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In this case the current is flowing in forward direction due to which the force due to magnetic field acts towards the right and the rod moves towards right.

Here in this case, I say force causing motion is due to magnetic field and motion is too in its direction but all the time at all the places I have heard and read that "Work done by magnetic field is always zero" . If you say the statement is true then can you please help me in finding my solution.

Nihar Karve
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JAZZ
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1 Answers1

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read that "Work done by magnetic field is always zero"

This is misleading / not true in general. In macroscopic physics, (macroscopic) magnetic forces do work, for example magnetic force due to external field acting on current-carrying conductor $BIL$ does work when the conductor moves (such as in your example) and also this force can do work on magnetic bodies such as iron/steel bodies or magnets.

However, the quoted statement has a core of truth: work of magnetic force acting on a moving point charged particle (so-called Lorentz force) is always zero, because the force is always perpendicular to velocity.

This is consistent with the macroscopic physics, because work of macroscopic magnetic force on moving current-carrying conductor or magnetic body isn's simply sum of works of Lorentz forces due to external magnetic field; there are involved also induced internal forces between the current-forming charged particles and the rest of the conductor and these do the work. In current-carrying conductor, you can imagine them to be all the electric and non-electric forces that act between the mobile charge carriers (electrons) and the rest of the conductor.

Ján Lalinský
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  • So can I conclude that work done by magnetic forces are not always zero – JAZZ Jun 02 '21 at 17:12
  • It can be non-zero if by "magnetic force" we mean macroscopic force due to external magnetic field. It is always zero if we mean the microscopic Lorentz force. – Ján Lalinský Jun 02 '21 at 17:14
  • Can you please explain me the microscopic meaning in the above diagram because I think I am too close to my conclusion. if possible can you please have a diagram – JAZZ Jun 02 '21 at 17:16
  • Your diagram belongs to macroscopic physics, there are macroscopic objects like rod, wires, voltage source etc. It can be explained using microscopic electric and magnetic Lorentz forces, but it is not trivial. – Ján Lalinský Jun 02 '21 at 17:18
  • So, each individual contribution to the magnetic work is zero but the total contribution is not? –  Jun 02 '21 at 17:20
  • @Wolphramjonny no, there are non-zero contributions to work of macroscopic magnetic force (ponderomotive force pushing the rod) that are not due to Lorentz magnetic forces acting on current-carrying particles. The non-zero work contributions are due to work of internal forces between the lattice and the current-carrying particles. – Ján Lalinský Jun 02 '21 at 17:22
  • @Wolphramjonny You said my words. That's what differentiates the macroscopic and microscopic Physics – JAZZ Jun 02 '21 at 17:23
  • Thanks, I am going to try to find something more detailed, as I fail to imagine how such thing could be possible –  Jun 02 '21 at 17:30
  • @Wolphramjonny maybe this answer will help: https://physics.stackexchange.com/questions/114627/what-does-the-work-on-a-current-carrying-wire-in-a-magnetic-field/114638#114638 – Ján Lalinský Jun 02 '21 at 18:46