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Magnetic Field and the resulting force

  1. Mar 8, 2014 #1

    adjacent

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    1. The problem statement, all variables and given/known data
    This is not a homework Problem.Just a doubt.
    attachment.php?attachmentid=67416&stc=1&d=1394279376.png
    The cross represents current-IN.
    1-Magnetic field lines are produced in clockwise direction when current is going IN.
    2-When wires carry currents IN,they form clockwise magnetic fields.So the magnetic fields of two wires join,forming a loop which attracts them(This is what my teacher said)
    3-What happens in the case of a magnet?It won't form a "loop" because the field direction of the outer magnetic field of the magnet is opposite to that of the wire(A).But the inner field is in the same direction.
     

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  3. Mar 8, 2014 #2

    Simon Bridge

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    That description from your teacher was a bit glib and incomplete. Hopefully you will study the forces on wire in an external magnetic field very soon.

    A wire carrying a current does experience a force in the field of a magnet.
    i.e. wire A will be deflected (roughly) upwards and to the right, wire B will be deflected upwards and to the left.

    For two current carrying wires, consider the force on one current due to the field of the other wire: you'll see it follows the same rules.
    http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forwir2.html
     
  4. Mar 9, 2014 #3

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    Thank you so much.I have another question.
    How was that rule and right hand thumb rule discovered?(I assume by an experiment)
    Why does it always obey that rule?
     
  5. Mar 9, 2014 #4

    Simon Bridge

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    The hand-rule is a kinesthetic mnemonic for remembering the relationship for a vector cross product.
    Math is the language of physics, used to describe the relationships we discover in Nature - this particular relationship, which was discovered through many experiments, happens to be well described by a cross product.
     
  6. Mar 9, 2014 #5

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    Another question.
    If a moving charge produce magnetic fields,will it lose energy?
    Fr example,it attracts an iron nail and in that process the iron nail hits something and some of the energy becomes heat.
    So the charge will slow down?

    I haven't studied these in detail.Just the basics :smile:
     
  7. Mar 9, 2014 #6

    Simon Bridge

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    The magnetic field is part of what a moving charge is so no.
    An accelerating charge loses energy though - except in special circumstances.
    Charges flowing in a wire can slow down as a result of the overall interaction.
    How magnetic fields can be exploited to "do work" is subtle - see link at the bottom.

    In your example:
    The nail - falling towards the magnet, exchanges EM potential energy for kinetic energy.
    On impact, the kinetic energy is exchanged for elastic potential energy and heat.

    http://van.physics.illinois.edu/qa/listing.php?id=17176 ... for better details.
     
  8. Mar 9, 2014 #7

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    Thanks.
    I guess I will need to learn these things before thinking so much :wink:
     
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