# Electromagnetic Induction

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#1
Hi,

I've just been doing an electromagnetic induction question and, looking at the mark scheme and the examiner's report, I'm a bit confused. Please could someone explain it to me.

For 4ai. I got that the current flows through coil P. This induces a magnetic field in the iron bar. This causes a change in the field through Q which induces an emf in Q. This induces a current in Q.

I don't understand the last part which says ' So the ammeter deflects then returns to zero, when the flux becomes steady'

4aii. I also really don't understand this part. I get that if the resistance is high then the electromagnet's current will be lower but I don't understand why this would reduce the flux linkage or why a reduce in flux linkage would induce an emf in the opposite direction.

Any help would be really appreciated, thanks
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4 years ago
#2
Electromagnetic induction only happens when there's a change in the amount of flux being linked - when the rate of change of linked flux is zero the induced EMF is zero.

it's not the amount of flux being linked that matters for this question, it's the rate of change (with respect to time) - so opening or closing the switch is particularly interesting because you get a change in current in the primary (LHS) coil happening in a very short time... which causes an abrupt change in the amount of flux linked by the secondary (RHS) coil... which is to say a high rate of change of flux linkage... which will cause a large EMF in the secondary.

once you've closed the switch you could leave the switch closed all day and nothing interesting would happen because the flux is constant... i.e. the rate of change is zero so nothing is induced inthe secondary.

constant high flux has the same rate of change as constant no-flux,.. because it's constant the rate of change is zero.
you only get the induction when the rate isn't zero so opening and closing that switch will only ever cause pulses of induced EMF on the secondary at the instants the current flow in the primary is changing.
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