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Electromagnetic induction open cuircuit and how to determine direction

  1. Feb 23, 2012 #1
    When a magnet is pushed into a solenoid an induced emf occurs. So if it is a closed circuit then an induced current will flow. But if it is an open circuit then an emf will still be induced but no current this time. So will any poles be formed to repel it?

    What do they mean by the current flow in a clockwise/anti clockwise direction?

    Thanks for the help!
  2. jcsd
  3. Feb 23, 2012 #2


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    Staff: Mentor

    A solenoid is a coil of wire. If you push the magnet in one direction the current will flow through the wire. If you then push it in the opposite direction the current will flow the other way in the wire, in the direction reverse of the original. Since it's a coil, you can label these as clockwise or counterclockwise.
  4. Feb 23, 2012 #3
    But where do I look to tell the clockwise/ anti clockwise? Do I look at the solenoid in a vertical way meaning that the long solenoid is facing me or do I look at the cross sectional area of the solenoid, meaning I see a circle? I think I should see it in the vertical way right? Because if I look at the cross sectional area it changes if I look at it from the other side... but I'm unsure about this.

    2) when I have a magnet pushed into a oprn circuit solenoid, thrn an induced emf will occur but no induced current right? So I will feel no resistance right? I'm doing my GCE O levels so I don't know anything about that small flow of charge even when there is an open circuit.

    thanks for the help you guys rock!
  5. Feb 24, 2012 #4

    Philip Wood

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    Your first point: if there is no current because the circuit isn't closed, there will still be an emf, and its direction can still be figured out from Lenz's law, e.g. by considering the poles that the solenoid would have, if the circuit were completed!

    As for which way to look at the solenoid, I'd look end-on. See diagram. Statements in terms of clockwise or anticlockwise are, imo, of little use without diagrams.

    Attached Files:

  6. Feb 24, 2012 #5
    But if there is no current then how can there be a induced pole to oppose the magnet?
  7. Feb 24, 2012 #6

    Philip Wood

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    Please read my first paragraph again, carefully. The rule is based on the current that there WOULD be, and the consequences of that current. The rule gives you the direction of the emf even if there is no current actually flowing.

    Note that I'm not attempting to give you a mechanism for HOW the emf is induced. That's another story, which I didn't think you were asking about.
  8. Feb 24, 2012 #7
    I thought that the emf has no direction? Sorry if I'm unclear on this but will there be a force opposing me? Because since its the current that results in the poles being induced then since no current means no force? Thanks dorbtje help!
  9. Feb 24, 2012 #8

    Philip Wood

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    Imagine you move a rod of conductor containing positive charge carriers through a magnetic field. The field is vertically down, the rod lies East-West, and you are pushing it North (in a direction at right angles to itself). The charge carriers will experience forces to the West, along the wire, even though they can't flow (because we haven't given them a complete circuit). These forces arise from the so-called Motor Effect – the force on a charge carrier moving in a magnetic field.
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