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Q & A: Finding out which is the North pole of an electromagnet

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Q:
I can create an elctromagnet by winding a wire round a nail and connecting a battery. OK. But how will I find which is the north & south pole?
- Ajish.M.Varghese
India
A:
Simple! Get a compass and put it near one of the electromagnet's poles. The end of the compass needle which usually points north will be repelled by the north pole of your electromagnet and attracted by the south pole.

You can check this against a permanent magnet with labeled north and south poles by bringing the same compass close to it.

You can predict which way this will go by using the "right-hand-rule". Taking the convention that current flows from the positive end of your battery to the negative end (yes, the electrons go the other way, but they are negatively charged), you wrap the fingers of your right hand in the direction positive current flows through the coils of your electromagnet. Then your thumb, when you stick it out, will point in the direction of the magnetic field, which points along local magnetic north.

Tom

(republished on 07/13/06)

Follow-Up #1: whoops

Q:
The north magnetic pole of the Earth is actually the south pole (i.e. negative pole) of the Earth"s magnetic dipole See picture at:

www.physics.sjsu.edu/becker/physics51/images/28_03_Earth_magnetic_field.jpg

Thus, the tip of a compass needle that points to the Earth"s magnetic north pole is the north pole (i.e. positive pole) of the compass magnetic arm and it will be repelled by the north pole of the electromagnet.
- Mark Grosenbaugh (age 54)
Woods Hole, MA 02543
A:
Thanks Mark. We've fixed the original answer, which used to have it backwards before we got your note.
Mike W.

(published on 01/09/08)

Follow-up on this answer.