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4.5.2Electricity and Magnetism

THE A.C. Generator — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 54 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

Generator — IGCSE PHYSICS notes: Describe the structure of a simple a.c. generator, explain how an alternating e.m.f.

  • 4.5.2 The A.C. Generator — IGCSE PHYSICS notes
  • Energy transfer
  • Over one rotation
  • Faster rotation
  • More turns
  • Soft iron core

Part 1 of 7Structure of a simple a.c. generator

ComponentFunction
Permanent magnetProvides a uniform magnetic field
Rotating coilCuts the field as it rotates, so an induced current flows
Slip ringsTwo complete rings, each fixed to one end of the coil, allowing current to flow between the coil and the external circuit
Carbon brushesPress on the slip rings to give a good sliding electrical contact without twisting the wires
  • Energy transfer
    • kinetic energy is converted into electrical energy.

Part 2 of 7How an alternating e.m.f. is produced

1
An external energy source, such as a turbine, turns the coil in the magnetic field.
2
The sides of the coil cut magnetic field lines, inducing an e.m.f. (electromagnetic induction).
3
Each side moves up then down through the field every half turn.
4
After half a turn each side moves the opposite way, so the e.m.f. reverses.
5
Slip rings and brushes pass the alternating current to the external circuit.
  • Meter
    • a centre-zero galvanometer in the circuit swings one way, then the other, showing the e.m.f. changes size and direction.

Part 3 of 7Coil position and e.m.f. (Extended)

Coil positionMotion of the sidesE.m.f.
Plane parallel to the field (horizontal)Perpendicular to the field, cutting the most linesMaximum
Plane perpendicular to the field (vertical)Parallel to the field, cutting no linesZero

Part 4 of 7Graph of e.m.f. against time (Extended)

1Shape
  • Shape a sine or cosine curve, depending on the starting position of the coil.
  • Starting vertical (e.m.f. zero) gives a sine curve.
  • Starting horizontal (e.m.f. maximum) gives a cosine curve.
2Over one rotation
  • 0° gives zero, 90° gives a maximum, 180° gives zero, 270° gives a maximum in the opposite direction and 360° gives zero.
3One cycle
  • One cycle one full rotation gives two peaks and two zeros.
    • (one positive, one negative)
4Frequency
  • Frequency f in hertz equals the number of revolutions per second, and the period is T = 1/f.

Part 5 of 7Increasing the output

Faster rotation
  • Faster rotation raises the e.m.f. and also increases the frequency.
More turns
  • More turns on the coil.
Stronger magnet.
Soft iron core
  • Soft iron core wound inside the coil.
Magnet rotating
  • Magnet rotating an alternating current can also come from a magnet rotating within a coil.

Part 6 of 7Direction of the current

1Fleming's right-hand rule
  • Fleming's right-hand rule thumb gives the motion, first finger the field and second finger the induced current in each side.
2d.c. dynamo
  • d.c. dynamo uses a split-ring commutator instead of slip rings.

Part 7 of 7More from the specification

1
A.C. generator
  • A coil rotating in a magnetic field cuts the field lines, inducing an alternating emf, which is collected through slip rings and brushes.
2
Faraday's law
  • The induced voltage is proportional to the rate of change of magnetic flux linkage, so faster rotation or more turns gives a larger voltage.
3
Moving conductor
  • When a wire moves across a magnetic field an emf is induced,
  • and its direction is given by Fleming's right-hand rule; a voltmeter can detect it.
4
Motor effect
  • A current-carrying conductor in a field experiences a force, which is the principle of the motor, the reverse of a generator.
5
Transformer
  • Two coils, primary and secondary, on an iron core change alternating voltage in the ratio of turns, Vp/Vs = Np/Ns,
  • and for 100% efficiency power in equals power out.
6
Potential difference
  • Work done by a unit charge passing through a component.
7
Step-up transformer
  • Transformer with more turns on the secondary, raising the voltage.
8
Step-down transformer
  • Transformer with fewer turns on the secondary, lowering the voltage.
9
Transformer
  • Device that changes the size of an alternating voltage.
10
Primary
  • Input coil.
11
Solenoid
  • Long coil of wire carrying a current
12
Conductor
  • Material that allows current to flow.
13
Voltmeter
  • Instrument that measures p.d., connected in parallel.
14
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil

Key terms

TermMeaning
a.c. generatorDevice that converts kinetic energy into alternating electrical output.
Slip ringsRings that carry current between a rotating coil and the external circuit.
BrushesCarbon contacts pressing on the slip rings.
Alternating e.m.f.E.m.f. that repeatedly changes direction.
FrequencyNumber of cycles per second, equal to the revolutions per second of the coil.

Avoid these mistakes

1
Confusing slip rings (a.c. generator) with the split-ring commutator (d.c. motor or dynamo).
2
Saying the e.m.f. is greatest when the coil is perpendicular to the field — it is greatest when the coil plane is parallel to the field.
3
Forgetting that faster rotation increases both the size and the frequency of the output.
4
Describing the e.m.f. as constant in direction — it reverses every half turn.

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