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What is the difference between magnetic dipole moment and torque ?

  1. Oct 12, 2013 #1
    imagine a door shaped circuit in the between a south and north pole
    such that the long sides are perpendicular to the magnetic field created by the magnet , now
    in my physics book , it says that the torque that rotates the door is equal to B * Md where Md is the magnetic dipole moment which equals Intensity * Area * Number of turns * for a coil *
    now what is the difference between the magnetic dipole moment ? isn't moment the same thing as torque ?
     
  2. jcsd
  3. Oct 12, 2013 #2

    Simon Bridge

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    You have an equation that says: ##\tau=BMd## (making some assumptions about geometry) and you are asking if ##M## is not the same as ##\tau##??? Doesn't the equation tell you?

    You should be aware that words can be used to mean different things in different contexts.
    A "torque" is a "mechanical moment" - when there is no risk of confusion, you get to leave off the "mechanical" part to save typing. Notice that there is also a "moment of inertia" or "inertial moment" - which is also a moment that is not the same as a torque.
     
  4. Oct 13, 2013 #3

    jtbell

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    I think you mean (electric) current, not intensity. Perhaps the symbol I for current is confusing you.

    Magnetic dipole moment is a property of the current loop which is independent of the presence of an externally-produced magnetic field. Because of that property, the loop experiences a torque when placed in an externally-produced magnetic field. That torque depends on both the magnetic dipole moment of the loop, and the strength of the magnetic field.
     
  5. Oct 13, 2013 #4

    Simon Bridge

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    The reason the symbol for electric current is "I" is that it used to be called "current intensity" or just "intensity". The use of an archaic term usually signifies someone who is self-taught from old text books found for free online.


    There are other uses for the word moment, for instance we can talk about an event turning or hinging on a particular moment of time. Also not a torque. There are lots of moments that are not torques. :)
     
  6. Oct 13, 2013 #5
    mr simon , i do understand the equation
    but what was really bothering me is the word MOMENT , i have been taught that moment is the same as torque , so for me it was like saying
    magnetic torque = magnetic * torque * X Magnetic flux density of the field
    but now i understand that the the magnetic moment is somekind of an intrinsic property of a magnet * or loop* to align itself to another magnetic field , or to rotate to another magnetic field
    more like the affinity or strength of a magnet to align to another magnet field , is that true ?
    would it be correct if i think of it as the " arm " of the magnet ?
     
    Last edited: Oct 13, 2013
  7. Oct 13, 2013 #6

    Simon Bridge

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    Yeah - I got that, which is why I went to some trouble to point out that this is the wrong impression.

    The lesson where torque and moment was the same was supposed to be taken in context of mechanical moments only. You are expected to figure this stuff out as you advance - definitions will change as you learn more stuff.

    That would be one way of thinking about it for now. Be aware that this understanding will have to change later depending on what you end up doing.

    No.
     
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