Inducing a potential difference
Definition
Electromagnetic induction
The production of a potential difference across a conductor when the magnetic field around it changes.
Definition
Induced potential difference
A potential difference produced across the ends of a conductor by a change in the magnetic field around it.
Definition
Induced current
A current driven around a complete circuit by an induced potential difference.
- When the magnetic field around a conductor changes, a potential difference is produced across the ends of the conductor.
- This effect is called electromagnetic induction, and the potential difference it produces is an induced potential difference.
- One way to change the field is to move a magnet towards or away from a coil of wire, so the field passing through the coil grows or shrinks.
- Another way is to move a wire across the field lines of a magnet, which is often described as the wire cutting the field lines.
- Only relative motion matters, so moving a magnet past a still coil has the same effect as moving the coil past a still magnet.
- A wire moving along the field lines does not cut them, so no potential difference is induced across it.
- If there is no change in the field, nothing is induced, so a magnet resting inside a coil induces no potential difference however strong the magnet is.
- If the conductor is part of a complete circuit, the induced potential difference drives an induced current around the circuit.
- If the circuit is broken, a potential difference still appears across the ends of the conductor, but no current flows.
Size and direction of the effect
- The induced potential difference is larger when the magnetic field changes faster, so moving the magnet more quickly gives a larger reading.
- A stronger magnet gives a larger induced potential difference for the same movement, because the field changes by a greater amount.
- A coil with more turns gives a larger induced potential difference, because a potential difference is induced in every turn and these add together along the coil.
- Reversing the direction of movement, such as pulling the magnet out instead of pushing it in, reverses the direction of the induced potential difference.
- Reversing the pole that faces the coil, such as pushing in a south pole instead of a north pole, also reverses the induced potential difference.
- Reversing both the movement and the pole together gives an induced potential difference in the original direction.
Opposing the change
- An induced current flowing in a coil produces its own magnetic field, just like any other current in a coil.
- The induced current always flows in the direction that makes its magnetic field oppose the change that produced it.
- When a north pole is pushed towards one end of a coil, the induced current makes that end of the coil a north pole, which repels the approaching magnet.
- When the north pole is pulled away again, the induced current reverses and that end of the coil becomes a south pole, which attracts the magnet and resists it leaving.
- In both cases the coil pushes back against the movement, so you have to do work to move the magnet.
- The work you do against this opposing force is the energy that the induced current carries around the circuit.
- If the induced field helped the movement instead, the magnet would speed up on its own and energy would appear from nothing, which cannot happen.
- If the circuit is broken, no current flows and there is no opposing field, so the magnet moves freely even though a potential difference is still induced.
Practical
Inducing a current with a magnet
- Aim: to show that a changing magnetic field induces a potential difference and a current, and to find what affects the size and direction of the effect.
- Apparatus: a coil of insulated wire with about 600600600 turns, a second coil with about 300300300 turns, two strong bar magnets, a sensitive centre-zero ammeter (galvanometer), connecting leads and a cardboard tube that the magnet can slide through.
- Method:
- Connect the ends of the 600600600-turn coil directly to the centre-zero meter, with nothing else in the circuit, and place the meter about a metre away so the magnet cannot pull on its needle.
- Hold the bar magnet still with its north pole just outside one end of the coil, and check that the needle reads zero.
- Push the north pole into the coil at a steady speed, and note the direction in which the needle moves.
- Hold the magnet still inside the coil and watch the needle.
- Pull the magnet back out and note the direction in which the needle moves.
- Repeat with the south pole facing the coil.
- Hold the magnet still and move the coil over it instead.
- Stand the coil upright with the tube passing through it, and drop the magnet through the tube from heights of 10 cm10\ \text{cm}10 cm, 20 cm20\ \text{cm}20 cm and 40 cm40\ \text{cm}40 cm, recording the largest reading each time, so that the speed is controlled by the drop height.
- Repeat the drops with two magnets held together, and then with the 300300300-turn coil in place of the 600600600-turn coil.
- Results:
- The needle moves only while the magnet or the coil is moving, and it returns to zero whenever both are still.
- Pushing the magnet in and pulling it out move the needle in opposite directions.
- Turning the magnet round to use the south pole reverses each of these readings.
- Moving the coil instead of the magnet gives the same readings, which shows that it is the relative movement that matters.
- A larger drop height, two magnets and the coil with more turns each give a larger reading.
- Watch out: a coil with only a few turns gives a reading too small to see, and a magnet held near the meter moves its needle directly, so keep the two well apart.
- Safety: strong magnets can snap together and trap fingers, and should be kept away from phones and bank cards.
Example
Magnet falling towards a coil
- A bar magnet falls with its south pole downwards towards the top of a vertical coil connected to a meter.
- As the south pole approaches, the magnetic field through the coil increases, so a current is induced in the coil.
- The induced field opposes the approach, so the top of the coil becomes a south pole.
- The two south poles repel, so there is an upward force on the falling magnet and it speeds up less than it would in free fall.
- If the ends of the coil are disconnected, no current flows, there is no opposing field, and the magnet falls freely.
Common Mistake
- Do not say that a magnet held still inside a coil induces a current, because the magnetic field must be changing.
- Do not say that the induced field helps the magnet along, because it always opposes the change that produced it.
- Do not say that a broken circuit has no induced potential difference, because the potential difference is still induced and only the current is missing.
Exam technique
Describing induction
- Use the word induced whenever you describe a potential difference or current produced by a changing magnetic field.
- Link the cause and the effect in one sentence: a change in the magnetic field around the conductor induces a potential difference across its ends.
- For ways to increase the reading, give specific changes, such as moving the magnet faster, using a stronger magnet or using a coil with more turns.
- When asked why a magnet is harder to push into a coil in a complete circuit, explain that the induced current produces a magnetic field that opposes the movement.
Self review
- What is electromagnetic induction?
- Why does a magnet held still inside a coil induce no potential difference?
- State three ways of increasing the induced potential difference.
- What is needed for an induced potential difference to drive a current?
- Explain why it is harder to push a magnet into a coil when the coil is part of a complete circuit.
