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    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.

    My answer to 1)

    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
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    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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