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Advancing Physics A2... Help needed with Question watch

1. Hello,

I’m an OCR Advancing Physics student in need of help…

I have a question that I am struggling to complete myself, and neither of my two teachers have been able to solve it either. I would be so grateful if someone would be prepared to take the time to explain this for me, I’m desperately trying to get an A in Physics and I only really have one teacher that is capable of reaching that standard so I am often left to do a lot of the work myself.

The question below is from the Advancing Physics A2 textbook: Chapter 15: Electromagnetic Machines p134.

The question that I cannot solve is q4, however below is q4 and q1, which is also needed to calculate the solution.

1) A 250 turn coil is wound round a closed iron core, carrying magnetic flux 4 * 10^-4 Wb. If this flux falls to zero in 1/100s, calculate the average emf induced in the coil.

4) The flux, maximum value 4 * 10^-4, in the core of question 1 now varies sinusoidally at a frequency f of 50Hz. Show that the maximum rate of change of flux is 2*Pi*f the maximum flux, and calculate the maximum emf in the 250 turn coil.

Emf = 250 / (0.01 / 4*10^-4) = 10V (correct)

The answer to 4) is 31V….

I can see that 31V must be 2*Pi*F / 10V……But I do not understand why; if indeed it is the correct method at all. Also how do I “show” the 2*Pi*f part???

You can imagine that this has taken a long time to type-out, so please be kind enough to reply if you can help me!

Many, many, thanks

Thomas Hitchings
2. Hi. I did this question a couple of weeks ago but I think I can remember the answer (I haven't got my book with me so I can't be totally sure) but the equation for the flux will be:
Flux = sin (2pi*50*t)
You need to differentiate this to get the rate of change of flux so :
rate of change of flux = 2pi*50*cos (2pi*50*t)
the max rate will be when cos (2pi*50*t) = 1
so the max rate is 2pi*50
Hope this helps!

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Updated: February 24, 2004
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