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Magnetism - Work and Energy

  1. Aug 27, 2009 #1
    I'm confused as to wither or not magnetic fields do work. From reading the forums, as well as my textbook and online resources there seems to be conflicting answers. We talk about magnetic energy, and how work relates to change in energy but then I hear that magnetic fields do no work. Could someone please clarify this for me.

    Thank you :)

    ps. My question stems from reading about so called free-energy devices that use magnetic fields. I was trying to debunk them by stating that according to my textbook magnetic fields do no work, but as I looked into it I kept seeing that I was really confused on the matter and I believe it stems from my confusion over terms.
     
  2. jcsd
  3. Aug 27, 2009 #2

    Born2bwire

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    Magnetic fields can do work. The easiest demonstration of this is the fact that two magnets will pull each other together when in close enough proximity. I think your confusion may deal with the fact that magnetic fields are conservative. If I hold two magnets at a distance of 5 inches apart, release them and they pull together, then they do work. If I then separate them and move them back to a distance of 5 inches, I do the exact opposite amount of work that they did pulling themselves together. So the net work of the system is 0.

    In most systems, we will end up having the magnets do a net work of 0, eventually the energy that we extract from the fields gets put back in or no real work is done in the first place. Otherwise, we will eventually exhaust the magnet. Another example is that of the Lorentz force. The force contribution from the magnetic field is always perpendicular to the velocity of the charged particle. So in that case the work is always zero since it is perpendicular to the path of motion.
     
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