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Non-Conservative E-field

  1. Feb 25, 2012 #1
    Greetings everyone,

    While I was studying on my own using MIT OCW, I came across the following document. In that document in the last part you are asked to calculate the ratio of the reading of the two voltmeters positioned to the left and right of the loop. I did some work on them but I am unsure if I interpreted the question correctly. Any help would be appreciated.


    P.S: I found it appropriate to post it in this section since it is related with classical physics and it is not a homework question.

    Attached Files:

  2. jcsd
  3. Feb 25, 2012 #2
    Thanks for sharing this interesting problem. According to my calculation, the ratio becomes -R2/(2R1+R2).
    Last edited: Feb 25, 2012
  4. Feb 25, 2012 #3
    Shouldn't it change when the loop goes ne times around the magnetic field can you write the Kirchhoff's Loop Rule equations for the loops?

  5. Feb 25, 2012 #4
    It's simple in fact. Did you take account of the emf induced on the outer loop?
  6. Feb 25, 2012 #5
    The outer loop that includes V_1 and the middle loop only, am I wrong ?
  7. Feb 25, 2012 #6
    Here are the equations




  8. Feb 25, 2012 #7
    Suppose the magnetic field is coming out of the plane as it is decreasing. This induces ε volt emf which causes a current passing through R2 from A terminal to D terminal. The same emf is induced in the outer loop only that the current is negligible due to the high resistance of the voltmeter. The potential of the upper terminal will be ε volt higher than the potential of terminal D .

    I hope its clear.
  9. Feb 25, 2012 #8
    Thank you I think it is clear now! In my expression V1/V2 = (ε + IR1)/IR2, and it gives the same answer if I plug in the approximated value of I into it. Then this is some sort of transformer if I am not mistaken. If the loop was wrapped N times the coefficient of R1 in the denominator of your expression would be (n+1), am I right ?
  10. Feb 25, 2012 #9
    When wrapped N times, V1/V2=- (Nε + IR1)/IR2=-(N+1)R1/R2+N.

    I am hesitant to call it a transformer because in a transformer we have input and output terminals. Where is the input terminal here? Also his ratio was calculated with the assumption than voltmeters have impedance much higher than R1 and R2. If we replace , for example V2, with a voltage source, things will change. Beside, although the ratio is independent of the flux, the voltages do depend on it.
  11. Feb 25, 2012 #10
    Thanks for the reply but I think the ratio would be ((n+1)R1+nR2)/R2.
  12. Feb 25, 2012 #11
    It's the same. ((n+1)R1+nR2)/R2= (n+1)R1/R2+nR2/R2=(n+1)R1/R2+n
  13. Feb 25, 2012 #12
    Sorry my bad, I am indebted.
  14. Feb 26, 2012 #13
    Btw. the lecture that goes with that pdf is on youtube.
    Last edited by a moderator: Sep 25, 2014
  15. Feb 27, 2012 #14
    Thanks a lot. That was entertaining as well as informative.

    The main point of such a problem is that in a time-varying magnetic field, the voltage difference between two points in a conductive material ( here wires) can be non-zero.
    Last edited by a moderator: Sep 25, 2014
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