Question about magnetic fields (electromagnet)

[mburt]

Hi guys,

So far I am aware of these rules regarding magnetic fields/direction of current:

A compass needle point is attracted to a south pole (or inversely repelled by a north pole).

Left hand rule #2: For an electromagnet, point thumb toward north pole, fingers curl in direction of current (up or down the front).

However there is a diagram I've been looking at for some time and I can't figure out how it's possible! I have the answer I just can't work in reverse to try to discover this rule: what effect does moving a permanent AWAY from an electromagnet, with its north pole closest to the electromagnet, regard current direction, and compass deflection (compass placed on the opposite side).

I've attached an image of the diagram with the answer included (multiple choice, it has a check by it).

Thanks,
Mike.

 

[zhermes]

This type of problem is a good deal more complicated than the examples you gave. This involved "Induction", and changing currents, and electric & magnetic fields.

Try reading up on the phenomenon of http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction" which tells you in which direction the currents will be induced.

Once you've done that, ask about anything that doesn't make sense; or if you still have trouble with the problem.

 

[mburt]

Thanks, I'll check that out.

 

[mburt]

Ah, I figured it out! As a magnet moves towards an electromagnet, Lenz' law tells us that the electromagnet's magnetic field will attempt to REPEL the magnet.. (oppose the force)

So if a magnet with a north pole facing the electromagnet was moving AWAY from the electromagnet, the side of the electromagnet facing the moving magnet would attempt to attract it, thus being south. Once that's established the question is easy.

Thanks!