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4.5.1Electricity and Magnetism

Electromagnetic Induction — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 52 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.5.1 Electromagnetic Induction — IGCSE PHYSICS notes: Describe how an e.m.f. is induced when a conductor cuts magnetic field lines or the field linking a coil changes, and state the factors that affect it, Lenz's law and the right-hand rule.Electromagnetic induction — an e.m.f.

  • Parallel to the field
  • Number of turns
  • Size of the coil
  • Strength of the field
  • Length of wire
  • Pole orientation

Part 1 of 8Induced e.m.f.

1Electromagnetic induction
  • Electromagnetic induction an e.m.f. is induced in a conductor whenever there is relative movement between the conductor and a magnetic field.
  • A conductor moving in a stationary field cuts the field lines.
  • A stationary conductor in a changing field has the field lines cut through its turns.
2Current
  • Current if the conductor is part of a complete circuit, a current is induced;
    • without a complete circuit there is an e.m.f. but no current.
3Detection
  • Detection a sensitive voltmeter measures the e.m.f. and an ammeter detects the current.
4Uses
  • Uses electromagnetic induction is the basis of generators and transformers.

Part 2 of 8Demonstrating induction with a magnet and coil

1
Connect a coil to a sensitive centre-zero voltmeter.
2
Hold the bar magnet still inside or outside the coil: the reading is zero because no field lines are cut.
3
Move the magnet into the coil: a momentary reading shows an e.m.f. is induced.
4
Move the magnet out: a reading of opposite sign shows the e.m.f. is reversed.

Part 3 of 8Demonstrating induction with a wire

1
Connect a long wire to a voltmeter and place it between the poles of a U-shaped magnet.
2
Hold the wire still: the reading is zero because no field lines are cut.
3
Move the wire between the poles: the voltmeter shows a reading.
4
Move the wire back out: the reading has the opposite sign.
  • Parallel to the field
    • a wire moved parallel to the field lines cuts none, so no e.m.f. is induced.

Part 4 of 8Factors affecting the size of the e.m.f.

Speed
  • Speed faster movement cuts field lines at a greater rate.
Number of turns
  • Number of turns each turn of the coil adds to the e.m.f.
Size of the coil
  • Size of the coil a larger area has more wire cutting the field.
Strength of the field
  • Strength of the field a stronger magnet has more field lines in a given area.
Length of wire
  • Length of wire a longer wire in the field gives a larger e.m.f.
Extended
  • Extended the e.m.f. is proportional to the rate at which field lines are cut.

Part 5 of 8Factors affecting the direction of the e.m.f.

Pole orientation
  • Pole orientation switching the poles of the magnet reverses the e.m.f.
Direction of movement
  • Direction of movement reversing the movement of the wire, coil or magnet reverses the e.m.f.
Wording
  • Wording say "more turns on the coil" not "more coils", and "a stronger magnet" not "a bigger magnet".

Part 6 of 8Lenz's law (Extended)

Lenz's law
Lenz's law the direction of an induced e.m.f. always opposes the change causing it.
North pole pushed in
  • North pole pushed in the end of the coil nearest the magnet becomes a north pole and repels it.
North pole pulled out
  • North pole pulled out that end becomes a south pole and attracts it.
Energy
  • Energy work must be done against this force, so energy is conserved.

Part 7 of 8Right-hand dynamo rule (Extended)

1First finger
  • First finger the direction of the field (N to S).
2Thumb
  • Thumb the direction of motion of the wire.
3Second finger
  • Second finger the direction of the induced current (conventional).
4Use
  • Use it gives the direction of the induced e.m.f. when a wire moves through a magnetic field.

Part 8 of 8More from the specification

1
Electromagnetic induction
  • Electromagnetic induction a voltage is induced in a conductor when it cuts magnetic field lines or the field linking a coil changes
2
Factors affecting induced voltage
  • Factors affecting induced voltage a faster movement, stronger magnet or more turns on the coil increase the induced voltage
3
Generator
  • Generator a coil rotating in a magnetic field gives an alternating current
4
Transformer
  • Transformer a primary and secondary coil on an iron core;
    • Vp/Vs = Np/Ns, and for an ideal transformer Vp x Ip = Vs x Is
5
Solenoid and motor effect
  • Solenoid and motor effect a current-carrying wire in a field experiences a force,
  • which is how a motor turns; step-up transformers increase voltage and reduce current in power lines
6
Electromagnetic induction
  • Electromagnetic induction a voltage is induced in a conductor when it cuts magnetic field lines or the field linking a coil changes
7
Factors affecting induced voltage
  • Factors affecting induced voltage a faster movement, stronger magnet or more turns on the coil increase the induced voltage
8
Generator
  • Generator a coil rotating in a magnetic field gives an alternating current
9
Transformer
  • Transformer a primary and secondary coil on an iron core;
    • Vp/Vs = Np/Ns, and for an ideal transformer Vp x Ip = Vs x Is
10
Solenoid and motor effect
  • Solenoid and motor effect a current-carrying wire in a field experiences a force,
  • which is how a motor turns; step-up transformers increase voltage and reduce current in power lines
11
Alternating current
  • Current that continually reverses direction.
12
Potential difference
  • Work done by a unit charge passing through a component.
13
Step-up transformer
  • Transformer with more turns on the secondary, raising the voltage.
14
Step-down transformer
  • Transformer with fewer turns on the secondary, lowering the voltage.
15
Permanent magnet
  • Magnet whose field is always there.
16
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
17
Primary
  • Input coil.
18
Solenoid
  • Long coil of wire carrying a current
19
Brushes
  • Carbon contacts pressing on the slip rings.

Key terms

TermMeaning
Electromagnetic inductionProduction of an e.m.f. by relative movement of a conductor and a magnetic field.
Induced e.m.f.E.m.f. produced by electromagnetic induction.
Lenz's lawAn induced e.m.f. opposes the change that causes it.
Right-hand dynamo ruleRule giving the direction of induced current: thuMb motion, First finger field, seCond finger current.
Centre-zero meterMeter whose zero is in the centre so it shows direction.

Avoid these mistakes

1
Saying a stationary magnet in a coil induces an e.m.f. — it needs relative motion or a changing field.
2
Saying "add more coils" instead of "more turns on the coil".
3
Using the left hand for the generator rule — the dynamo rule uses the right hand.
4
Forgetting that reversing the motion or the poles reverses the e.m.f.

Last updated:

Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.
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