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Magnet Force Help

  1. Dec 19, 2006 #1
    Can anyone help me determine how much force is created when a one tesla permanent magnet is allowed to make contact with a flat piece of iron. The distance that this size magnet can be released and will be drawn to the iron is about 3 centimeters. Is there any way to express this force in joules or watts?
  2. jcsd
  3. Dec 19, 2006 #2


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    There's not enough info to do a calculation, but a typical 1T magnet that's about the size of a quarter, will stick to a flat piece of iron with a force of about 20lbs (about 100N). You can't express a force in units of joules or watts (those are units for energy and power). You can, however, calculate the potential energy of the magnet-iron system, and that will be a number in units of joules.
  4. Dec 19, 2006 #3
    What other info is needed? Here's what is listed on this magnet:


    Pull Force: 231.55 lbs
    Surface Field: 7100 Gauss
    Brmax: 13,200 Gauss
    BHmax: 42 MGOe

    With the above magnet,

    if 20 lbs. equals 100 newtons


    231 lbs. equals 1155 newtons.

    Is this correct?

    When I go to online unit converters, I find that 1 watt = 1 joule = 1 newton.

    I realize these are different terms relating to different things; power, energy and force. But is this how you would calculate the potential energy?
  5. Dec 19, 2006 #4


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    :bugeye: :eek: :surprised :mad:

    Which converters give you that, so we can steer people away from them in the future? :grumpy:
  6. Dec 19, 2006 #5
    They clearly state that newtons are per meter and watts are per second, but there seems to be a clear relationship.



  7. Dec 19, 2006 #6


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    Woah! That's a big one!

    Well, isn't this the information you are looking for?

    Close, but a little high! To convert lbs to Newtons(N), multiply by 9.8/2.2 (or about 4.45).

    This is completely nonsensical. What's true is that 1 Watt-second = 1 Joule = 1 Newton-meter. But this doesn't mean you can convert a force into an energy!

    No, it's not. Any real calculation of PE will be far more involved.
    Last edited: Dec 19, 2006
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