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Electromagnetic induction and Faraday's Law

  1. Mar 4, 2006 #1
    I need to be enlightened on this. If a magnet with north pole facing towards a coil of wire is pushed in does it induce a current clockwise, or counterclockwise? If you push the coil in towards the magnet will it induce current in the opposite direction? How can I use the right hand rule with this?

    The book I have gives pictures in 3 dimensions, and I'm having trouble imagining it.
     
  2. jcsd
  3. Mar 4, 2006 #2
    If you look at wire coil down it's central axis (so that coil looks like a single ring) then shove a magnet in this hole, the current induced in the coil will go counterclockwise.

    If you hold a wire with your right thumb lying along the wire in the direction of current and wrap your fingers around the wire, your fingers show the direction of the circular magnetic field around the wire.

    When the coil is by itself, there's no magnetic field inside the coil, but when you bring the north end of the magnet unto the coil, a magnetic field goes into the coil. All objects tend to resist a change in magnetic flux, so the wire will try to "cancel" out the magnetic field going into the coil, but creating a current in the wire such that the induced current creates it's own magnetic field moves outward from the coil.

    If you hold a hoola hoop in front of you with your right hand in such a way that likee a big perfect circle, this can be used to represent the end of the coil. Let your thumb lie along the edge of the hoola hoop and your fingers should now be curled TOWARDS YOURSELF. This represents the magnetic field the coil creates, and the direction of your thumb is the direction of the induced current.
     
    Last edited: Mar 4, 2006
  4. Mar 4, 2006 #3
    [​IMG]

    This is the image that confused people. I said the direction of the current was counterclockwise, some people agreed with me, but most said it was clockwise, even the teacher. Now you said if you bring the north pole towards the coil it will induce a current counterclockwise, is this counterclockwise then?
     
  5. Mar 4, 2006 #4

    ZapperZ

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    If you look it from the top, it should be clockwise, i.e. the diagram is correct.

    By applying Lenz's law, the coil will try to oppose the change in magnetic field. Since the N end of the magnet is getting closer, the coil will try to oppose it by producing a field in the opposite direction. This means that the coil will generate a current in the clockwise direction, so that your right-hand rule will produce a direction pointing in the opposite direction to the field from the magnet.

    Note that if you pull the magnet out, i.e. reserve it, THEN you'll get a counterclockwise direction of the current.

    Zz.
     
  6. Mar 4, 2006 #5
    I'm still not getting it. If I push the magnet in from the other side will it produce a counterclockwise current? SpeeDFX said if you have the ring on the central axis and push a magnet through it, the current will go counterclockwise. If the coil was pushed through the magnet will the current go in a different direction than if the magnet was pushed through the coil? I'm really fuzzy on this. My teacher said he isn't 100% sure.
     
  7. Mar 4, 2006 #6

    ZapperZ

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    Do this one step at a time!

    Did you buy my explanation of the case present in the figure you cited? You can't go running to a different situation without understanding this. This will do nothing but create confusion.

    Do you see why, per the situation in the figure, that the current, when viewed from the top, is clockwise?

    Zz.
     
  8. Mar 4, 2006 #7
    Yes, I see why that is clockwise when looking at it from the top. I guess I was wrong then.. hate when that happens :)
     
  9. Mar 4, 2006 #8
    Ok, so what about the other situation?
     
  10. Mar 4, 2006 #9
    Ok, so that diagram represents what I was saying perfectly. If you pretend you are the small face of the magnet (on the north side) that is entering the coil as you enter the coil, you will see the current moving counterclockwise as you enter the loop. Looking at the coil from the top (as depicted) reverses the perspective, and the current moves appears to go clockwise.
     
    Last edited: Mar 4, 2006
  11. Mar 6, 2006 #10
    So in reality which way is the current going?? In the perspective of the north pole of the magnet - counterclockwise, in the perspective at the top - (appears) clockwise. But ZapperZ says its clockwise.
     
  12. Mar 6, 2006 #11

    ZapperZ

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    Where exactly is the contradiction/confusion in what you just said here? Something that is clockwise from one side appears counterclockwish from the other. Try it yourself. Look at your clock from the wrong side.

    Zz.
     
