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I saw this question which asks for the condition for flow of electricity through the conductor i know that electricity will flow only when there is a potential difference achieved between the conductor and the ground is at 0 potential .

My approach

I feel that when there is fluctuation of current through the wire there will be development of a magnetic field for short duration so electricity should flow but option 1 and 2 are wrong(as given in the book) , presence of electron beam develops a positive charge on plate so a potential is developed and electricity flows and alpha beam develops a negative charge on plate so potential is achieved so 3 and 4 are correct but I think all should be correct.

What's wrong with my concept? (Given answers are 3 and 4.)

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  • $\begingroup$ Pls tell what's wrong in my concept $\endgroup$
    – imposter
    Commented Jul 24, 2020 at 6:57
  • $\begingroup$ Why do u think dev of magnetic field will help in flowing of current through C? $\endgroup$
    – Danny LeBeau
    Commented Jul 24, 2020 at 7:35
  • $\begingroup$ No its vice versa when we change the current an induced end will be developed and changing end is associated with magnetic field isnt it? $\endgroup$
    – imposter
    Commented Jul 24, 2020 at 7:43
  • $\begingroup$ But for that current should fluctuate continuously.Look for transient current. $\endgroup$
    – Danny LeBeau
    Commented Jul 24, 2020 at 7:56
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    $\begingroup$ I eliminated c and d because I assumed that the beams were "beams" which to me imply that the current is constant in space ... practically continuous. I also assumed steady-state current because a and b explicitly mention changing currents. So to my eye the question is poorly written. I say this with empathy for the author of the question, having written many faulty questions myself, only to find the fault on students' responses to an exam question. $\endgroup$
    – garyp
    Commented Jul 24, 2020 at 17:28

3 Answers 3

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In the given options, the first and the second resemble each other (option 2 is the symmetrical case of 1). So we only need to prove that current will not flow through C if there's a flow of charge and NOT A beam.

Your concept is correct, as there's change of magnetic field, eddy currents will be induced and that will solely depend on the orientation of the metallic plate.

We say that in Orientation 1: Normal of the symmetric plate intersects the axis of the wire.

Here, the magnetic flux will be zero.

And for any other Orientations, a current will be induced in the plane of metal plate. These currents are called eddy currents and you'll find them relevant when you look up electrical generators and their heating causes. Option C and D are correct with the electrostatic concept.

Thank you

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I think I agree with Anusha. Current in the thread produces a magnetic field. Changing the current changes the magnetic field, and a changing magnetic field produces an electric field which should produce some current in the grounding wire.

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  • $\begingroup$ You mean the book is wrong? $\endgroup$
    – imposter
    Commented Jul 24, 2020 at 12:53
  • $\begingroup$ This question may have appeared in the book before the discussion of electromagnetic induction. $\endgroup$
    – R.W. Bird
    Commented Jul 24, 2020 at 15:56
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Since it's an open plate , that is it's top and bottom are not connected from a wire it will behave as an open circuit. So current will flow from c only through electrostatic induction only.

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