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Magnetism and energy

  1. Aug 16, 2008 #1
    This is something I can't get my head around, and hopefully someone here could help.

    Say you have a fixed permanent magnet (not an electromagnet), and you place a metallic object with mass nearby. The object will begin to accelerate toward the magnet due to magnetic force. At the moment before impact with the magnet, the object has an instantaneous velocity greater than zero, meaning that it has kinetic energy. Where does that energy come from? I know magnets don't "deplete" the more they are used, so would that mean that a magnet has infinite potential energy?
  2. jcsd
  3. Aug 16, 2008 #2
    aren't all magnetic feilds , electrical feilds, and therefore there is no magnet thats not elctromagnetic? Oh and they would not have infinite potential energy, becaue the magnet hit at a certain force which is not infinity and yes they do have a kinetic energy but i tjhink that energy doesn't have enough force to break between the magnets
    this is because:
    picture 2 magnets one + and one -, picture no ther force at work but newton's lawas of motion (meaning no gravity) and the laws that govern electromagnetics. Picture the magnets headed toward each other at the same force (note this was the exact sam emass and sape the magnets) since each weigh x, their kinetic energy is deposited and instead of moving backward, they move forward due to the magnets agian pulling each other ofter the potential energy taking place.
    Last edited: Aug 16, 2008
  4. Aug 16, 2008 #3


    Staff: Mentor

    There is energy in the magnetic field itself. When a ferromagnetic object is in a magnetic field the external field is reduced. This reduced field implies reduced energy, which is equal to the work done on the object.
  5. Aug 16, 2008 #4


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    Welcome to PF!

    Hi ndenison! Welcome to PF! :smile:

    As DaleSpam says, there is energy in the magnetic field itself.

    It's the same as gravity …

    place an object near a star … it will begin to accelerate toward the star due to gravitational force … but the star doesn't have any less gravity … so where does that kinetic energy come from? :confused:

    It comes from depleting the potential energy (either gravitational or magnetic). :smile:
  6. Aug 17, 2008 #5
    what about a diamagnetic superconductor? where does the energy come from when you cool it below its critical temp and it levitates off the magnet it was sitting on?
  7. Aug 17, 2008 #6


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    It comes from cooling it below its critical temperature. :wink:
  8. Aug 17, 2008 #7
    You just need to look at what you wrote: “you place a metallic object with mass nearby.” You expend energy when you place the object in the field which gives the object potential energy. When it moves it is translating that potential energy into kinetic energy, and when it finally hits the magnet the kinetic energy is translated into heat energy. All of that energy came from the bag of peanuts you ate before placing the object in the field.:smile:
  9. Aug 17, 2008 #8


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    mmm … peanuts!

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