What is the shape of the magnetic field around a straight current-carrying wire?

circles around the wire. The field lines are concentric circles centred on the wire.

Which change makes an electromagnet weaker?

a smaller current. 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 force reverses direction. 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?

reverses the current in the coil every half turn. Without it the coil would stop after half a turn.

Which would make a d.c. motor turn faster?

a larger current. A bigger current gives a bigger force on the coil.

Why is soft iron used as the core of an electromagnet?

It is easily magnetised and demagnetised. The magnet can then be switched on and off.

Electromagnets and motors · Edexcel IGCSE Science (Single Award)

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.

6 questions, about 2 minutes.

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?
- straight lines along the wire
- circles around the wire (the answer)
- lines pointing away from the wire
- there is no field
The field lines are concentric circles centred on the wire.
Which change makes an electromagnet weaker?
- more turns
- a larger current
- adding an iron core
- a smaller current (the answer)
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 force disappears.
- The force doubles.
- The force reverses direction. (the answer)
- Nothing changes.
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?
- increases the voltage
- reverses the current in the coil every half turn (the answer)
- cools the coil
- stops the coil
Without it the coil would stop after half a turn.
Which would make a d.c. motor turn faster?
- a weaker magnet
- fewer turns on the coil
- a larger current (the answer)
- reversing the battery
A bigger current gives a bigger force on the coil.
Why is soft iron used as the core of an electromagnet?
- It is a permanent magnet.
- It is easily magnetised and demagnetised. (the answer)
- It is an insulator.
- It is light.
The magnet can then be switched on and off.

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.