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Recently I came across a problem:

A metal rod of length $L$, connected to a vertical shaft is rotating in a horizontal plane with a constant angular velocity $\omega$ (anticlockwise as seen from above) about one of its fixed ends. Find the potential difference between the ends of the rod. Now, a uniform magnetic field in vertically upward direction is switched on. What should be the magnitude of said field so that the potential difference between the ends of the rod doubles?

The first part of the problem seems easy enough, the free electrons in the wire would move in a circle owing to an electric field that would be induced in the rod which would provide the centripetal force for the same (Please correct me if I am wrong). So we have $eE=m\omega^2x$, where $e$ is the electronic charge, $m$ is the mass of the electron, and $E$ is the electric field at a distance of $x$ from the wire. So to find the potential difference, I integrate the $E$ over the length of the rod, to get the potential difference as $m\omega^2L^2/2e$, where the free end is at a lower potential difference.

Now for the second part, I figure that the centripetal force is now provided by the resultant Lorentz force, but since the magnetic force is also towards the centre we should have $$eB\omega x-eE=m\omega^2x$$ Now clearly, since the potential difference doubles, so does the electric field, but I believe that in this case, the direction of electric field is the opposite, so I integrate both sides with respect to $dx$, and I get $B=3m\omega/e$, but the answer given is $m\omega/e$. I'd like to know what is incorrect about my approach, and if there are any other effects that I am neglecting in my solution, as the intended solution simply equates the motional emf to the difference of the required potential difference in the second and first part. I'd also like a bit more insight into if there are any tangential electric fields in the rod as well, or any other fields that my solution is missing.

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  • $\begingroup$ In the first case, (x) is the distance from the axle (not from the wire). $\endgroup$
    – R.W. Bird
    Sep 5 at 16:29
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Assuming the free electrons actually shift outward (away from the axle) as a result of their circular motion, then the separation of charge produces an electric field pointing outward in the rod. This field exerts a centripetal force on the electrons preventing any further shift. For the given directions of motion and applied magnetic field, the magnetic field also produces a centripetal force on the negative electrons. This would reduce the charge separation and potential difference. To double the potential difference, it will have to reversed, and then doubled. I agree with your factor of three.

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