How do magnets and electromagnets work, and what is the motor effect?
Permanent and induced magnets, magnetic fields, the magnetic field around a current-carrying wire and a solenoid, electromagnets, and the motor effect.
A focused answer to Edexcel GCSE Combined Science Topics 12 and 13 (CP12 to CP13), covering permanent and induced magnets, magnetic fields, the field around a wire and a solenoid, electromagnets, and the motor effect.
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What this dot point is asking
Edexcel wants you to distinguish permanent and induced magnets, describe magnetic fields, describe the field around a current-carrying wire and a solenoid, explain electromagnets, and describe the motor effect.
Permanent and induced magnets
Like poles repel and unlike poles attract. The force between two magnets is a non-contact force.
Magnetic fields
A magnetic field is the region around a magnet where another magnet or magnetic material feels a force. Field lines run from north to south outside the magnet, and the field is strongest where the lines are closest together, at the poles. A compass needle lines up with the field, which is how we know the Earth has its own magnetic field (its core is magnetic).
Fields from currents and electromagnets
Electromagnets are used in scrapyard cranes, electric bells, relays and loudspeakers.
The motor effect
The size of the force increases if the current increases or the magnetic field is made stronger. The direction of the force is reversed by reversing either the current or the magnetic field. This effect is used in electric motors and loudspeakers.
In a simple electric motor, a coil of wire carrying a current sits between the poles of a magnet. The two sides of the coil carry current in opposite directions, so the motor effect pushes one side up and the other side down, making the coil rotate. A device called a split-ring commutator swaps the current direction every half turn, so the coil keeps spinning the same way rather than stopping. This turns electrical energy into kinetic energy and is the basis of many everyday devices, from fans to electric vehicles.
The strength of the magnetic field is called the magnetic flux density, measured in tesla. A stronger field, a larger current and a longer length of wire in the field all increase the motor-effect force, which is why powerful motors use strong magnets and many turns of wire in the coil. Being able to explain how to change the size and direction of the force, and how a motor uses it, is a common exam requirement.
Try this
Q1. State the difference between a permanent and an induced magnet. [2 marks]
- Cue. A permanent magnet is always magnetic; an induced magnet is only magnetic while in a magnetic field.
Q2. Name the effect that produces a force on a current-carrying wire in a magnetic field. [1 mark]
- Cue. The motor effect.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksDescribe the difference between a permanent magnet and an induced magnet, and explain why a solenoid is used in an electromagnet rather than a single straight wire.Show worked answer →
A 4-mark question on magnets and electromagnets.
A permanent magnet produces its own magnetic field all the time, whereas an induced magnet only becomes magnetic when it is placed in a magnetic field and loses most of its magnetism when removed (2 marks). A solenoid (a coil of wire) is used because the magnetic fields of the many turns add together to make a much stronger, more uniform field than a single straight wire (2 marks).
Markers reward the always-magnetic versus temporarily-magnetic distinction, and the idea that the coils' fields add to strengthen the field.
Edexcel 20224 marksA current-carrying wire is placed at right angles to a magnetic field and experiences a force. State the name of this effect, describe how to increase the size of the force, and state how to reverse its direction.Show worked answer →
A 4-mark question on the motor effect.
This is the motor effect (1 mark). The force can be increased by increasing the current or by increasing the strength of the magnetic field (1 mark). The direction of the force can be reversed by reversing the direction of the current, or by reversing the direction of the magnetic field (poles) (2 marks).
Markers reward naming the motor effect, a valid way to increase the force, and reversing either the current or the field to reverse the force.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)