Inducing a potential difference

1. A conductor has a potential difference induced across its ends whenever the magnetic field passing through that conductor changes.
2. The change can be produced by relative movement between the conductor and a magnet, or by a magnetic field that grows or collapses in strength while everything stays still.
3. A stationary conductor sitting in a steady magnetic field has nothing induced across it, however strong that field is, because nothing about the field through it is changing.
4. The induced potential difference exists across the ends of the conductor whether or not any current flows, in the same way that a cell has a potential difference across its terminals before it is connected to anything.
5. For a straight wire, the field through the wire only changes if the wire cuts across the magnetic field lines, so moving the wire along the field lines induces nothing.

When an induced current flows

1. An induced current flows only when the conductor forms part of a complete circuit.
2. Connecting the ends of a coil to a sensitive ammeter completes the circuit, so the needle deflects while the magnet is moving and returns to zero when the movement stops.
3. Leaving the ends of the coil unconnected still gives an induced potential difference, but no charge can circulate, so no current is measured.
4. The size of the induced current follows $I=\frac{V}{R}$, so a coil made of thicker wire or a circuit with less resistance gives a larger current for the same induced potential difference.
5. The needle deflects in one direction while the magnet approaches and in the opposite direction while it recedes, which shows that the direction of the induced current depends on the direction of the change.

Ways of producing the change

1. Pushing a bar magnet into a coil of wire increases the field through the coil, and pulling it out decreases the field, so both movements induce a potential difference.
2. Holding the magnet still and moving the coil over it works exactly as well, because only the relative movement of the two matters.
3. Moving a straight wire downwards between the poles of a horseshoe magnet makes the wire cut across the field lines and induces a potential difference along its length.
4. Rotating a coil continuously inside a magnetic field keeps the field through the coil changing, which is how a generator produces a continuous output.
5. Switching a current on or off in a nearby coil makes that coil's magnetic field grow or collapse, and the changing field induces a potential difference in the second coil even though nothing moves.
6. Moving a magnet sideways past a coil, so that the field through the coil barely changes, induces very little.

Direction of the induced potential difference

1. Reversing the direction of movement reverses the induced potential difference, so pushing a magnet in and pulling it out give deflections on opposite sides of zero.
2. Reversing the poles of the magnet, so that the south pole leads instead of the north pole, also reverses the induced potential difference.
3. Reversing the movement and the poles together leaves the direction of the induced potential difference unchanged, because the two reversals cancel.
4. Moving the magnet in and out repeatedly produces a potential difference that repeatedly changes direction, which is an alternating potential difference.

Energy transfer during induction

1. The induced current in the coil is itself a current in a loop of wire, so it produces its own magnetic field.
2. This induced field always acts to oppose the change that produced it, so the coil repels an approaching pole and attracts a receding pole.
3. A force must therefore be applied to keep the magnet moving, and the work done against that opposing force is the source of the electrical energy.
4. Energy is transferred from the kinetic store of the moving magnet, through the electrical working of the circuit, and finally to the thermal store of the surroundings as the induced current heats the wires.
5. If the induced field helped the change rather than opposing it, the magnet would accelerate on its own and energy would appear from nowhere, which conservation of energy forbids.
6. This is why a bicycle dynamo makes the pedals harder to turn once its lamp is switched on, and why a generator supplying more current needs more driving power.

Reading an induction trace

1. A data logger connected across the coil records how the induced potential difference changes with time, giving a pulse rather than a steady value.
2. Each pulse begins at zero, rises to a peak while the field is changing fastest, and returns to zero when the movement stops.
3. A faster movement gives a pulse that is taller and narrower, because the same change in the field is squeezed into a shorter time.
4. A pulse below the zero line means the induced potential difference is in the opposite direction to one above the line, not that it is smaller.