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Homework Help: How to find magnetic dipole force?

  1. Sep 9, 2013 #1
    1. The problem statement, all variables and given/known data
    The relationship of dipole force and distance is noted by F ∝ ##d^{-3}##. One magnet is placed on a weighing scale and set the mass is set to equilibrium. Another magnet, with its positive end pointing down, is lowered from directly above towards the positive end of the magnet at rest. As the weighing scale goes out of balance, the distance between the two magnets as well as the change in mass from the original equilibrium position is noted and recorded. With the moving magnet at rest, the weighing scale is then set to equilibrium again and the process is repeated by moving the upper magnet closer to the magnet at rest and observing the change in distance and mass difference from the first equilibrium position.

    A graph is plotted with the logarithm of the mass difference from the initial equilibrium position versus the distance between the magnets at those respective points of mass change.

    a) Why is this graph a straight line? What does the slope indicate?
    b) Calculate the theoretical masses using F ∝ ##d^{-3}##. Plot a graph based on this data.

    2. Relevant equations
    F ∝ ##d^{-3}##

    3. The attempt at a solution

    First of all, I don't know where the logarithm comes from, or why you take the logarithm of the mass difference. I don't understand where the magnetic dipole force is involved using mass and distance. Therefore, I can't explain why the graph is what it is, or what the slope represents. I don't see any force involved, just mass change and distance.

    To find the Force, I looked up the equation for force due to magnetic dipoles, and it appears to be del(dipole moment1 dot dipole moment2). I looked up magnetic dipole moment, but I don't think we have enough information from the given problem to solve for the magnetic dipole force. I could be wrong, but I'm completely stuck here which is why I am asking on here. Thanks in advance
  2. jcsd
  3. Sep 10, 2013 #2


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    Science Advisor

    A balance does not measure mass - despite what the question says. It measures weight.

    I'd suggest drawing a diagram of the forces acting on the first magnet in equilibrium without the second magnet, then with the second magnet. Then write down an equation relating the forces and see if that gets you started.
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