Moving a conductor and a magnetic field past each other creates an e.m.f.
Definition
Electromagnetic induction
The production of an e.m.f. across a conductor when it moves across a magnetic field, or when the magnetic field linking it changes.
- There are two ways to get the same result: move the conductor across a stationary field, or hold the conductor still and change the field linking it.
- Push a bar magnet into a coil connected to a sensitive centre-zero meter and the needle kicks, which shows an e.m.f. has been induced.
- Pull the magnet back out and the needle kicks the other way, because the change is now in the opposite direction.
- Hold the magnet still inside the coil and the needle drops back to zero, because nothing is changing any more.
- Relative movement is the whole point: it is the change in the field that induces the e.m.f., not the presence of a field.
- Moving the coil over a stationary magnet works just as well, which confirms that only the relative motion matters.
- This single effect is what generators, transformers, induction hobs and contactless card readers all run on.
Four things make the induced e.m.f. bigger
- Moving the magnet or the conductor faster gives a larger e.m.f., because the field linking the coil changes more quickly.
- Using a stronger magnet gives a larger e.m.f., because there is more field to change.
- Using more turns on the coil gives a larger e.m.f., since every turn contributes its own share.
- Using a coil of larger area gives a larger e.m.f., because more of the field passes through it.
- All four come down to the same rule: the faster the field linking the conductor changes, the larger the e.m.f. induced.
Practical
Investigation: inducing an e.m.f. with a magnet and a coil
- Aim: to show that an e.m.f. is induced only during relative movement, and to find what makes it larger.
- Apparatus: a coil of insulated copper wire, a second coil with more turns, a bar magnet, a sensitive centre-zero meter and connecting leads.
- Method:
- Connect the coil directly to the centre-zero meter, so that the needle can swing either side of zero and show the direction as well as the size.
- Push the N pole of the bar magnet quickly into the coil, watching which way the needle kicks and how far it goes.
- Hold the magnet completely still inside the coil and note that the needle falls back to zero.
- Pull the magnet back out at the same speed and note that the needle now kicks the opposite way.
- Repeat, pushing the magnet in slowly and then quickly, and compare the size of the two deflections.
- Turn the magnet round so the S pole goes in first, and note the direction of the kick.
- Swap in the coil with more turns and repeat one movement, keeping the speed and the magnet exactly the same.
- Finally hold the magnet still and move the coil over it instead, to show that only the relative movement matters.
- Measure and record: the readings are comparative, so tabulate the size and the direction of the deflection under movement and deflection; no graph is needed.
- Result: an e.m.f. is induced only while there is relative movement; a faster movement, a stronger magnet or more turns all give a larger deflection; reversing the movement or reversing the pole reverses the direction of the deflection.
- Watch out:
- Move the magnet at a controlled, repeatable speed, since comparing a fast push with a slow one is the whole point of the experiment.
- Keep the meter on a sensitive range so that a small e.m.f. still gives a readable deflection.
- Change only one thing at a time when comparing speed, turns and magnet strength, or you cannot say which caused the difference.
- Watch the needle at the moment of movement, because the kick is brief and easy to miss.
- Safety: minimal risk, although the magnet can snap onto the metal meter case and damage it, so keep it clear of the instrument.
An e.m.f. appears even when no current can flow
- The induced e.m.f. appears across the ends of the conductor whether or not the circuit is complete.
- Join those ends through a meter and the e.m.f. drives an induced current round the loop.
- Leave the circuit open and the e.m.f. is still there, but with no path there can be no current.
- The wording matters: the changing field induces an e.m.f., and the e.m.f. is what drives the current.
- A bicycle dynamo shows it in practice: spinning it against the tyre induces an e.m.f., and the lamp lights only once the circuit through it is complete.
Common Mistake
- A magnet resting inside a coil induces nothing, because an e.m.f. needs the field to be changing.
- Moving a conductor along the field lines rather than across them induces nothing, so the direction of the movement matters.
- Do not say the magnet "gives" the coil a current; it induces an e.m.f., which then drives a current if there is a complete circuit.
- Pushing the magnet twice as far does not double the e.m.f.; it is the rate of change that counts, not the distance.
Exam technique
- Describing the experiment earns marks for the coil and centre-zero meter, the movement of the magnet, and what the needle does when the magnet is moved and when it is held still.
- A centre-zero meter is worth naming, because it shows the direction reversing as well as the size.
- When you list the factors, give the direction of the effect each time, as in "a faster movement gives a larger e.m.f.".
- Use "e.m.f." rather than "voltage" in an induction answer, since that is the quantity being induced.
Self review
- What is electromagnetic induction?
- Describe an experiment that demonstrates it.
- Why does the meter read zero when the magnet is held still inside the coil?
- State three factors that increase the size of an induced e.m.f.
- What is the difference between an induced e.m.f. and an induced current?
