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Electromagnetic induction can occur in a coil when the current in a neighbouring coil changes.

This is called mutual induction and is the principle behind transformers, which are used to change the size of an a.c. voltages in many household appliances.

Mutual induction can be demonstrated using the apparatus shown below.

Two coils of wire are placed beside each other.

One is connected to a power supply and switch.

This is called the primary coil.

The neighbouring coil is connected to a sensitive, centre-zero meter.

This is called the secondary coil.

The power supply is switched off, but set to 3 V.

An iron core links the two coils but there is no electrical connection between them.

They are linked magnetically by the iron core, but not electrically.

Observations

• When the power supply is switched on, the needle on the ammeter flicks in one direction, and then returns to zero.
• If the switch remains closed, the needle of the ammeter remains at zero.
• When the power supply is switched off, the needle on the ammeter flicks in the other direction and then returns to zero.
• If the switch remains open, the needle of the ammeter remains at zero.

The primary coil is simply an electromagnet connected to a d.c. power supply.

• When the switch is closed, its magnetic field strength increases from zero to its maximum steady value. This changing magnetic field links with the secondary coil. The secondary coil is then a conductor in a changing magnetic field. Hence, a current is induced in the secondary coil and the needle on the ammeter flicks to one side. This is called mutual induction.
• The current is only induced when the magnetic field of the primary coil is changing. As soon as the magnetic field is at full strength it stops changing, current is no longer induced in the secondary, and the needle of the ammeter returns to zero.
• The needle of the ammeter will remain at zero until the current in the primary, and hence its magnetic field, changes again.
• When the switch is opened, the reverse happens. Current in the primary coil falls to zero, which causes its magnetic field to collapse from its maximum value to zero. This changing (decreasing) magnetic field links with the secondary coil and induces a current to flow in it in the opposite direction. The needle on the ammeter flicks to the other side.
• As soon as the magnetic field is at zero it stops changing, current is no longer induced in the secondary coil, and the needle of the ammeter returns to zero.
• If the switch is left open, no current is induced in the secondary coil. There is no current in the primary coil and hence no changing magnetic field to link the secondary coil.
• If the switch remains closed, no current is induced in the secondary coil. The magnetic field of the primary coil is not changing and so current is not induced in the secondary coil.
• The purpose of the soft iron core is to connect the coils magnetically. The core lets the changing magnetic field of the primary easily, and efficiently, link with the secondary coil.
• The d.c. supply and switch can be replaced by an a.c. supply. When the a.c. supply is switched on the needle of the ammeter moves from side to side repeatedly.

Key facts

• A changing magnetic field in the primary coil, links with the secondary coil, and induces a current to flow in the secondary coil.
• This principle is the basis of how a transformer changes the size of an a.c. current or voltage.