# Magnetic field due to an electric current in a wire

##### Class practical

Iron filings reveal the pattern of magnetism around a current-carrying wire, and factors that affect it.

#### Apparatus and materials

For each student group

Copper wire, PVC-covered, 26 SWG, 50-100 cm with bare ends

Cardboard, white, piece of

Wire strippers

Plotting compass

#### Health & Safety and Technical notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2 volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils will become hot.

#### Procedure

a Make a hole in the centre of the board. Pass a length of insulated copper wire through the hole, and connect it to the black and red DC terminals on the low-voltage power supply.

b Fix the white board in a horizontal position, perhaps by clipping it under a terminal on the power supply (use one of the yellow a.c. terminals not otherwise used in this experiment).

c Switch on the power supply and sprinkle iron filings onto the white board.

d Tap the board gently with a pencil and watch the pattern develop.

e Pour away the iron filings and explore the field with the small plotting compass. Reverse the supply connections to see what effect this has on the compass needle.

#### Teaching notes

1 Fast students could also try the effect of a larger number of turns.

2 Students will see that:

• a current-carrying wire produces a magnetic field around itself;
• the field reverses when the current is reversed;
• the field is stronger with more turns of the wire.

3 A simple rule to use to show the direction of the current in a wire and the direction of its associated field is the 'right hand grip rule'. With the thumb of a clenched right hand pointing in the direction of the conventional current (positive flow), then the fingers point around the current in the direction of the conventional magnetic field.

The field direction is defined as the way in which the north-seeking pole of a compass needle points.

This experiment was safety-checked in July 2007