR ANSWER

A motor and a generator are

A. similar devices.

B. very different devices with different applications. C. forms of transformers.

D. energy sources.

Motor and Generator

CHECK YOUR ANSWER

A motor and a generator are

A. similar devices.

B. very different devices with different applications. C. forms of transformers.

D. energy sources.

Electromagnetic Induction

Electromagnetic induction

• discovered by Faraday and Henry

• voltage is induced with change of magnetic field strength

in a coil of wire

Electromagnetic induction (continued)

• induced voltage can be increased by

– increasing the number of loops of wire in a coil

– increasing the speed of the magnet entering andleaving the coil

• slow motion produces hardly any voltage

• rapid motion produces greater voltage

Induction occurs whether the magnetic field moves

past the wire or the wire moves through the magnetic

field.

More loops; more induction

Faraday’s law

• the induced voltage in a coil is proportional to the

number of loops, multiplied by the rate at which the

magnetic field changes within those loops

• amount of current produced by electromagnetic induction

is dependent on

– resistance of the coil

– circuit that it connects – induced voltage

More difficult to

push the

magnet into a

coil with many

loops becausethe magnetic

field of each

current loop

resists themotion of the

magnet.

CHECK YOUR ANSWER The resistance you feel when pushing a piece of iron into a

coil involves

A. repulsion by the magnetic field you produce.

B. energy transfer between the iron and coil. C. Newton’s third law.

D. resistance to domain alignment in the iron.

CHECK YOUR ANSWER The resistance you feel when pushing a piece of iron into a

coil involves

A. repulsion by the magnetic field you produce.

B. energy transfer between the iron and coil. C. Newton’s third law.

D. resistance to domain alignment in the iron.

Generators and Alternating

Current

Generator • opposite of a motor

• converts mechanical energy into electrical

energy via coil motion

• produces alternating voltage and current

Generators and Alternating

CurrentThe frequency of alternating voltage induced

in a loop is equal to the frequency of the

changing magnetic field within the loop.

Power Production

Using Faraday and Henry’s discovery of

electromagnetic induction, Nikola Tesla and

George Westinghouse showed that electricity

could be generated in sufficient quantities to lightcities.

The Transformer —Boosting or

Lowering Voltage

• input coil of wire —primary powered by AC voltage

source

• output coil of wire —secondary connected to external

circuit

The Transformer

Transformer (continued)

• both wound on a common iron core

• then magnetic field of primary passes through secondary

• uses ac in one coil to induce ac in second coil

The Transformer

• Transformer relationship:

primary voltagenumber of primary turns

=secondary voltage

number of secondary turns

Transformers Everywhere

• This commontransformer lowers120V to 6V or 9V. Italso converts AC to

DC by means of adiode inside.

• A commonneighborhood

transformer thattypically steps 2400Vdown to 240V.

Transformer Power

CHECK YOUR ANSWER

A step-up transformer in an electrical circuit can step up

A. voltage.

B. energy. C. Both A and B.

D. Neither A nor B.

Transformer Power

CHECK YOUR NEIGHBOR

A step-up transformer in an electrical circuit can step up

A. voltage.

B. energy. C. Both A and B.

D. Neither A nor B.

Explanation:

Stepping up energy is a conservation of energy no-no!

Electric Power

• Electric power is equal to the product of

the voltage and current.

Electric Power Voltage curren

Electric Grid uses Transformers

• Voltage generated in power stations is stepped

up with transformers prior to being transferred

across the country by overhead cables.

• Then other transformers reduce the voltagebefore supplying it to homes, offices, and

factories.

Transformer Power

• Neglecting heat losses, power into a

transformer = power out of transformer.

secondary primary currentxVoltagecurrentxVoltage

Electric Field Induction

Basic to electromagnetic induction is that electric

and magnetic fields can induce each other.

An electric field is induced in any region of spacein which a magnetic field is changing with time.

A magnetic field is induced in any region of space

in which an electric field is changing with time.

Electric and Magnetic Field Induction

The mutual induction of electric and magnetic fields can

produce

A. light.

B. energy.

C. sound.

CHECK YOUR NEIGHBOR

Electric and Magnetic Field Induction

CHECK YOUR ANSWER The mutual induction of electric and magnetic fields can

produce

A. light.

B. energy.

C. sound.

Field Induction

• Light is produced by the mutual induction

of electric and magnetic fields

• speed of light is the speed of emanation of

these fields

– too slow, the regenerating fields die out

– too fast, fields build up in a crescendo of ever-

increasing energy – at speed c , just right! And, there is light!

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