7.3.1 Induced potential (HT)
The generator effect and induced potential difference
Generator effect
The generator effect is the induction of a potential difference across a conductor when the conductor moves relative to a magnetic field, or when the magnetic field around it changes.
- When a conductor moves relative to a magnetic field, a potential difference is induced across the ends of the conductor.
- A potential difference is also induced if the conductor is stationary but the magnetic field around it changes.
- This happens because the conductor cuts through magnetic field lines, or feels a changing field, which separates charge and produces the induced potential difference.
- If the conductor is part of a complete circuit, the induced potential difference makes an induced current flow.
- For example, pushing a bar magnet into a coil connected to a sensitive ammeter makes the ammeter read a current for as long as the field around the coil is changing.
A potential difference can be induced in an incomplete circuit, but an induced current flows only if the circuit is complete.
The induced current opposes the change
- An induced current produces its own magnetic field, which opposes the change that caused the current.
- If a magnet is pushed into a coil, the induced current's field opposes the magnet moving in; if the magnet is pulled out, the current reverses and its field opposes the magnet moving out.
- This does not stop the motion, but work must be done to keep the magnet moving, so energy is transferred mechanically to the generator and then electrically to the circuit.
Question: A student pushes the north pole of a bar magnet into a coil connected to an ammeter, and the needle deflects. The student then holds the magnet still inside the coil. Explain what happens.
Answer: While the magnet moves in, the field around the coil changes, so a potential difference is induced; because the circuit is complete, an induced current flows and the ammeter deflects. When the magnet is held still, the field is no longer changing, so no potential difference is induced and the current falls to zero.
Factors affecting the size of the induced potential difference
- The induced potential difference is larger if the conductor or magnet moves faster.
- It is larger if a stronger magnetic field is used.
- It is larger if the coil has more turns.
- A larger induced potential difference gives a larger induced current, provided the circuit's resistance is unchanged.
- If the magnet is stationary relative to the coil, no potential difference is induced, even in a strong field: there must be relative movement or a changing field.
Factors affecting the direction of the induced potential difference
- The direction of the induced potential difference, and of the induced current, depends on the direction of the change.
- The direction reverses if the movement is reversed, if the magnetic field is reversed, or if a magnet is moved out of a coil instead of in.
- For example, if pushing a magnet in deflects the ammeter one way, pulling it out deflects the needle the other way, because the induced current must oppose the new change.
- Do not say a current is induced just because a conductor sits in a magnetic field; there must be relative movement or a changing field.
- An induced potential difference can exist with no current if the circuit is incomplete.
- Use the chain: movement or changing field, change in field around the conductor, potential difference induced, current if the circuit is complete.
- State that the induced current makes a field that opposes the original change, and name the factor (speed, field strength or turns) the question is testing.
- What is the generator effect?
- When does an induced potential difference produce a current?
- Name three factors that increase the size of the induced potential difference.
- Why is no potential difference induced when a magnet is held still inside a coil?
- What three changes reverse the direction of the induced potential difference?
- Where does the energy come from when an induced current flows?
