Electromagnets and motors Cambridge IGCSE Physics revision
Not started
Learn it
In plain words
Electricity and magnetism are linked. A current in a wire makes a magnetic field around it; coil the wire up and you have an electromagnet you can switch on and off. Put a current-carrying wire in another magnet's field and it gets pushed. That push is what turns an electric motor.
Three things to know
- A current in a wire makes a magnetic field: circles round a straight wire, and like a bar magnet's for a coil (solenoid). More current, more turns or an iron core make an electromagnet stronger.
- A current-carrying wire in a magnetic field feels a force (the motor effect). Reversing the current or the field reverses the force.
- In a d.c. motor a coil turns because the forces on its two sides are in opposite directions. The split-ring commutator reverses the current every half turn to keep it turning the same way.
Tips and tricks
- Three ways to make an electromagnet stronger: more current, more turns, an iron core. Learn them as a set.
- Fleming's left-hand rule gives the direction of the push: First finger Field, seCond finger Current, thuMb Motion.
It lands in your notebook with its questions as flashcards.
Electromagnets and motors: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the shape of the magnetic field around a straight current-carrying wire?
The field lines are concentric circles centred on the wire.
Which change makes an electromagnet weaker?
Field strength depends on current, turns and the core.
A wire in a magnetic field feels a force when a current flows. What happens if the current is reversed?
The direction of the force depends on the direction of the current and of the field.
What does the commutator do in a d.c. motor?
Without it the coil would stop after half a turn.
Which would make a d.c. motor turn faster?
A bigger current gives a bigger force on the coil.
Why is soft iron used as the core of an electromagnet?
The magnet can then be switched on and off.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Electromagnets and motors
Cambridge IGCSE Physics 0625 · 9 marks · papermunch.org
Name ______________________________ Date ______________
State three ways of making an electromagnet stronger.[3]
Show answerHide answer
Increase the current, put more turns on the coil, and use a soft-iron core.
A wire carrying a current is placed between the poles of a magnet and moves upwards. State two ways to make it move downwards.[2]
Show answerHide answer
Reverse the direction of the current, or reverse the direction of the magnetic field.
Explain the job of the split-ring commutator in a d.c. motor.[2]
Show answerHide answer
It reverses the current in the coil every half turn, so the coil keeps turning in the same direction.
Describe the shape of the magnetic field around a straight wire carrying a current.[2]
Show answerHide answer
Concentric circles around the wire, closer together nearer the wire.
Answers: Electromagnets and motors
- 1. Increase the current, put more turns on the coil, and use a soft-iron core.
- 2. Reverse the direction of the current, or reverse the direction of the magnetic field.
- 3. It reverses the current in the coil every half turn, so the coil keeps turning in the same direction.
- 4. Concentric circles around the wire, closer together nearer the wire.