electromagnetic inductionb

5. GENERATORS5. GENERATORS5. GENERATORSApplications ofApplications of

Electromagnetic InductionElectromagnetic Induction

Unit 22b Unit 22b

Useful websites**Generatorhttp://www.sciencejoywagon.com/physicszone/lesson/otherp

ub/wfendt/generatorengl.htmhttp://micro.magnet.fsu.edu/electromag/java/generator/ac.html

** Transformerhttp://micro.magnet.fsu.edu/electromag/java/transformer/index.html

*Faraday’s Lawhttp://micro.magnet.fsu.edu/electromag/java/faraday/index.html

http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm

http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm

http://micro.magnet.fsu.edu/electromag/java/generator/ac.html

http://micro.magnet.fsu.edu/electromag/java/transformer/index.html

http://micro.magnet.fsu.edu/electromag/java/faraday/index.html

OBJECTIVESOBJECTIVES•

Describe a simple form of a.c. generator (rotating coil or rotating magnet), the use of carbon brushes and slip rings (where needed).

•

Sketch a graph of voltage output against time for a simple a.c. generator.

OBJECTIVESOBJECTIVES•

Describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations.

•

Recall the equations Vp

/Vs

= Np

/Ns and Vp

Ip= Vs

Is

(ideal transformers).•

Apply the relationships between Vp

, Vs

, Np

, Ns

, Ip

and Is

to new situations or to solve related problems.

5. Alternating Current 5. Alternating Current GeneratorsGenerators

•

The direction of the induced current in the coil can be found using Fleming’s Right-Hand RuleManchester Football Club

Motion

Field

Current

OR Mother

Father Child

5.1 Simple Alternating Current 5.1 Simple Alternating Current Generator consists ofGenerator consists of

a. Carbon brushes provide electrical contact with the slip ring so that current can flow out to an external circuit.

b. Slip rings are to ensure that the voltage is alternating every half cycle.

C. A rectangular coil of wire which rotates in the magnetic field of a permanent magnet.

Explanation:As the coil rotates, it sides cut the magnetic field, and an e.m.f. and hence a current is induced.

a. Carbon brushes provide electrical contact with the slip ring so that current can flow out to an external circuit.

b. Slip rings are to ensure that the voltage is alternating every half cycle.

BMotion and Field are at right angle=> Maximum induced e.m.f.

Motion and Field are parallel => ZERO induced e.m.f.

B

Alternating Current Alternating Current GeneratorsGenerators

B

Motion and Field are at right angle=> Maximum induced e.m.f.

Alternating Current Alternating Current GeneratorsGenerators

Motion and Field are parallel => ZERO induced e.m.f.

B

Maximum current from B to A

‘Maximum

current ‘ from A to B

-

voltage output of an a.c. generatorvoltage output of an a.c. generatorwhen the coil is horizontal,when the coil is horizontal,

one cycle

amplitude

time

induce

d

e.m

.f.

positions of coil

field

the a.c. generatorthe a.c. generator

o rate at which it cuts the magnetic lines of force is greatest since motion and field are at right angle.

o induced eletromotive force (e.m.f.) is maximum and hence induced current is the maximum.

o rate at which it cuts the magnetic lines of force is greatest since motion and field are at right angle.

o induced eletromotive force (e.m.f.) is maximum and hence induced current is the maximum.

when the coil is vertical,when the coil is vertical,

o rate at which it cuts the magnetic lines of force is least since motion and field are parallel.

o induced eletromotive force (e.m.f.) is minimum(i.e.0) and hence induced current is the minimum (i.e.0).

o rate at which it cuts the magnetic lines of force is least since motion and field are parallel.

o induced eletromotive force (e.m.f.) is minimum(i.e.0) and hence induced current is the minimum (i.e.0).

5.2 Graph of Voltage5.2 Graph of Voltage Against TimeAgainst Time

Doubling the number of turns in coil

OriginalNo of coils doubled

Time /s

Voltage

/v

5.2 Graph of Voltage5.2 Graph of Voltage Against TimeAgainst Time

Doubling the speed of rotation

OriginalRotation doubled

Time /s

Voltage

/v

principle of a.c. generatorsprinciple of a.c. generators

Induced e.m.f. of a generator can be increased by:Induced e.m.f. of a generator can be increased by:

increasing the speed of rotation of the coil

increasing the number of turns in the coil

increasing the speed of rotation of the coil

increasing the number of turns in the coil

the a.c. generatorthe a.c. generator

B C

DA

Y

XP

Q

N S

winding the coil around a soft iron core to concentrate the magnetic lines of force through the coil

using stronger magnets

winding the coil around a soft iron core to concentrate the magnetic lines of force through the coil

using stronger magnets

6. TRANSFORMER6. TRANSFORMER6. TRANSFORMERApplication ofApplication of

Electromagnetic InductionElectromagnetic Induction

transformerstransformers

Different electrical appliances need different working voltages. Transformers are used to supply the different voltage requirements of the appliances we use from the mains supply of 240V.

Different electrical appliances need different working voltages. Transformers are used to supply the different voltage requirements of the appliances we use from the mains supply of 240V.

transformers used in electrical appliances

step-up transformer in a power station


Faraday’s iron ring experimentFaraday’s iron ring experiment

when the switch was closed, the needle of the compass above wire XY deflected

when the switch was opened, the compass needle deflected again

when a steady current was supplied in coil A, no deflection occurred. Therefore if we want a constant current in coil B, we have to use an A.C. supply as the magnitude of the current in the coil A will be constantly varying.

when the switch was closed, the needle of the compass above wire XY deflected

when the switch was opened, the compass needle deflected again

when a steady current was supplied in coil A, no deflection occurred. Therefore if we want a constant current in coil B, we have to use an A.C. supply as the magnitude of the current in the coil A will be constantly varying.


