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Question about lenz law

  1. May 23, 2014 #1
    So say you have a square loop of wire which has a current of some positive value going clockwise and it enters a magnetic field going into the board. Now since lenz law states that the induced emf will make a current going counterclockwise (to oppose changing mag flux) then what is the new current value? Does the current just keep getting smaller and smaller and then start to actually go the other way?
     
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  3. May 23, 2014 #2

    Matterwave

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    The induced EMF counteracts the EMF that is driving your current, so yes it will reduce the current. Whether it will force the current to go the other way depends on the specifics (how strong your current driving EMF is, how fast the loop enters the field, how strong the field is, etc.)
     
  4. May 23, 2014 #3

    Ok well this same situation was presented in my class and it asked what direction the force was when the right side of the square just entered the magnetic field. Since it was going clockwise and the induced emf would not bring the current the other way (I predicted from common sense exactly what you said), then I said the force was to the right (right hand rule). It was a conventional current but he said the answer was to the left! Any explanation? Thanks
     
  5. May 23, 2014 #4

    Matterwave

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    By your description, the answer should be that the force on the right side wire of the loop is going to the right.

    I don't see why it would be left. Did the professor give you a reason?
     
  6. May 23, 2014 #5

    It's a high school teacher and he didn't specify but I didn't have time to think about it because class was ending
     
  7. May 23, 2014 #6

    Matterwave

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    Perhaps the teacher just did the right hand rule wrong. By your description, you are correct in thinking the force on the right side wire is acting to the right.
     
  8. May 23, 2014 #7

    Yes perhaps. So is an inductor basically the same concept as this situation right here?
     
  9. May 23, 2014 #8

    Matterwave

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    An inductor usually has many loops rather than just one (usually a solenoid). But you basically do induce a current from one source to the other via the Faraday's law.
     
  10. May 23, 2014 #9
    You don't need any hand rule.
    The flux of the external field through the coil is increasing because the coil enters the field.
    It enters the field moving to the right. So moving more to the right will increase the flux. To oppose this a force should act to the left, opposite to the motion of the coil. This is all.
    If the force were to the right, the coil will move faster and the flux will have a higher rate of change.
     
  11. May 23, 2014 #10

    But that contradicts the right hand rule? Even if that does sound bad, are you saying the RHR only applies at certain times?
     
  12. May 23, 2014 #11

    Matterwave

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    Nasu's answer is true if there was no current in the loop to begin with. If you had a current to begin with, the loop would initially accelerate towards the right, but then slow in its acceleration as the counter EMF is produced. The equilibrium condition would then depend on the EMF that is providing the initial current. If, for example, the loop's initial EMF is very strong, then this EMF would effectively be driving the loop towards the right.
     
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