Rolling a NIB Magnet Down an Aluminium Sheet

Rolling a NIB magnet down an aluminium sheet

A typical behaviour is for the magnet to be pulled to one side as shown.

If the magnet is reversed it is pulled to the opposite direction.

Question: Why is it pulled to one side?

Answer: Must have something to do with the earth’s field. If so, what is the mechanism?

Check: Do it outside well away from any steel. Still works.

However …

Change the orientation of the incline and see what happens!!

If I roll the magnet towards the south, I get the behaviour described. If I roll it to the east or west it is not quite as marked. If I roll it to the north, it doesn’t bend AT ALL. (You may have to try a few different orientations before you find the exact direction for that to happen.

So I have two questions for some spark brighter than me out there.

(1) What mechanism produces the curved deflection when rolled towards the south?

(2) Why no deflection when rolling north? Something to do with the tilt of the earth’s field?

John Joosten, email -

After a couple of days thinking about it I came up with a possible hypothesis which would probably keep the students happy. It goes something like this …

In the situation at the right, the magnet is rolling down the hill towards the south.

As the magnet rolls down the hill it induces eddy currents within the aluminium sheet which will tend to oppose the motion. Therefore, looking from above, we should have this situation.

This should produce a uniform slowing-down effect with no bias to the left or the right.

Now, if the earth’s field is take into effect the situation with the induced ‘magnets’ on either side needs to be examined.

The effect will be to produce stronger induced north and south poles as shown in the third diagram. In effect, the earth’s field makes the induced ‘magnet’ on the right weaker while it makes the one on the left stronger. This will produce a resultant force to the left as shown by the green arrow.

This will be enough to steer the rolling magnet to the west.

Now, the only way in which the influence of the earth’s field can be eliminated is to ensure that it is oriented perpendicularly to the incline of the aluminium sheet. The only way in which this can be achieved is to roll the magnet northwards. If you manage to point the sheet precisely towards local magnetic north, you will find that you have a incline of about 90°- α where α is the magnetic dip angle. You will also find that the magnet rolls down the hill without deviating. (If you have not got magnetic north, the angle will deviate slightly to compensate).