🔄 Reverse the movement
Pushing the magnet in gives a voltage one way. Pulling it out gives a voltage the opposite way.
Electromagnetic Induction » Electromagnetic Induction
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.15
- How a voltage can be made by moving a magnet and a coil past each other
- What a sensitive voltmeter shows when a magnet goes in and out of a coil
- Three ways to make the induced voltage bigger
- How to reverse the voltage, and when a current flows
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Unlock This CourseMaking a voltage with a magnet
So far in this topic, a current has made a magnetic field, and a field has pushed on a current. Now we go the other way round. A magnet can make a voltage, with no battery at all. This is called electromagnetic induction.
Key terms:
- Electromagnetic induction: making a voltage in a conductor or a coil using a magnetic field that moves or changes.
- Induced voltage: the voltage that is made in the conductor or coil.
Learn this
A voltage is induced in a conductor or a coil when it moves through a magnetic field, or when the magnetic field changes through it.
The conductor is usually a wire. A coil is a wire wound into many turns. Either way, the key idea is that the wire and the magnetic field are moving relative to each other. It does not matter whether the wire moves or the magnet moves.
The magnet and coil experiment
You can see induction with very simple apparatus. To draw and describe it in the exam:
- Join the two ends of a coil of insulated wire to a sensitive voltmeter. This makes a complete circuit with no battery.
- Hold a bar magnet still, near the coil. The needle stays at zero.
- Push the north pole of the magnet into the coil. The needle flicks to one side.
- Hold the magnet still inside the coil. The needle goes back to zero.
- Pull the magnet out. The needle flicks the other way.
The needle only moves while the magnet is moving. When nothing moves, the magnetic field through the coil is not changing, so no voltage is induced. Even a strong magnet gives nothing if it is held still.
| What you do | What the needle does |
|---|---|
| Magnet pushed into the coil | Flicks to one side |
| Magnet held still (in, out or beside the coil) | Stays at zero |
| Magnet pulled out of the coil | Flicks to the opposite side |
| Magnet turned round, so the other pole goes in | Flicks to the opposite side |
The same thing happens if the magnet stays still and you move the coil over it. Moving a single wire up and down across a magnetic field, between the poles of two magnets, also induces a voltage in the wire.
Reversing the voltage
🧭 Reverse the pole
Push in the south pole instead of the north pole, moving the same way, and the voltage is reversed.
If you reverse both the movement and the pole, the two reversals cancel out and the voltage is in the original direction.
What changes the size of the voltage?
The needle can flick a little or a lot. Three things make the induced voltage bigger:
| Change | Effect on the induced voltage |
|---|---|
| Move the magnet (or coil) faster | Bigger |
| Use a stronger magnet | Bigger |
| Use a coil with more turns | Bigger |
Moving faster makes the field change more quickly, and that gives a bigger voltage. A slow push gives a small flick. A fast push gives a big flick.
Voltage and current
Induction makes a voltage. A voltage is what pushes charge round a circuit. A current flows only if the circuit is complete. If the coil is not connected to anything, a voltage is still induced across its ends, but no current flows. When the coil is joined to the voltmeter, or to a lamp, the circuit is complete and a small current flows while the magnet is moving.
A changing field
The spec says a voltage is induced when a magnetic field changes through a conductor or coil. A moving magnet is one way to make the field change. The key idea is always the same: no change in the field through the coil, no induced voltage.
Optional investigation
Edexcel suggests an optional practical (it is not required): investigate the factors affecting the generation of an electric current by induction. Move a magnet into a coil connected to a sensitive voltmeter. Change one factor at a time (the speed, the strength of the magnet, or the number of turns) and keep the others the same. Repeat each reading and take a mean. The needle swing shows the size of the induced voltage.
Worked example
Worked example
A student pushes a magnet into a coil joined to a sensitive voltmeter. The needle moves 2 divisions to the right. Describe what happens to the needle in each case.
(a) The magnet is held still in the coil. The field is not changing, so no voltage is induced. The needle returns to zero.
(b) The magnet is pulled out again, at the same speed. The movement is reversed, so the needle moves to the left, about 2 divisions.
(c) The magnet is pushed in twice as fast. The induced voltage is bigger, so the needle moves further to the right than 2 divisions.
Common mistakes
Common mistakes
- Saying a still magnet in a coil makes a voltage. It only does so while the magnet or coil is moving (the field is changing).
- Saying a current flows without a complete circuit. A voltage is induced, but the current needs a complete circuit.
- Forgetting to say the direction reverses when the magnet is pulled out or turned round.
- Writing "more voltage" without a reason. Name the change: faster, stronger magnet or more turns.
Exam-style question
A student connects a coil of wire to a sensitive voltmeter. She pushes the north pole of a bar magnet into the coil.
(a) State what the voltmeter shows while the magnet is moving into the coil, and when the magnet is held still inside it. [2 marks]
(b) Describe what happens to the reading when she pulls the magnet out of the coil. [1 mark]
(c) State two changes that would increase the size of the induced voltage. [2 marks]
Model answer
(a) The voltmeter shows a reading (the needle moves) while the magnet is moving in (1). It shows zero when the magnet is held still (1).
(b) The reading is in the opposite direction (1).
(c) Move the magnet faster (1). Use a stronger magnet, or use a coil with more turns (1).
Exam tip
In induction questions, always link the voltage to movement or a changing field. "The magnet is moving" earns the mark; "the magnet is near the coil" does not.