# Force between electromagnets

##### Demonstration

To introduce and revise electromagnets.

#### Apparatus and materials

For each student group

Unilab coils (or equivalent), 60 + 60 turns, 2

Clamp stand & boss, 2

Power supply, low voltage, variable

Aluminium rod (e.g. from clamp stand)

Steel rod (e.g. from clamp stand)

Ruler (30 cm, 50 cm or metre rule)

#### Health & Safety and Technical notes

This involves switching on the lab pack with the voltage turned up. Though warned against doing this in the literature accompanying our Unilab power supplies, we haven't had any problem over the years we've been doing this.

This involves running the coils at a greater current than they are rated for, though only for a brief time. If students were to try, they could melt the coils.

The coils initially "kick" apart, then "creep" apart if the supply is left on. Students shouldn't leave the supply on: this will either make the lab pack trip out, or melt the coils. The experiment investigates the "kick", not the "creep".

#### Procedure

a Clamp the steel bar horizontally between two clamp stands, after putting on it two 60 + 60 Unilab coils. Adjust the coils so that they are a few mm apart.

b Connect the coils to the power supply so that, when the supply is switched on, the coils attract. Switch off.

c Reset the distance between the coils to a few mm and change the connections so that, when the supply is switched on, the coils repel. Switch off and note the ‘kick distance’, i.e. the new distance between the coils.

d Repeat step c, for a range of outputs from the power supply, starting with 12 V and working down. Record the kick distance for each output.

e Replace the steel rod with one made of aluminium and repeat step d

#### Teaching notes

1 Steps a - c can be used to introduce this as a class experiment. It provides a good quick activity if the results are not recorded, but also produces good graphs if results are recorded.

2 The point of doing step b before c is that students may have to change their connections to make the coils repel. It is important to start with the maximum potential difference (pd) across the coils, so that students see something significant happen. If you start with a small pd across the coils, nothing happens and students are likely to lose interest.

Likewise, it is best to start with the steel rod before trying the aluminium rod.

3 This experiment is particularly good for helping students appreciate the effect of core material on the strength of an electromagnet.

This experiment was submitted by John Myers from Ilkley Grammar School.