So either this past paper is wrong or my ideas about induction have a problem.
The old exam I am doing asks what happens when you drop a magnet through a coil connected to an Amperemeter, and then how it might be different were the Amperemeter not properly connected (i.e. the circuit is incomplete).
My understanding is this: When you drop a magnet through a copper coil the acceleration is reduced as it enters (and leaves) the coil as these are the points where the flux linkage change occurs, and so EMF will be induced. This EMF will oppose the direction of motion (Lenz's Law) and thus it will slow the magnet.
Hopefully I'm right so far. I was under the impression that having an incomplete circuit wouldn't change a thing. But the mark scheme says this:
- Magnet now falls with acceleration g
- EMF induced
- No current
- So no energy lost from circuit [Or no opposing force on magnet, or no force from magnetic field, or no magnetic field produced]
I can sort of convince myself that a break in the circuit will stop electrons flowing round the coil which might stop the EMF being induced in the first place, but it certainly doesn't sit well with me. Also the fact that the MS says that EMF is induced, I thought Lenz's law stated that the induced EMF will always oppose the motion that caused it? If it's not opposing the motion, what the hell is it doing?? Also if the mark scheme is correct and I'm just way more misguided than I thought, can someone explain how a coil that doesn't form a complete circuit is different to a copper tube, because I'm pretty sure I've seen this done with a copper tube and the magnet definitely didn't fall at g.