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Energy Loss From Permanent Magnets

  1. Dec 12, 2011 #1
    Here's a question I posted about 10 years ago as a member of the forum. No one seemed to provide a satisfactory answer then so maybe someone might have some ideas on the subject:

    When a permanent magnet attracts an object it does work and loses energy.
    When the object is pulled away is the magnetic field rejuvenated to some extent and energy transfered back to the magnet?
    I often wondered about this because the magnetic catches on cupboard doors never seem to wear out which would happen if all the magnetic potential energy was used up.
    The magnet is in a fixed position and exerts a force on the piece of metal on the door. The door is accelerated by the force so it gains kinetic energy and the only source of energy in the system is from the magnetic field of the magnet.
    There must be energy transfer from the magnet in order to accelerate the door and give it kinetic energy.

    This is the conversation in the forum from then:

    http://physicspost.com/physicsforums/topic.asp-ARCHIVE=&TOPIC_ID=4161.htm
     
  2. jcsd
  3. Dec 12, 2011 #2

    Drakkith

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    Staff: Mentor

    The force from a magnet does not require an expenditure of energy to function. In a similar way, gravity doesn't require it either. Note that you cannot get repeatable work from a permanent magnet if all you are doing is allowing an object to be attracted to it in such a manner.

    Edit: I don't know for sure, but it would appear to me to be similar to potential energy related to gravity. The potential energy is converted to kinetic and then lost when the object impacts the magnet. But again, I'm not sure.
     
    Last edited: Dec 12, 2011
  4. Dec 13, 2011 #3
    When the door is moved close to the magnet and is let go, the magnet does work as it moves the mass of the door through a distance. That is the definition of work. The energy to do this must come from the magnet.
    Presumably the same thing would happen if a distant object is moved into the influence of Earths gravity. The gravitational force would attract the body giving it velocity and kinetic energy. On impact, some of energy would be dissipated as heat.
     
    Last edited: Dec 13, 2011
  5. Dec 13, 2011 #4

    clem

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    Science Advisor

    The energy does not come from the magnet. The magnetic field energy changes.
    It is the same when when a proton attracts an electron, the field energy changes, but not the proton.
     
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