4.4Electricity and Magnetism
Electromagnetic Effects — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
30 min read 49 parts Lesson 46 of 65 Unit quiz
Lesson summary
4.4 Electromagnetic Effects (Part 1) — IGCSE PHYSICS notes: Explain how an e.m.f. is induced, apply Lenz’s law and the right-hand dynamo rule, and predict results of moving a magnet through a coil.Condition — an e.m.f.
- Moving conductor
- Moving field
- Detecting it
- Magnet stationary
- Magnet moved into the coil
- Magnet moved back out
Part 1 of 6Induced e.m.f.
Condition
- Condition an e.m.f. is induced in a conductor whenever there is relative movement between the conductor and a magnetic field.
Moving conductor
- Moving conductor a wire moving through a fixed field cuts the field lines, and an e.m.f. is induced in the wire.
Moving field
- Moving field when a magnet moves through a fixed coil, the field lines cut through the turns of the coil and an e.m.f. is induced in the coil.
Detecting it
- Detecting it a sensitive voltmeter measures the e.m.f.;
- if the conductor is part of a complete circuit, a current is induced and an ammeter detects it.
- No movement, no e.m.f. — if the magnet and coil are both stationary, no field lines are cut and the meter shows zero.
Part 2 of 6Lenz’s law (Extended)
Statement
- Statement the direction of an induced e.m.f. always opposes the change causing it.
Meaning
- Meaning the magnetic field produced by the induced current acts so as to try to stop the wire or magnet moving.
Part 3 of 6Demonstrating Lenz’s law (Extended)
| Action | Pole formed on the coil end nearest the magnet | Effect |
|---|---|---|
| North pole pushed into the coil | North | Repels the magnet and opposes its motion |
| North pole pulled away from the coil | South | Attracts the magnet and opposes its motion |
- In both cases the induced e.m.f. drives a current whose magnetic field opposes the movement of the magnet.
Part 4 of 6Right-hand dynamo rule (Extended)
- Point the First finger of the right hand in the direction of the Field.
- Point the thuMb in the direction of Motion of the wire.
- The seCond finger now points in the direction of the induced Current (strictly the e.m.f.).
- Remember that current is the direction of positive charge, from the positive terminal to the negative terminal.
Part 5 of 6Induction in everyday devices
Induction in everyday devices
Generators
- Generators power stations turn coils in magnetic fields to induce an alternating e.m.f.
Transformers
- Transformers a changing current in one coil induces an e.m.f. in another.
Microphones and induction hobs
- Microphones and induction hobs moving coils and changing fields induce currents in the same way.
Part 6 of 6More from the specification
1
Electromagnetic induction and mechanical energy
- Electromagnetic induction and mechanical energy moving a conductor through magnetic flux cuts field lines, converting mechanical input work into electrical energy.
2
Magnetic Field of a Solenoid
- When an electrical current flows through a long coiled wire (solenoid), it generates a uniform magnetic field inside, with field strength proportional to the current and number of turns per unit length.
3
Motor Effect on a Current-Carrying Conductor
- When a current-carrying wire is placed in a magnetic field, the interaction between fields produces a mechanical force F = B·I·L sin θ;
- this magnetic force drives the continuous rotation of an electric motor coil.
4
Transformer Voltage Equation
- A transformer alters alternating voltage;
- alternating current in the primary coil generates a changing magnetic flux in the core that induces an alternating output voltage across the secondary coil according to the equation Vp / Vs = Np / Ns.
5
Potential difference
- Work done by a unit charge passing through a component.
6
Step-up transformer
- Transformer with more turns on the secondary, raising the voltage.
7
Step-down transformer
- Transformer with fewer turns on the secondary, lowering the voltage.
8
Permanent magnet
- Magnet whose field is always there.
9
Primary
- Input coil.
10
Solenoid
- Long coil of wire carrying a current
11
Brushes
- Carbon contacts pressing on the slip rings.
12
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
Key terms
| Term | Meaning |
|---|---|
| Electromagnetic induction | The production of an e.m.f. when a conductor and a magnetic field move relative to each other. |
| Induced e.m.f. | The e.m.f. produced in a conductor by induction. |
| Lenz’s law | An induced e.m.f. acts in a direction that opposes the change causing it. |
| Right-hand dynamo rule | Finger rule giving the direction of induced current from field and motion. |
| Turns | Individual loops of wire in a coil. |
| Field lines cut | Magnetic field lines crossed by a moving conductor, giving induction. |
Avoid these mistakes
1
Believing a stationary magnet in a coil induces a current — there must be relative movement.
2
Saying "more coils" instead of "more turns on the coil".
3
Saying "a bigger magnet" instead of "a stronger magnet".
4
Forgetting that Lenz’s law means the induced effect opposes the change.
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.