A half-ring shaped conductor is being placed around a solenoid. This solenoid has a changing magnetic field.

a) There is a current and EMF (Electromagnetic force) in the half-ring shaped conductor b) There is a current but no EMF in the half-ring shaped conductor c) There is no current but there is an EMF in the half-ring shaped conductor d) There is no current but there is no EMF in the half-ring shaped conductor

My thoughts about this: First of all the field outside a solenoid isn't very big in contrast to the field inside the solenoid. But I assume that it can't be neglected. I think there won't be a current as we would need a full ring-shaped conductor for this. I do think that there will be an EMF in the conductor, and this will generate some eddy currents in the conductor. But there will be no "full-on current" so to speak. So I would say the answer is C. Is this correct?

A Little side-question; Is it at all possible for a non-conducting material to have an induced EMF. For example when we move a non-conducting material (ring-shaped) into a magnetic field. The magnetic flux in the ring changes, but will there be an induced EMF? I know that there won't be a current in the loop, but I'm not certain about the EMF.

Thank you very much!

  • $\begingroup$ I think the answer must depend on how the magnetic flux is changing. Although your conclusion that there wouldn't be a current since there isn't a closed path seems reasonable, I think you're forgetting that electrons can "slosh" back and forth along the length of the conductor as in, for example, an antenna. So, how is the magnetic flux changing? Is it a steady change, variable change or even perhaps alternating? $\endgroup$ – Alfred Centauri Jun 24 '12 at 22:54
  • $\begingroup$ I assume the magnetic field just increases linearly with time $\endgroup$ – user10088 Jun 25 '12 at 18:48

Since there is a steadily changing magnetic flux, there are static closed loops of electric field lines circling the solenoid.

If the half-ring shaped conductor is placed around the solenoid, the mobile charge carriers in the conductor will redistribute in such a way that there is zero net E field inside the conductor.

Thus, there is no current (except for the more or less initial near instantaneous current required for the redistribution of charge within the conductor). So, that rules out answers a) and b).

If you place a voltmeter across the ends of the conductor (make sure that the leads do not form a loop threaded by the the magnetic flux!), you will measure a voltage across the conductor that is numerically equal to the emf. (In fact, you're measuring the strength of the opposition to the emf due to the charge distribution in the conductor). So, you are correct, the answer is c).

If you had a half-ring of a non-conductor, no charge redistribution occurs (no free carriers) so you would not measure a voltage across the non-conductor.


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