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# Magnetic fields question (coil, ammeter, wut??) watch

1. Can somebody please help a croissant out and tell me what I should be writing for these two questions?
I don't recall being taught this in class, I can't find anything on it in either of my textbooks and I don't see what relevance the switch being open and closed would have on the ammeter reading.
I know that when the circuit is complete (so the switch is closed), the direction of current flow will be in the same direction of the induced e.m.f., but again, I'm not sure if that's relevant.
So basically, what kind of things to I have to say in part (i) and (ii)?

Thank you
2. Hopefully you'll recall that electro magnetic induction depends on the amount of flux linked changing over time.
Operating the switch creates a large rate of change of linked flux in the secondary coil.
leaving the switch open or closed does not produce a changing amount of flux in the secondary coil.
3. (Original post by Joinedup)
Hopefully you'll recall that electro magnetic induction depends on the amount of flux linked changing over time.
Operating the switch creates a large rate of change of linked flux in the secondary coil.
leaving the switch open or closed does not produce a changing amount of flux in the secondary coil.
How would that affect the ammeter reading?
4. (Original post by LeCroissant)
How would that affect the ammeter reading?
Well it'll read 0 when the amount of flux linked is constant over time - which is the same as saying zero rate of change... this will be the situation if you leave the switch open or leave the switch closed.

it'll read either +ve or -ve when the amount of flux linked is changing over time - so you'll get current spikes in the secondary at the instant the switch closes or opens

try this video
5. (Original post by Joinedup)
Well it'll read 0 when the amount of flux linked is constant over time - which is the same as saying zero rate of change... this will be the situation if you leave the switch open or leave the switch closed.

it'll read either +ve or -ve when the amount of flux linked is changing over time - so you'll get current spikes in the secondary at the instant the switch closes or opens

try this video
Thank you

What would happen in part (ii) where the switch remains closed but the resistance of the variable resistor is increased?
Would the reading on the ammeter stay at zero because the amount of flux is not changing?
6. (Original post by LeCroissant)
Thank you

What would happen in part (ii) where the switch remains closed but the resistance of the variable resistor is increased?
Would the reading on the ammeter stay at zero because the amount of flux is not changing?
The flux is proportional to the current in the primary ... and the variable resistor will alter the current and so cause change in the flux- so while the variable resistor is moving there will be an induced emf in the secondary (but not while the variable resistor is stationary)
7. (Original post by Joinedup)
The flux is proportional to the current in the primary ... and the variable resistor will alter the current and so cause change in the flux- so while the variable resistor is moving there will be an induced emf in the secondary (but not while the variable resistor is stationary)
Thank you, do you know what this would show up as on the ammeter? In particular, how would what you now observe on the ammeter differ from what you observed previously when the switch was closed?
8. (Original post by LeCroissant)
Thank you, do you know what this would show up as on the ammeter? In particular, how would what you now observe on the ammeter differ from what you observed previously when the switch was closed?
1. no changes in the current going through the primary means no EMF is induced in the secondary.

2. rapid change in the current going through the primary (by switching on and off) means rapid, high amplitude spikes of EMF are induced in the secondary

what you might expect from gradual changes of current in the primary is something intermediate between the previous two situations - lower amplitude pulses of induced EMF on the secondary that last for the duration of the movement of the wiper on the variable resistor.

The amplitude of the induced EMF is proportional to the rate the flux is changing so if you measured the induced EMF on the secondary with the variable resistor moving from max to min over a time of say 1 second it'd be twice the value you measured with the variable resistor making the same movement over 2 seconds (and so on)

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