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Finding energy from dipole moment - Helmholtz pair?

  1. May 14, 2015 #1
    Firstly apologies for not typing this out - but I need the diagram. And I have no idea where to start. I 'think' most of it is correct. BUT - I have no idea what to do with the last part of c. I thought I could just double the energy. But I'm going to get a negative energy for the system... Clearly that isn't right. I have also considered that the energies just cancel each other out... But that doesn't seem to make sense either. Any ideas?

    17631688445_e0915d965e_b.jpg

    17444099010_438e7b2a08_k.jpg

    17631761911_0b93ee8e7b_b.jpg
     
  2. jcsd
  3. May 14, 2015 #2

    TSny

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    Your work looks very good to me. But what should be the sign of the dot product of m and B?
     
  4. May 14, 2015 #3
    The sign shoud be: U = -B.m

    So I still get a minus sign... Or am I missing something obvious? Should I just double the U and make it positive to get the answer?
     
  5. May 14, 2015 #4

    TSny

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    Sorry, I wasn't very clear.

    ##U = -\vec{m} \cdot \vec{B}= -m \; B \cos \theta##. What is the value of ##\theta##?
     
  6. May 14, 2015 #5
    I though ti was 90 degrees. But I am guessing I am wrong...
     
  7. May 14, 2015 #6

    TSny

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    Note that ##\cos 90^o = 0##, so that's not what you would like to have.

    Consider the loop on the left. What is the direction of ##\vec{m}## for that loop? What is the direction of the magnetic field ##\vec{B}## produced by the loop on the right at the location of the loop on the left?
     
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