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[someth1ng]

CSSA 2007 Question 6 (MC)

A bar magnet is moving quickly into a coil of wire connected to an ammeter. A current of 8A is produced. If this experiment is repeated using two bar magnets identical to the first one (that are taped together, side by side) and a different wire coil containing twice as many turns as the first one, what would be the expected current?

(A) 4A
(B) 8A
(C) 16A
(D) 32A

I thought it was B, 8A because wouldn't double the coils cause the current to halve?

 

[My Name is Khan]

isn't that D) ?

 

[My Name is Khan]

CSSA 2007 Question 6 (MC)



I thought it was B, 8A because wouldn't double the coils cause the current to halve?

they double the current :L

 

[Parvee]

Im guessing the answer is D because you are practically doubling the magnetic field strength and the number of coils doubles, hence 8x2x2

 

[someth1ng]

I was just thinking like, in a transformer, when the you double the coils, emf doubles but current halves.

 

[My Name is Khan]

what is the answer? is it D?

 

[barbernator]

its D. can u explain why you thought it was B so we can explain why not?

 

[My Name is Khan]

increasing the relative motion b/w conductor or magent increases the induced emf.
decreasing the distance b/w conductor and magnet increases the induced emf.
increasing the no. of turns of coils on the conductor increases the induced emf.

initially, it is 8A, doubling T doubles the current. so we have 16A.
then doubling I doubles the current, therefore we have 32A.

so answer should be D

 

[someth1ng]

Well, in a transformer, the secondary coil, if you double the number of turns, you halve the current where the change of flux is the same.

 

[My Name is Khan]

Well, in a transformer, the secondary coil, if you double the number of turns, you halve the current where the change of flux is the same.

why are involving a transformer here?

 

[barbernator]

took me a while to think this one through, but i know why not.

Within a transformer, the magnetic flux is changing continuously and the current drops.

The movement of the magnet in and out required a certain amount of force in and out. The generated current within the wire opposes the change. If the same amount of FORCE was used when the coil length was doubles, the current and emf would remain the same. But seeing as we are doubling the coils and keeping the motion of the magnets the same, we are doubling the FORCE in and out and hence current doubles. not sure if this is totally correct but it seems reasonable.

 

[someth1ng]

why are involving a transformer here?

It's clearly a similar concept.

 

[My Name is Khan]

yes the electromagnetic induction but there are two types of transformers.
Here we are only considering a magnet moving relative to a conductor.

 

[My Name is Khan]

its similar to Faraday's experiment where he moves a magnet inside a solenoid

 

[someth1ng]

It just seems counter-intuitive that if you have a lot more coils, you'll end up with a lot of power...(higher emf and higher current)

 

[barbernator]

It just seems counter-intuitive that if you have a lot more coils, you'll end up with a lot of power...(higher emf and higher current)

according to the solutions, was the answer D?

 

[someth1ng]

Okay, to clarify, if a higher emf (and hence, current) is induced in the coil due to more coil, this creates higher magnetic repulsion (due to higher current) meaning that more work must be done on the magnet - right? This would be the only intuitively-correct scenario (for me, at least).

 

[Parvee]

Okay, to clarify, if a higher emf (and hence, current) is induced in the coil, this creates higher magnetic repulsion (due to higher current) meaning that more work must be done on the magnet - right?

Yes, magnitude of current depends on the speed of the magnet/coil

 

[My Name is Khan]

is the answer D?

 

[someth1ng]

Yes, magnitude of current depends on the speed of the magnet/coil

If more coils are present, the magnet will be repelled more and more work must be done, right?

Yes, the answer is D