How can you find out the strength of a magnet?
1. Take two identical magnets one in each hand.
2. Stretch out your arms in front of you and point the poles of the magnet inwards towards one another (choose two identical poles so that the magnets repel one another).
3. Bring the magnets closer and closer together until they touch pole to pole.
Even if the magnets arenít all that powerful you should notice the pushing force trying to keep them apart and that it is very difficult to force them to touch in a straight line face to face.
We know that the repulsive force exists and that it is quite strong but how can it be measured?
Here are some ideas which could lead to various experiments.
1. Toy cars or trains pushing apart or pulling themselves together.
Use blu tac to attach identical magnets to the front of two identical lightweight toy cars and try gradually bringing them closer and closer together. What happens? Try matching up similar poles then repeat with opposite poles.
If you had a ruler could you make a measurement which helped determine how strong these magnets were compared to another pair of magnets?
2. Ring magnets with centre holes
Place two ring magnets on a wooden rod so that they repel one another.
How far apart are they?
How can we measure the force needed to bring them together?
3. Magnet and paper clips
How many paper clips can a magnet pick up in one go? Can this be used to compare the strengths of different magnets?
4. Magnets and weighing scales
Use blue tac to fix a magnet onto the pan of some plastic scales. How much does it weigh?
Bring a piece of iron near to the magnet but do not touch it. What happens to the reading on the scales?
Bring an identical magnet near to the magnet in the scales. What happens? Try similar and opposite poles of the magnets.
All of these experiments can lead to ways of finding out or comparing magnetic strength and are great fun for children to perform. Labeling or colour coding the magnets is a good idea since it allows comparison and a rank order to be established.
Using marbles as masses to help find out the attractive force of a magnet or pair of magnets
The toy cars can be used in this investigation by attaching a string and bag for marbles. The bag is then hung over the edge of the bench and marbles added one at a time until the cars are pulled apart.
Alternatively two horseshoe magnets or two magnets fitted with hooks can be pulled apart using marbles or conventional masses to provide the force required by simply hanging them up from a bench or stand. It can be very surprising how much force is holding them together!
Using a Force-Meter containing a spring might be dangerous since it reaches a point when it flies off suddenly! Marbles in a bag (to stop them spilling out when they fall) is a safer way.