Question 11 from the 2010 HSC

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tammyhumphrey
Posts: 3
Joined: Sat Jul 09, 2011 2:30 pm

Question 11 from the 2010 HSC

Postby tammyhumphrey » Sat Jul 09, 2011 2:50 pm

Hi All,
Following on from Joe's discussion of the 2009 paper I thought I should point out that there is insufficient information given in question 11 from the 2010 paper to answer the question.

The question is as follows:
Two copper rings lie in the same plane as shown (figure shows an inner ring centered inside an outer ring, through which it is indicated that a current is travelling clockwise).
An increasing current flows clockwise around the outer ring.
What happens in the inner ring?
(A) A decreasing clockwise current flows.
(B) A decreasing anticlockwise current flows.
(C) An increasing clockwise current flows.
(D) An increasing anticlockwise current flows

As I will run out of space I'll discuss this in the next post

tammyhumphrey
Posts: 3
Joined: Sat Jul 09, 2011 2:30 pm

Re: Question 11 from the 2010 HSC

Postby tammyhumphrey » Sat Jul 09, 2011 2:52 pm

The emf induced in the inner ring is given by Faraday's law, that an emf is induced with the opposite sign to the rate of change of the magnetic flux. The question tells us that the magnitude of the magnetic flux is increasing with time, thus an emf is induced which causes current flows in the opposite direction in the inner ring to the outer ring. What the question does not tell us is the sign of the second time derivative of the current in the outer ring. There are 3 possibilities:

1. The current in the outer ring is increasing at constant rate, in which case the current in the inner ring is also constant.

2. The current in the outer ring is increasing at an increasing rate in which case the current in the inner ring is increasing in magnitude with time, or

3. The current in the outer ring is increasing at a decreasing rate, in which case the current in the inner ring is decreasing in magnitude with time.

So (B) and (D) are equally correct.

tammyhumphrey
Posts: 3
Joined: Sat Jul 09, 2011 2:30 pm

Re: Question 11 from the 2010 HSC

Postby tammyhumphrey » Sat Jul 09, 2011 2:54 pm

Sorry, I shouldn't say equally correct, but rather that we cannot determine between (B) and (D) from the limited information given in the question.


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