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Magnetic force on a conductor

  1. Sep 12, 2013 #1
    When a conductor carries current, due to Lorentz force, the conductor experiences a force. However I was wondering what causes electromagnetic induction. Moving a conductor in a magnetic field causes its charges to experience a Lorentz force, but unlike the case of the current, here both the positive and the negative charges are moving, so both of them feel a force. So how is a current set up? Why don't the positive charges move?
  2. jcsd
  3. Sep 12, 2013 #2
    The opposite charges will be under the action of forces in opposite directions. The positive charges won't move because they are attached to the fixed nuclei. The electron's are free to move
  4. Sep 12, 2013 #3
    Yes, they both feel Lorentz forces. And if they are "free", they both move.
    The motions will be in opposite directions so the currents will add up.

    In a metal the positive charges (ionic cores) are bound together in the lattice and the Lorentz force cannot "move" them. The electrons are the only free charge carriers in this case.
  5. Sep 12, 2013 #4
    What do you mean 'in a metal the positive charges are bound together in the lattice'?
  6. Sep 12, 2013 #5
    The positive charges are fixed. The nuclei of the atoms are not free to move around. If they were free to move you would have a liquid, not a solid
  7. Sep 13, 2013 #6
    Conductors only have free electrons because of their electronic configuration.Positive charges in conductors are present inside the nucleus of the atom where they are bound by strong nuclear forces and this nucleus is constrained inside the metal lattice and hence these positive charges cannot move across the conductor .So the positive charges also experience lorentz force but that force is not enough to move the protons or the nucleus.
    Last edited: Sep 13, 2013
  8. Sep 13, 2013 #7
    The nuclear force binds the protons and neutrons together in the nucleus. But this is not so relevant here. The nucleus (positive) could move under the effect of the Lorentz force if it were not bound to the other nuclei in the crystal. They also are more massive than the electrons so even if they were free their motion will be much slower.

    In a ionic solution both positive and negative ions (their nuclei included) move under the effect of external fields.
  9. Sep 13, 2013 #8
    Ok thanks. I was wondering how the nuclear force can hold the protons in place. If the protons are subject to force and accelerate they can drag the neutrons along with them. Could you please elaborate on how 'if they were not bound to the other nuclei in the crystal'? Which force binds them exactly?
  10. Sep 14, 2013 #9


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    The nuclear force is very strong compared to the electromagnetic forces from magnetic fields in your lab. It's like picking up an apple: you pick up the whole apple and not just the shell (where you apply the force), as the internal forces (holding the apple together) are very strong compared to the force of you, lifting the apple.

    If you try to move a nucleus in a solid, the electrons surrounding this nucleus will move as well (to stay in their orbits), but then they get repelled by electrons of other atoms nearby.
    In addition, there are electrons in chemical bonds, keeping the atoms together and at a fixed distance (again, due to electromagnetic forces), but that needs some quantum mechanics for a proper description.
  11. Sep 18, 2013 #10
    Okay thanks!
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