Electromagnetic effects

O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A current always has a magnetic field around it, and a wire carrying a current across a magnetic field feels a force. This is the motor effect. In a d.c. motor the forces on the two sides of a coil turn it, and a split-ring commutator reverses the coil current every half turn so that it keeps turning the same way.

The opposite also happens. When a conductor cuts magnetic field lines, or the field through a coil changes, an e.m.f. is induced. This is electromagnetic induction, and it needs change: a magnet at rest induces nothing. Generators and transformers work this way.

A transformer changes the size of an alternating p.d. using two coils on an iron core.

Rules to remember

Induced e.m.f. is larger when the magnet or coil moves faster, the magnet is stronger, or the coil has more turns
Lenz's law: the induced current flows in the direction that opposes the change producing it
Field of a straight wire: circles round the wire. Right-hand grip rule: thumb along the current, fingers curl the way the field goes
Left-hand rule: First finger = Field, seCond finger = Current, thuMb = Motion (force), all at right angles
Reversing the current or the field reverses the force. More turns, more current or a stronger field gives a motor coil a bigger turning effect
Transformer: V_p/V_s = N_p/N_s. It works only with a.c.
High-voltage transmission: the same power at a higher voltage needs a smaller current, so less energy is wasted heating the cables

Common mistake

Mixing up slip rings and the split-ring commutator. Slip rings are on an a.c. generator and keep each end of the coil joined to the same brush; the split ring is on a d.c. motor and reverses the coil current every half turn.

Worked example

A transformer has 4000 turns on its primary coil and 200 turns on its secondary coil. The primary coil is connected to a 230 V a.c. supply. What is the secondary p.d.?

Use V_p/V_s = N_p/N_s, so V_s = V_p × N_s ÷ N_p.
V_s = 230 × 200 ÷ 4000 = 46 000 ÷ 4000 = 11.5 V.
Check: fewer turns on the secondary, so it is a step-down transformer and the p.d. should be smaller.

Answer: 11.5 V

Practice questions

Try each one, then open the answer.

1. In a simple a.c. generator, what is the purpose of the slip rings?

A
to reverse the current every half turn
B
to keep the coil connected to the external circuit while it rotates, giving an alternating output
C
to increase the magnetic field
D
to step up the voltage

Show answer

Answer: B. Slip rings keep a continuous connection to the rotating coil. The output alternates naturally as the coil turns; the split-ring commutator of a d.c. generator would rectify it.

2. Which change does not increase the turning effect on the coil of a d.c. motor?

A
using more turns on the coil
B
increasing the current in the coil
C
using a stronger magnet
D
reversing the direction of the current

Show answer

Answer: D. The turning effect increases with more turns, a larger current and a stronger magnetic field. Reversing the current only reverses the direction of turning; its size is unchanged.

3. Which change reverses the direction of the magnetic field around a straight current-carrying wire?

A
increasing the current
B
reversing the current
C
using a thicker wire
D
moving the wire faster

Show answer

Answer: B. The field direction depends on the current direction (right-hand grip rule). Reversing the current reverses the field.

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