Investigating the magnetic force

What the Activity is for

The purpose of this activity is to allow pupils to design and carry out a simple investigation in the context of magnetism. You and the pupils should be encouraged that the correct answer is not a straight line through the origin!

What to Prepare

Per pair:

• bar magnet
• newtonmeter, 0–10 newton
• short strip of masking tape
• retort stand base
• 5 pieces of card (choose the thickness of the card to match the strength of your bar magnets: the force should fall significantly from the situation where there is no card between the magnet and the base to the situation where all five pieces of card separate the base from the magnet)
• graph paper

What Happens During this Activity

Pupils work together to investigate the force needed to drag the bar magnet from the iron base with 0,1,2,3,4,5 pieces of card between the magnet and base.

Before doing the experiment in full, you should get the pupils to sketch the shape of the force-separation graph that they expect, based on direct physical experience of pulling the magnet away from the base by hand. The experiment then acts as a more refined check on their first approximation.

Each group makes careful measurements of the forces needed with different thicknesses, preferably plotting the results as they go. This will help them to see the emerging shape. Point to the need to make several measurements for each data point, as they seek to find a true value.

Later you might ask them to produce a clean version of their graph, together with a written explanation of what it shows. They might usefully compare this to a force-extension graph for a spring, if they have already met that.