1
$\begingroup$

Lenz's law was explained to my class using an example wherein there is a bar magnet passing through a current carrying loop in a gravity free space. Our professor told us that the induced magnetic force is repulsive in nature in order to perform negative work on the bar magnet. The kinetic energy lost by the bar magnet as a result of the repulsive force would be lost in the form of heat in the wires of some internal resistance.

If the induced repulsion is because of the heating effect of current, won't Lenz's Law be invalid for resistance free wires, where resistance is zero ? If it is valid, why ?

$\endgroup$
6
  • $\begingroup$ The kinetic energy of the bar magnetic is transformed into the kinetic energy of the electrons in the wire, by making a current. The repulsive force is due the magnetic field of this current, not its losses as heat. $\endgroup$ Commented Aug 19, 2023 at 16:22
  • $\begingroup$ @jensenpaull what would happen to the energy lost as negative work then ? If it can't be lost as heat, what would happen to that excess energy ? $\endgroup$ Commented Aug 19, 2023 at 16:28
  • $\begingroup$ Negative work does not mean energy lost in the system , simply a loss of kinetic energy in the magnet. the energy goes into the kinetic energy of the current. $\endgroup$ Commented Aug 19, 2023 at 17:06
  • $\begingroup$ So, would the electrons inside the current carrying wire be accelerated ? $\endgroup$ Commented Aug 19, 2023 at 18:06
  • $\begingroup$ When there is a changing magnetic field, an electric field is induced inside the copper. This electric field accelerates electrons creating current. This current creates an opposing magnetic field slowing down the magnet. Energy is conserved, the mechanism by which is it conserved is the magnetic field generated by the electrons, acting on the magnet ( larger experiments will also have a slight different mechanism due to the finite speed of light but that is negligable here) $\endgroup$ Commented Aug 20, 2023 at 13:57

1 Answer 1

1
$\begingroup$

Lenz's Law is a result of the conservation of energy, and the way I think about it is that there are only two directions in which the current can flow in the loop, clockwise and counterclockwise. The resultant magnetic field caused by the loop would either attract the bar magnet or repel it.

bar magnet moving towards a loop

If the magnetic field by the loop was to attract the bar magnet, a small movement in the magnet would cause the loop to attract it, which would make it move faster and increase the current in the loop, which increases the attraction and so on. This creates energy out of nowhere as not only does the loop have an increasing current flowing through it, the magnet now has an increasing kinetic energy.

So it is apparent by contradiction that the resultant emf must repel the bar magnet. This also makes intuitive sense as the kinetic energy lost by the magnet now manifests as the increase in kinetic energy of the charges in the wire. So the repulsion between the magnet and the coil is not due to the power dissipated in the wire, the power that would be dissipated and the repulsion between the coil and magnet are both a result of the induced emf in the coil. This holds true for a wire of 0 resistance as well.

As a side note, a current can flow in a loop of 0 resistance with no work required, as there is no resistance to oppose the flow. In a superconducting loop, any change in flux linkage would instantly produce an opposing magnetic field, such that the flux through the loop is always 0. Reference is this thread on induced currents in a superconducting loop.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.