Physical connection of primary and secondary coils in a transformer: Physical connection of primary and secondary coils in a transformer:

secondary coil

lamp

primary coil

soft iron core

A.C. Supply

primary coil is connected to a.c. supply and a

switch

secondary coil is connected to a small lamp

6.1 Basic iron-core transformer6.1 Basic iron-core transformer


When the switch is closed,When the switch is closed,

basic iron-core transformerbasic iron-core transformer

the current in the coil increases from zero to maximum, producing an increasing magnetic field in the primary coil

magnetic field induces a current in the secondary coil which then lights up the lamp.

the continuous supply of alternating current ensures a continuous e.m.f. and hence a current to be induced in the secondary coil.

the current in the coil increases from zero to maximum, producing an increasing magnetic field in the primary coil

magnetic field induces a current in the secondary coil which then lights up the lamp.

the continuous supply of alternating current ensures a continuous e.m.f. and hence a current to be induced in the secondary coil.

a.c. supply

secondary coil

lamp

primary coil

soft iron core


basic iron-core transformerbasic iron-core transformer

When the switch is opened,When the switch is opened,

the current in the coil decreases to zero, producing a decreasing magnetic field in the primary coil

magnetic field induces a current in the secondary coil in the opposite direction which then lights up the lamp

the current in the coil decreases to zero, producing a decreasing magnetic field in the primary coil

magnetic field induces a current in the secondary coil in the opposite direction which then lights up the lamp

a.c. supply

secondary coil

lamp

primary coil

soft iron core

Therefore an A.C. (alternating current) supply serves as an alternative to constantly opening and closing the circuit

can be used for the primary coil.

Therefore an A.C. (alternating current) supply serves as an alternative to constantly opening and closing the circuit

can be used for the primary coil.


The alternating current will create a continually changing magnetic field which will induce a current in the secondary coil, thus lighting up the lamp all the time.

The alternating current will create a continually changing magnetic field which will induce a current in the secondary coil, thus lighting up the lamp all the time.

secondary coil

lamp

primary coil

soft iron core

A.C. supply

primary coil is connected to an A.C. supply

secondary coil is connected to a small lamp

A transformer is used to change the voltage of an alternating current, by having different number of turns in the primary and secondary coils.



6.2 Types of transformer:6.2 Types of transformer:primary

coilsecondary

coil

principle of transformersprinciple of transformers

o has more turns in secondary coil than in the primary coil

o increases the voltage

o has more turns in secondary coil than in the primary coil

o increases the voltage

a. step-up transformera. step-up transformer




6.2 Types of transformer:6.2 Types of transformer:primary

coilsecondary

coil

principle of transformersprinciple of transformers

o has fewer turns in secondary coil than in the primary coil

o decreases the voltage

o has fewer turns in secondary coil than in the primary coil

o decreases the voltage

b. step-down transformerb. step-down transformer

For an ideal transformer (100% efficient),For an ideal transformer (100% efficient),


Turns ratioTurns ratio

==number of turns in primary coil

number of turns in secondary coilprimary input voltage

secondary output voltage

turns ratio =turns ratio = Np

Ns

==Np

Ns

Vp

Vs

primary coil

secondary coil

= == =Is

Ip

Np

Ns

Vp

Vs

V p I p = V s I s Vp Ip = Vs Is

If the transformer is 100% efficient,

input power = output power

If the transformer is 100% efficient,

input power = output power

turns ratioturns ratio


Design considerations can improve the efficiency of the transformer: Design considerations can improve the efficiency of the transformer:


practical transformer designpractical transformer design

effective soft magnetic material for the coreeffective soft magnetic material for the core

low-resistance copper wires to reduce losing energy in the form of heat

laminated core can reduce the flow of eddy currents

low-resistance copper wires to reduce losing energy in the form of heat

laminated core can reduce the flow of eddy currents

laminated core

both primary and secondary coils wound round the center core


practical transformer designpractical transformer design

special core design ensures that the magnetic field produced by the primary coil is linked completely to the secondary coil

Design considerations can improve the efficiency of the transformer: Design considerations can improve the efficiency of the transformer:

A pacemaker helps to trigger heartbeats in people with chronic heart conditions. This device works on the principle of electromagnetic induction.

A pacemaker helps to trigger heartbeats in people with chronic heart conditions. This device works on the principle of electromagnetic induction.


cardiac pacemakercardiac pacemaker

coil with electrodes wired to the heart


6.3 High voltage transmission6.3 High voltage transmission

electricity generated at the power station is transmitted to the mains through the use of cableselectricity generated at the power station is transmitted to the mains through the use of cables


power transmissionpower transmission

grid system for the transmission of electricity

generator produces a very high voltage having electrical power

power loss occurs during long distance power transmission; solved by reducing magnitude of the transmission current and stepping up the voltage

generator produces a very high voltage having electrical power

power loss occurs during long distance power transmission; solved by reducing magnitude of the transmission current and stepping up the voltage

power loss occurs during long distance power transmission

partly solved by reducing magnitude of the transmission current and stepping up the voltage

power loss occurs during long distance power transmission

partly solved by reducing magnitude of the transmission current and stepping up the voltage


power transmissionpower transmission

grid system for the transmission of electricity

Why are practical transformers not 100% efficient?

•

The coils of wire have some resistance. Energy is lost in the form of heat produced by current flowing in the coils.

•

Current known as eddy currents are induced in the iron core because the core is in a region of changing magnetic field. Heat is therefore produced in the core. This is reduced by using a laminated core.

•

Not all the magnetic field lines may be ‘cut’ by the secondary coil.

THE ENDTHE ENDTHE ENDThank You!!Thank You!!

electromagnetic inductionb

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