The magnetic force and the magnetic field
Classroom Activity for 11-14
What the Activity is for
You can use this activity to introduce the idea of a magnetic field as a region of space where forces are exerted. You introduce a tangible, visible analogue of the magnetic field, that fills a region of space and exerts a force on objects that enter that space.
What to Prepare
- a pair of model railway trucks on a short section of track (or low mass, low friction trolleys that track well) with a strong button magnet mounted on the end of each, with similar poles facing
- a pair of model railway trucks on a short section of track (or low mass, low friction trolleys that track well) with a piece of very soft foam mounted on the end of each
What Happens During this Activity
Push the magnetic trucks together and feel them resisting, even when the trucks are not touching. There seems to be something there, even though we cannot see it. You find it hard to make one magnet invade the space around the other. Release the trucks and let them spring apart, noticing that one magnet throws the other out of its space. As you do this you'll want to talk the pupils through these thoughts.
Repeat the sequence with the two trucks that have foam on the ends. Here, draw attention to the things in the space around the end of the truck becoming pushed out of shape, and show that it pops back when the trolleys spring apart again.
You can draw parallels between the two situations. There is something in the space beyond the truck. If you try and invade that volume of space, then a force is acting on the thing that does the invading. Once the invader has retreated, then the space is again filled by the thing, back in its original shape.
You might then suggest, with some confidence, that the magnetic fields and the foam have plenty in common. A fruitful way of thinking with magnetic fields is to consider their shape and how it is distorted and pops back, so exerting a force.
You might take this further by:
- Reversing one magnet and swapping the foam for rubber bands, so modelling attraction rather than repulsion.
- Comparing this attraction to gravity modelled by a spring.
- Exploring similarities with a spring of compressed air, felt in a sealed syringe.