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Moving-coil galvanometer graph

  1. Oct 12, 2016 #1
    1. The problem statement, all variables and given/known data
    A square coil of side a and consisting of N turns is free to rotate about a vertical axis through the mid-points of two opposite sides. It is situated in a uniform horizontal magnetic field of flux density B so that the plane of the coil makes an angle θ with the field. Draw a diagram of his arrangement as seen from above and show the couple acting on the coil when a current I flows through it. Write down an expression for the magnitude of this couple.

    Explain how and why this simple arrangement is modified in most moving-coil galvanometers.

    2. The attempt at a solution
    I think this should be the correct representation of the problem:

    1540.jpg

    Though I don't quite understand what "couple" means. What does "show the couple acting on the coil" mean?
     
    Last edited: Oct 12, 2016
  2. jcsd
  3. Oct 12, 2016 #2

    cnh1995

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    Yes.
    https://en.m.wikipedia.org/wiki/Couple_(mechanics)
    What can you say about the angle between the coil and the magnetic field as the coil rotates? How is that related to the torque acting on the coil?
     
  4. Oct 12, 2016 #3
    So couple is the couple of forces which move in different directions on the graph?

    T = B I a N -- an expression for the magnitude of this couple? (T = torque, B = field, I = current, a = coil side, N = number of turns.)

    Well, the coil rotates with the arrow on it. So the angle decreases or increases depending on (?) current going through the magnet? Current increases and so the torque increases and the other way around (?).
     
  5. Oct 12, 2016 #4

    cnh1995

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    You can see that the driving torque varies with the angle if the magnetic field is horizontal. To have an almost constant driving torque, the magnetic field is made radial by providing a curvature to the magnetic poles.
     
  6. Oct 12, 2016 #5
    You mean that in the problem we had to draw not circled poles of the magnet but just regular ones (squared).

    And in real life the magnet is modified to the one in the drawing, to make the magnetic field radial, right?
     
  7. Oct 12, 2016 #6

    cnh1995

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    Yes.
     
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