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Conservation of Energy and current

  1. May 2, 2015 #1
    Assume there is a table with an infinite long wire passing through the centre and few iron pieces lying around the wire on the table. Now I pass current through the wire which creates electro magnetic field due to which all the iron particles get magnetic potential energy. They are attracted towards the electro magnet and potential energy gets converted into kinetic energy

    Now lets assume negligible resistance wire so we can create extremely large magnetic field from small potential difference and loss of energy will be minimal.

    So, how does the conservation of energy holds in this ? We are creating magnetic potential energy in iron pieces with minimal heat loss from current carrying wire(which is anyways independent of number of iron pieces)
  2. jcsd
  3. May 2, 2015 #2


    Staff: Mentor

    Hi KronosZ, welcome to PF!
    It doesn't work that way. Even with 0 resistance a wire has inductance and energy must be supplied or removed to change the magnetic field. This is well known and demonstrated in superconducting magnets like those used in MRI.
    Last edited: May 2, 2015
  4. May 2, 2015 #3


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    Science Advisor
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    Gold Member

    +1 to that.

    The energy stored in an inductor is 0.5 L I2. Adding iron increases the inductance compared to the case without iron present.
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