# Play with magnets

##### Class practical

These investigations provide an introduction to magnets and their interactions.

#### Apparatus and materials

For each student group

Permanent bar magnets

Cotton

Plotting compasses

Wooden burette stand (optional)

#### Health & Safety and Technical notes

'Permanent bar magnets' are bar-shaped magnets with opposite poles at either end. 'Magnadur magnets' are ceramic magnets with their poles on their flat faces. (It is interesting to see if the students can discover this for themselves.) See the illustrations. These magnets can come to no harm magnetically, but being ceramic can chip and fracture like china. If two are placed near to each other, oriented to attract, they may move together with sufficient violence to do damage.

Small plotting compasses are very cheap. Badly balanced ones or those with sticky pivots should be discarded. Polarity is easily reversed; students should check which is the north-seeking pole of the compass needle. If their magnetisation is weak, they will oscillate slowly in the Earth's field. To strengthen their magnetisation, place them in a strong field, for example between poles of a pair of strong bar magnets.

#### Procedure

a Play with bar magnets, studying the feel of attraction and repulsion. Avoid banging the magnets together or forcing them very close together against their mutual repulsion as this might ultimately weaken them

b Suspend a magnet freely in a stirrup. This can consist of two loops of cotton, as shown in the picture, arranged to hang the magnet as indicated.

A pencil projecting over the edge of a desk or bench and held by weights (for example, books) serves for a support. Alternatively a pencil or wooden rod can be fixed in a non-magnetic retort stand (brass, wood or stainless steel). This should be used to see how the magnet sets by itself. Let them find the effect of bringing another magnet near by.

c Use the small compass instead of the suspended magnet. This is more convenient. It can be moved around the bigger magnet to show the direction of pull at different places.

d Arrange two Magnadur magnets as shown; show that the poles are on the large faces.

#### Teaching notes

1 Students should be able to find out the following.

• Permanent magnets normally have two poles, a north-seeking pole and a south-seeking pole (using 'seeking pole' helps with the problem of what is at the Earth's magnetic poles).
• Similar poles repel and dissimilar poles attract. Students should have the opportunity to feel the forces of attraction and repulsion.
• A suspended magnet will settle in a particular direction in the Earth's field (and at a particular angle to the horizontal).
• A suspended magnet will be deflected by another magnet brought near to it.
• A compass needle is deflected by a magnet.

2 How Science Works Extension: You might like to put these findings in a historical context. Compasses were used as practical devices for many centuries without a scientific understanding of how they work. One idea was that the north magnetic pole of a magnet was attracted to the Pole Star. This was a credible idea for people who were used to using the stars for navigation. However, compass-makers noted an interesting phenomenon. When they made an iron compass needle, they carefully balanced it before magnetising it. When it was magnetised, they found that it was no longer balanced; rather, it tipped downwards with its north pole tilting into the Earth. This gave William Gilbert (1544-1603) the idea that magnetism arose from inside the Earth, not in the stars. He made a model Earth with a magnet inside it and showed that small compasses held nearby reproduced sailors’ observations of the Earth’s field.

Once the Earth’s magnetism was established, scientists went on to use the idea elsewhere. For example, magnetism is an example of action-at-a-distance, so theories were developed that suggested that the Earth was held in its orbit around the Sun by magnetic attraction between them.

This experiment was safety-checked in December 2004