# Calculating the Field strength of a magnet at a certain distance

1. Apr 15, 2013

### kaikalii

Is there a way to calculate the magnetic field strength, B, of a magnet (permanent magnet, not an electromagnet) at a certain distance? I assume it follows the inverse square law, but other than that, I do not know what to do.

Tools I have available:
• permanent magnet
• ammeter/voltmeter
• wire
• batteries
• spring scale
• ruler

I am aware of the equations: F=8.99*109q1q2/r2, F=qVB(sinθ), and F=BIL(sinθ), but I do not know how I would use those to calculate the field strength with just the tools listed above.

Also, if you were to cut a magnet in half, would that half the strength of the magnetic field? Does every type of permanent magnet material have some kind of Tesla per meter2 per kilogram constant?

2. Apr 16, 2013

### Philip Wood

Do you really want to calculate the field? Your inclusion of a list of tools suggests that you want to measure it.

The equations you have listed won't help you calculate the field strength at a distance from the magnet, though $F = Bqv sin \theta$ is the basis of the Hall probe for measuring the field strength, and $F = BIL sin \theta$ could also be used - with considerable practical difficulties - for this purpose. Your list of tools seems to be chosen with this last method (measuring the force due to the field on a current-carrying wire) in mind.

Close to the magnet the field will vary in a complicated way, partly dependent on the magnet's shape. Far from the magnet (say > 5 times the magnet's greatest dimension. e.g. length) I would expect the field to fall off roughly as the inverse cube of the distance from the centre of the magnet.

Last edited: Apr 16, 2013