Electromagnetism » The Magnetic Field Around a Current

A current makes a magnetic field

So far the magnetic fields in this topic have come from magnets. There is a second way to make a magnetic field: send an electric current through a conductor, such as a copper wire. The moving charge creates a magnetic field in the space around the wire.

You can show this with a simple test. Hold a plotting compass close to a wire, then switch the current on and off.

1. Set up a circuit with a power supply, a switch and a long straight wire.
2. Place a plotting compass next to the wire. With the switch open, the needle points north because of the Earth's magnetic field.
3. Close the switch. The needle moves away from north and turns to line up with the new field.
4. Open the switch. The needle swings back to north, because the field of the wire has disappeared.
5. Swap the connections to the power supply so the current flows the other way. The needle now turns the opposite way.

Keep the switch closed only for a moment. A wire joined straight across a power supply gets warm and drains the cells quickly.

Units reminder

You will meet three units again and again in this topic. You learnt them in Electrical Units and Circuit Symbols, so this is only a reminder.

Quantity	Unit	Symbol
Current	ampere	A
Voltage	volt	V
Power	watt	W

It is the current, measured in amperes, that makes the magnetic field around a wire.

Paper 2 only: the field around a straight wire

The field around a long straight wire is made of circles, with the wire at the centre. Picture the wire standing upright through a flat sheet of card. The field lines are rings drawn on the card, all around the wire.

The arrows on the circles show the direction of the field. You work out the direction with the right-hand grip rule.

Key terms:

• Right-hand grip rule: grip the wire with your right hand so your thumb points in the direction of the current. Your curled fingers then point in the direction of the magnetic field.

For a current coming out of the page, point your right thumb at yourself. Your fingers curl anticlockwise. For a current going into the page, the arrows are clockwise. Reverse the current and every arrow reverses.

Paper 2 only: the field around a flat circular coil

Now bend the wire into a single flat circle, a coil. Every small piece of the wire still has circular field lines around it. Because the wire is bent, the lines from all the pieces combine.

1. Draw the coil from the side. You see it as two small circles: one where the current comes out of the page and one where it goes in.
2. Close to each piece of wire, draw small circles around it, as for a straight wire.
3. Inside the coil, the field lines from the top and bottom of the wire point the same way, so they join into lines that pass straight through the middle of the coil.
4. Outside the coil, the lines curve round and come back to the other side, making closed loops.
5. Add arrows with the grip rule applied to the wire at the top or bottom.

The field is strongest at the centre of the coil, where the lines are closest together. Looked at from one side, if the current goes clockwise round the coil the field goes into the page through the middle. If the current goes anticlockwise, the field comes out of the page through the middle.

Worked example

Common mistakes

Exam-style question

A student places a plotting compass next to a straight wire and switches on a current.

(a) State what happens to the compass needle. [1 mark]

(b) The current is now reversed. State what happens to the field. [1 mark]

(c) The wire carries a current into the page. Describe how to draw the field pattern. [3 marks] (Part (c) is Paper 2 only.)