  13. Mar 6, 2006 #12
    Well today in class I proved it to myself and the class that the current was indeed counterclockwise when pushing the north pole of a bar magnet though a coil of wire. I took some copper wire and wound it in a counterclockwise fashion. Hooked the positive and negative leads of a multimeter to each end of the copper wire. Then took a bar magnet and inserted the north pole in the copper coils. The multimeter read +0.5 A when going in (which means it's going from the positive lead to the negative lead, along the counterclockwise coils), and -0.5A when going out (from negative to positive). When I inserted the bar magnet's north pole in the other side of the copper coils it gave a -0.5A reading along the clockwise fashion coils which still meant counterclockwise on the other side. All the directions were reversed when using the south pole.

    I've been wondering... is this the basics of AC and DC? A magnet moving up and down in and through coiled wires to create a positive and negative current to induce AC, and a magnet moving through a circle of coiled wires to induce DC. Kinetic energy transfered into electrical energy?
     
    Last edited: Mar 6, 2006
  14. Mar 6, 2006 #13

    Galileo

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    Firestrider, you seem to miss the point (but I could be wrong) that clockwise and counterclockwise are relative terms, like left and right. Something on the left for me is to the right for someone facing in the opposite direction of me. Looking from above the current will go clockwise, looking from below it will go counterclockwise.

    Take ZapperZ advice and try to understand Lenz' law. The current will always flow such as to oppose the change in flux.
     
  15. Mar 6, 2006 #14
    But when someone asks the question "The north pole of a magnet is pushed into a coil, what direction is the induced current?" what would be your answer?
     
  16. Mar 6, 2006 #15

    ZapperZ

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    You still don't see that such a thing depends on the perspective, do you? Why can't you answer that it is clockwise when viewed from one way, or counterclockwise when viewed the other way? These are NOT different answers. They are the same answer but looked from different perspective.

    I don't understand why you are having so much difficulty with this.

    Zz.
     
  17. Mar 6, 2006 #16
    I just don't see how there could be two answers on a test for the same question. I didn't see you saying that I'm right for one perspective either, that is why I was still questioning.
     
  18. Mar 6, 2006 #17

    ZapperZ

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    If you have a multiple choice question, and it doesn't say from WHICH perspective this "clockwise/anticlockwise" direction, then you have an ambiguous test. Complain to your instructor. If, instead, this is an answer you have to write down, then you should answer from which perspective.

    And what is there for me to say if you're right or not? I already given you my answer from the "top" view in that diagram. If you cannot translate that into whatever perspective you wish to adopt, then you have a more serious problem to contend with than this. Your fixation of having only ONE particular way of a correct answer no matter what the perspective is will give you lots of problems later on. What if I do a projectile problem but I shift my origin to the top of a cliff? Or maybe I must prefer to have that at the bottom of the cliff instead? Does that give me a different answer? What if I designate the downward direction as positive values instead of upwards direction? Would this mess up the problem?

    This has been one of the strangest issue I've seen in a long time. Of all the difficulties one can have in a physics problem, I've never seen anyone struggle with this type before.

    Zz.
     
    Last edited: Mar 7, 2006
  19. Mar 13, 2006 #18
    A simple answer would be "the opposite direction". But to determine which polarity the electrons flow (ie. positive/negative) you first need to know which direction the coil is wound.
     
  20. Apr 28, 2010 #19
    A coil has two sides,
    Through one side a magnet with N-pole enters or tends to enter.
    The current in the coil is such that it opposes the entry of the magnet.
    Hence, imagine the coil is replaced with another magnet {B) with its N pole pointing the N pole of the magnet that is inserted. N-pole repels N-pole.

    The current in the coil is such that its magnetic field in the side to which the magnet is inserted is north.

    To find the direction of current, again grip the magnet B with your right hand such that the thumb points the N-pole of the magnet. To make you clear, see the figure you have shown. Grip the magnet B as shown but with the thumb pointing the N-pole. In the figure it is not stretched.

    Now hold this magnet B over A such that N-pole opposes the N-pole. Your fingers round the magnet will show the direction of the current.
    =============================================

    If the magnet is pulled out of the coil, the current is such as to oppose this motion. To oppose the motion of magnet A, we must bring the S-pole of magnet B to the n-pole of the magnet A. (The two magnets attract each other preventing the motion away). For this you will tilt your right hand . Your fingers will show the direction.
    ====================================================
     
  21. Apr 28, 2010 #20

    ZapperZ

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    Please note that you are responding to a thread that had a last post in March, 2006!

    Zz.
     
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