How do magnetic fields arise from currents, and how is force produced by the motor effect?
Magnetic fields around magnets and current-carrying wires and coils, electromagnets, the motor effect, the force on a current-carrying conductor, and the electric motor.
A focused answer to WJEC GCSE Physics topic 1.9 on electromagnetism, covering magnetic fields around magnets, wires and solenoids, electromagnets, the motor effect, the force on a current-carrying conductor and how a simple electric motor works.
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What this topic is asking
WJEC wants you to describe magnetic fields around magnets, wires and coils, explain how electromagnets are made and strengthened, and use the motor effect to explain the force on a current-carrying conductor and the action of a simple electric motor. This is part of topic 1.9 Electromagnetism in Unit 1 of WJEC GCSE Physics (3420).
Magnetic fields
The strength and direction of a wire's field depend on the size and direction of the current. Reversing the current reverses the field.
Electromagnets
Electromagnets are used in scrapyard cranes, relays, circuit breakers, loudspeakers and electric bells.
The motor effect
The size of the force increases with a stronger magnetic field, a larger current, and a longer length of wire in the field. Reversing the current or the field reverses the force.
The electric motor
A simple d.c. motor uses the motor effect. A current-carrying coil sits between the poles of a magnet. The forces on the two sides of the coil act in opposite directions, creating a turning effect that spins the coil. A split-ring commutator reverses the current in the coil every half-turn, so the coil keeps rotating the same way. The motor turns faster with a larger current, more turns, or a stronger magnet.
Try this
Q1. State the direction of the magnetic field lines outside a bar magnet. [1 mark]
- Cue. From the north pole to the south pole.
Q2. State two ways to increase the force on a current-carrying wire in a magnetic field. [2 marks]
- Cue. Increase the current; use a stronger magnetic field (or a longer wire in the field).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20183 marksDescribe three ways to increase the strength of an electromagnet made from a coil of wire around an iron core.Show worked answer →
A topic 1.9 Describe question. Award one mark each for any three of: increase the current in the coil; increase the number of turns on the coil; use a soft iron core (or add an iron core) to concentrate the field; or bring the poles closer / wind the turns more tightly. Markers reward three distinct, correct factors. A common error is to repeat one idea in two ways, or to suggest changing the voltage without linking it to a larger current.
WJEC 20224 marksExplain how a force is produced on a current-carrying wire placed between the poles of a magnet, and how the force can be reversed.Show worked answer →
A topic 1.9 Explain question. The current in the wire creates its own magnetic field, which interacts with the field of the permanent magnet (1 mark). The two fields combine, and the wire experiences a force pushing it out of the field (the motor effect) (1 mark). The direction is given by Fleming's left-hand rule, and the force is greatest when the wire is at right angles to the field (1 mark). Reversing either the current or the magnetic field (swapping the poles) reverses the force (1 mark). Markers reward the interacting fields, the resulting force, the perpendicular condition and the reversal.
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)