A voltage is induced in a conductor when it cuts magnetic field lines or when the magnetic field through it changes. No change means no induced voltage. This idea, electromagnetic induction, appears in explanation questions on generators, transformers and simple coil experiments.
It is the starting point of induction and transformers, and it builds on the field-line pictures from magnetism and fields.
What has to happen for a voltage to be induced?
There must be relative movement between a conductor and a magnetic field, so that the conductor cuts field lines. Equivalent ways to say it: the magnetic field linking the coil changes, or the coil cuts across the field.
If the circuit is complete, the induced voltage drives a current. If the circuit is open, the voltage still exists but nothing flows. A sensitive meter connected to the coil shows the effect.
What changes the size and direction?
Three changes increase the induced voltage: a faster movement, a stronger magnet, or more turns of wire in the coil. Each one increases how quickly field lines are cut.
Reversing the direction of motion reverses the direction of the induced voltage. Reversing the magnet’s poles does the same. Doing both leaves the direction unchanged.
The induced current acts to oppose the change that produced it. Some syllabus years expect you to state this, so check the wording for your exam year.
Worked example
A student pushes the north pole of a bar magnet into a coil connected to a centre-zero meter. (Invented example data.) The meter deflects to the right. Explain what happens in each of these situations.
(a) The magnet stops inside the coil. The magnet is no longer moving, so no field lines are being cut. The meter returns to zero.
(b) The magnet is pulled out at the same speed. The movement is reversed, so the induced voltage is reversed. The meter deflects to the left, by about the same amount.
(c) The magnet is pushed in at twice the speed. Field lines are cut twice as quickly, so the induced voltage is larger and the deflection is bigger.
(d) The same push, but the coil has three times as many turns. Each turn cuts the lines, so the induced voltage is about three times larger.
Notice that every answer names the change: movement, reversal, speed, turns. That is what an examiner looks for.
The mistake to watch for
Mistaken answer: “A stronger magnet in the coil produces a larger voltage, so the meter stays deflected while the magnet sits inside.”
The student linked field strength to voltage but forgot that the field must be changing.
The correction: a stronger magnet increases the induced voltage only while it moves. Once it stops, the deflection falls to zero. A good habit is to write “while the magnet is moving” in every answer about induction.
Check yourself
Try these, then open each answer.
1. A coil is connected to a meter. The magnet is held still next to it. Why does the meter read zero?
Show answer
There is no relative movement, so no field lines are cut and the field through the coil is not changing. No voltage is induced.
2. Give two ways to increase the reading when a magnet is pushed into a coil.
Show answer
Any two of: push the magnet faster, use a stronger magnet, use a coil with more turns.
3. The south pole of the magnet is pushed in instead of the north pole, at the same speed. What happens to the direction of the deflection?
Show answer
Reversing the pole reverses the induced voltage, so the meter deflects the opposite way, by about the same amount.
Where this leads next
Once you can name the required change, move on to comparing generator and motor energy transfers, where a coil is turned inside a magnetic field. The induction practice set tests the wording as well as the ideas.
If your answers describe the right situation but miss the exact point the mark scheme wants, that is a habit that improves with live feedback. Our teachers give this in online one-to-one Physics tuition.