# Flipping the polarity of a compass needle. Why does this work?

1. Apr 1, 2013

### Ntstanch

Awhile back I had been messing around with batteries, copper wiring, compasses and small neodymium magnets; during this time I figured out how to flip the polarity of a compass needle. The most reliable way was to hold the two magnets above the compass needle and force, as best I could, the two repelling ends together towards the center of the compass needle. Once they were close enough to the center of the compass needle I would apply a breaking motion (like snapping a stick in two). And the compass needle would reverse.

Also, doing this with both magnets attracting is considerably less reliable than when they are repelling. However the primary factor seems to be the breaking motion (whether forcing repelling ends towards the center, or letting attracting ends snap towards the center).

Of course my question is how this works.

2. Apr 1, 2013

### DennisN

The magnetic needle aligns with the lines of the overall magnetic field in its presence. Whatever you do with the other two magnets, you do not flip the polarity of the magnetic needle, what you do is changing the magnetic field by moving your magnets.

See this picture (picture 2) with needles placed at various positions near a bar magnet.

3. Apr 1, 2013

### Ntstanch

Sorry if what I wrote was confusing... after the breaking motion the magnets are removed entirely and the north end of the needle then points south, and south north. At least until I repeat the experiment.

4. Apr 1, 2013

### DennisN

No problem. Ah, I see. Then the only explanation I can think of is that the two magnets have magnetized the needle (also some info here). Also, this clip shows how to magnetize a screwdriver with neodymium magnets.

5. Apr 1, 2013

### mikeph

The energy of interaction of the magnetic dipole of the needle and the external magnetic field is $E=-\mu.B$. When you put the magnets over the needle, you flipped the magnetic dipole, so its new direction is $\mu'=-\mu$ and the energy is $\mu'.B$. The needle still orientates itself in the direction which minimises this energy, which is now with the north-labelled side pointing south.