Devices that use the force on a current

More than motors

The force on a current in a field has many uses besides the motor. Here we study three.

The moving-coil galvanometer

A galvanometer is an instrument that detects a small current and shows its direction. An ammeter and a voltmeter are made from it.

It has a coil of wire on a soft iron core, that can turn between the poles of a strong magnet. A pointer is joined to the coil, and a spring pulls it back to the zero.

• When a current flows in the coil, the forces turn the coil, and the pointer moves along a scale.
• A larger current gives a larger force, so the pointer moves farther. The deflection is proportional to the current.
• When the current is reversed, the pointer moves the other way.

You should find that twice the current gives twice the turn, and that a negative current turns the pointer the other way.

The loudspeaker

A loudspeaker changes an electrical signal into a sound.

• A coil is placed in the field of a permanent magnet. It is joined to a paper or plastic cone.
• The signal of the music is a current that changes all the time, in size and in direction.
• The force on the coil changes with the current, so the coil moves to and fro, and it moves the cone with it.
• The cone pushes the air to and fro, and this is the sound wave that we hear.

A stronger current gives a larger movement, so a louder sound. A signal of a higher frequency gives a higher pitch.

The microphone

A moving-coil microphone works in the opposite way. A sound wave moves a thin sheet with a coil in a magnetic field. The moving coil makes a small current, which follows the sound. We study how a moving coil makes a current in the next course.

Other uses

• Electric motors in fans, pumps, trains, cranes and electric cars.
• Electric meters with pointers on a scale, for example in a car.
• Relays and electric locks, where a current moves a small part.

Practise with new numbers

Try it in Python

The program shows how the pointer of a galvanometer turns for different currents.

Remember these

• A galvanometer has a coil in a field, with a pointer. The deflection is proportional to the current.
• A loudspeaker has a coil in a field, joined to a cone. A changing current makes the cone vibrate, and this gives sound.
• A microphone uses the opposite idea: a sound moves a coil, and this makes a current.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What makes the pointer of a galvanometer move?

2. How does a loudspeaker make sound from a changing current?

3. What happens to the pointer of a galvanometer when the current is reversed?

4. A galvanometer turns $90^\circ$ for $100\ \text{mA}$. How far does it turn for $35\ \text{mA}$?