Induced magnetism or Induced e.m.f.

[Kyoma]

My classmates had an argument.

There are 2 trolleys; one of them (A) is attached to a solenoid wrapped around a soft-iron core, which is connected to a d.c. circuit. There's another trolley (B) which is attached to a solenoid wrapped around a soft-iron core but it is connected to a load resistor.

Apparently, according to the right-hand grip rule, there is a north-pole formed on the right side of (A). When (A) is pushed towards the right, towards (B), what is the pole of the left side of (B)? The north pole or the south pole?

My friend said it was the south pole, since the electromagnet in (A) would induce a south pole on the left side of (B). But, the other said it was the north pole, according to Lenz's laws.

So, which is which?

 

[Bill_K]

Since energy is being dissipated in the resistor, moving magnet A is doing work, and therefore encountering resistance. Consequently if A has a north pole on its right, B must acquire a north pole on its left to oppose it.

 

[BruceW]

Yeah, Bill is correct. The way I visualise it is that a magnetic field is created which is in opposite direction to the magnetic field which created it.

 

[Kyoma]

So, as long as a magnet does work, there will be an induced e.m.f.?

 

[BruceW]

yes, (as long as the solenoid attached to the resistor doesn't have a gap in the wire) - otherwise a current wouldn't be able to flow through it.

 

[Kyoma]

Thank you very much.

But, if the magnet is stationary, the left side of B will have to be a south-pole since there is a magnetic induction?

 

[BruceW]

If the magnet was stationary, and had already been on for a while, then there would be no current induced in B because a changing magnetic field is required for induction at B.
This changing magnetic field can either be caused by the magnet moving relative to B, or by the magnet being switched on.

 

[chrisbaird]

Great answers. Not to be pendantic, but strictly speaking, magnetic fields cannot do work, they can only redirect work. So for instance, when pushing a magnet into a solenoid, the force of your hand inwards is redirected into an electromotive force around the coils. That is why "free energy" perpetual motion machines using magnets will never succeed.

 

[Kyoma]

Yes, there's no current in the solenoid in B (there's no electromagnetic induction), but will there be a south-pole, a magnetic induction?

 

[BruceW]

If there is no current in solenoid B, then the only contribution to the magnetic field is from A.
A magnetic field can be made either by magnetisation of a material (for example a bar magnet), or by a moving current of charges.
Since I'm assuming the material which solenoid B is made out of can't be magnetised, and there is no current through B, then B doesn't contribute to the magnetic field.