A question.

by sridhar_n
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sridhar_n is offline
Oct17-03, 08:00 AM
P: 85
A question,

I have a plastic disc with a coil wound around is axis of rotation. The coil carries current I. Now the disc carries charges at its circumference. Now the current flowing in the coil is brought to 0 instantaneously. This creates Electric Field to be produced that travel in concentric circles and induce a force on the charges and make them rotate. However, by law of conservation of momentum, the disc must not rotate. The question is, will the disc rotate or not?

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Chi Meson
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Oct17-03, 09:43 PM
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Just to clarify:

I'm picturing something like a pencil with a coil of wire around it, this is the axis of rotation? And a thin, non-conducting disk that has a net charge at its outer edge?

And by shutting off the current, the collapse of the dipole magnetic field creates an electric field that is concentric with the disk. Is my image correct?

When the force is exerted on the charged rim of the disk, the charges begin to move. These moving charges create their own magnetic field which in turn exerts a force on the charges in the coil. If the coil was free to move, it would rotate in the opposite direction from the disk. If the coil was held in place then the entire world would share in the angular momentum and the effect would not be noticed.

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