Chapter 28 Electromagnetic Induction

Electricity and MagnetismElectricity and Magnetism

28 Electromagnetic Induction

Chapter 28Electromagnetic Induction



28.1 Induced e.m.f. and induced current

• An e.m.f. is induced whenever– the conductor cuts through magnetic field lines due to the

relative motion between the conductor and the magnet.

Induction by the relative motion between a coil and a magnet

Experiment 28.1




relative motion between the conductor and the magnet.

Induction by moving a wire across magnetic field lines

Experiment 28.2




relative motion between the conductor and the magnet.– the magnetic field through a coil changes.

Experiment 28.3Induction caused by a changing magnetic field



• This phenomenon is called electromagnetic induction.

• Such an e.m.f. is called induced e.m.f.

low potential

high potential

induced e.m.f.



• This phenomenon is called electromagnetic induction.

• Such an e.m.f. is called induced e.m.f.

• The current produced is called an induced current.

induced current if a closed loop is formed

wire frame

induced current



Faraday’s law of electromagnetic inductionThe magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes.

smaller

larger



Faraday’s law of electromagnetic inductionThe magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes.

Checkpoint (p.358) O

largersmaller



magnet approaching

N S

Lenz’s lawAn induced current always flows in a direction so as to oppose the change producing it.

Ring that cannot get through

opposing the change



magnet withdrawing

S N

Ring that cannot get through

Lenz’s lawAn induced current always flows in a direction so as to oppose the change producing it.

opposing the change



• Fleming’s right hand rule can also be used to determine the

direction of an induced current.

Example 28.2Experiment 28.5 Checkpoint (p.365) O

Conducting loop moving across a magnetic field



28.2 Faraday’s law and motional e.m.f.

• Magnetic flux is a measure of the number of magnetic field

lines through a surface.

Magnetic flux



• The magnetic flux through a planar surface in a magnetic fi

eld B is

• Magnetic flux is a scalar quantity with the unit weber (Wb).• The magnetic field is also called the magnetic flux density.

Example 28.3

= BA cos




• Faraday’s law can be expressed in mathematical form:

• For an N-turn coil, if the magnetic flux through each turn is

the same, the total flux through the coil, called the magnetic

flux linkage, is N.

t

The negative sign is another way to state Lenz’s law.

Example 28.6

Example 28.5Example 28.4


Mathematical form of Faraday’s law



• A motional e.m.f. is the induced e.m.f. arises from the motion

of a conductor in a magnetic field.

uniform magnetic field B

+++++

–––––

vmotional e.m.f.

magnetic force

Motional e.m.f.



• A motional e.m.f. is the induced e.m.f. arises from the motion

of a conductor in a magnetic field.

+++++

–––––

v

magnetic forceFB = qvB

electric forceFE = qE = qV / l

FB = FE

l


= Blv

At equilibrium,

motional e.m.f.



• If the velocity v makes an angle with the magnetic field B,

the motional e.m.f. is

Example 28.7

= Blv sin


v


straight conductor rod of length l



28.4 Applications of electromagnetic induction and generators

• Electromagnetic induction is used in

– moving-coil microphones





– magnetic storage

Writing data Reading data






– electric guitars






– electric guitars

– electrical generators

Generators in a power stationA bicycle alternator



• A generator is a device that

converts mechanical energy

into electrical energy.• The ones generating

alternating currents are called

a.c. generators (or alternators).• Those generating direct

currents are called d.c.

generators (or dynamos). A simple a.c. generator

Simple a.c. generator

Electrical generator



• The slip rings in an a.c. generator are used to prevent the

twisting of wires during the rotation of the coil.

slip rings



















Variation of the induced e.m.f. in a simple a.c. generator



• The commutator of a d.c. generator is used to reverse the co

nnection to the external circuit when the direction of the induc

ed e.m.f. in the coil reverses.

Simple d.c. generator

A simple d.c. generator



• The commutator of a d.c. generator is used to reverse the co

nnection to the external circuit when the direction of the induc

ed e.m.f. in the coil reverses.

Variation of the induced e.m.f. in a simple d.c. generator



• The induced e.m.f. of a generator can be increased by

– increasing the rotational speed of the coil,

– using stronger magnets,

– winding more turns of wire on the coil,

– increasing the area of the coil within the field, and

– winding the coil on a soft iron core.

Example 28.9Experiment 28.6



• In some generators, it is the magnets that rotate but not the

coil.

A bicycle alternator



• For an a.c. generator in a power station, there is an

electromagnet, called rotor, rotating in a set of fixed coils,

called the stator. Thus, e.m.f. is induced in the stator coils.




28.5 Eddy currents

• Eddy currents are induced currents circulating through a

conducting plate when the plate cuts through magnetic field

lines or is placed in a changing magnetic field.

uniform magnetic field

eddy currents

Coin detectorFalling magnet

Conducting plate leaving the field



• Eddy currents are induced currents circulating through a

conducting plate when the plate cuts through magnetic field

lines or is placed in a changing magnetic field.


eddy currents

Coin detectorFalling magnet

Conducting plate entering the field



• In general, magnetic braking effect arises whenever eddy

currents are induced due to the relative motion between a block

of conductor and a magnetic field.


external forcemagnetic force

Experiment 28.7

Braking effect

decelerates



• Eddy currents can heat up the conductor itself as a result of

the heating effect of current.• This phenomenon is called induction heating.


Induction heating

A floating aluminium foil

What makes the brass gong rotate

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Chapter 28 Electromagnetic Induction