Electromagnetic induction

A conductor can develop a voltage when it moves across a magnetic field or when the magnetic field linking it changes. This production of voltage is called electromagnetic induction. The voltage produced is an induced electromotive force, or induced e.m.f.

For example, moving a wire across a magnetic field can induce an e.m.f. in the wire. An e.m.f. can also be induced in a stationary coil when a magnet is moved towards it, changing the magnetic field linking the coil.

The induced e.m.f. can drive a current only if there is a complete circuit for charge to flow around. If the circuit is open, an e.m.f. can still be induced, but there is no continuous path for a current.

Demonstrating electromagnetic induction

A current can be produced in a coil without connecting it to a battery. Connect a coil of wire to a sensitive galvanometer, which detects small currents, and move a bar magnet into the coil. The galvanometer deflects, showing that a current flows while the magnet is moving.

The moving magnet causes the magnetic field linking the coil to change. This induces an electromotive force (e.m.f.) in the coil, which drives a current around the connected circuit. This production of an e.m.f. is called electromagnetic induction.

What the galvanometer shows

Pull the magnet out of the coil and the galvanometer deflects in the opposite direction. Reversing the magnet’s motion reverses the direction of the induced current.

Hold the magnet stationary relative to the coil and the deflection returns to zero. The magnet is still present, but the magnetic field linking the coil is no longer changing, so no e.m.f. is induced.

These observations show that it is a change in the magnetic field linking the coil, not simply the presence of a magnet, that produces the induced e.m.f.

Factors affecting induced e.m.f.

An induced e.m.f. is a voltage produced when the magnetic field linking a conductor changes. The faster this change occurs, the greater the magnitude of the induced e.m.f.

A change can occur because a conductor moves across a magnetic field, or because the magnetic field itself changes while the conductor is stationary.

Motion through a magnetic field

When a conductor or coil moves across a magnetic field, its movement changes the magnetic field linking it. The induced e.m.f. increases if:

• the conductor or coil moves faster across the field;
• the magnetic field is stronger, for the same motion.

Moving faster makes the field linking the conductor change more quickly. A stronger field produces a larger change for the same motion.

Number of turns in a coil

A coil with more turns produces a greater induced e.m.f., provided each turn experiences the same change in magnetic field. The e.m.f.s induced in the turns add together.

Changing the field around a stationary conductor

The conductor does not have to move. If the magnetic field linking a stationary conductor changes, an e.m.f. is induced. A more rapid change in that field produces a greater induced e.m.f.

Lenz's law

A changing magnetic field linking a conductor can induce an electromotive force (e.m.f.). Lenz’s law states that the induced e.m.f. has a direction that opposes the change causing it.

If the conductor forms a complete circuit, the induced e.m.f. drives a current. This current produces its own magnetic field, which opposes the change in the magnetic field linking the circuit:

• If the original field linking the circuit is getting stronger, the induced field acts against it.
• If the original field linking the circuit is getting weaker, the induced field acts in the same direction as it, opposing the decrease.

The induced field therefore opposes the change, not necessarily the original magnetic field.

For example, when a magnet moves towards a coil in a complete circuit, the magnetic field linking the coil increases. The induced current produces a field that opposes this increase. When the magnet moves away, the field linking the coil decreases, so the induced current reverses direction and produces a field that opposes the decrease.

If the circuit is incomplete, an e.m.f. can still be induced, but there is no current around the circuit to produce this opposing magnetic field.

Fleming's right-hand rule

When a conductor moves across a magnetic field, a current can be induced in it. Fleming's right-hand rule relates the direction of the conductor's motion, the magnetic field and the induced conventional current.

Hold the thumb, first finger and second finger of your right hand at right angles to one another:

• Thumb: the direction of the force producing the conductor's motion.
• First finger: the direction of the magnetic field, from north to south.
• Second finger: the direction of the induced conventional current.

If two directions are known, orient the corresponding fingers in those directions. The remaining finger or thumb then shows the third direction. The current direction given by the rule is the direction of conventional current.

An induced current flows only if the conductor is part of a complete circuit. If the circuit is open, there is no continuous path for current to flow.