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Help with lenz's law

  1. Jul 23, 2008 #1
    1. The problem statement, all variables and given/known data

    A small metel ring, as illustrated by the picture, is dropped into a region of constant magnetic field.

    For regions 1, 2 and 3, determine
    (a) whether the induced current is clockwise, counterclockwise, or zero
    (b) whether the magnetic force on the ring upward, downward, or zero


    2. Relevant equations

    3. The attempt at a solution
    I think that

    1. The induced current is counterclockwise
    2. The current is zero
    3. The current is clockwise

    I am not sure about these answers though, and I have no idea how to solve for the direction of force. Please explain your answers if you can help me thanks
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 23, 2008 #2
  4. Jul 24, 2008 #3
    Let's look at position 1. The ring is entering the magnetic field so the out-of-page magnetic field increases. The current starts to run in such a way that the induced magnetic field opposes the change in the actual field. So what will be the direction of the induced field? Consequently, using the RHR, what will be the direction of the current?
  5. Jul 24, 2008 #4
    okay so it looks like i had reversed it then

    a. should be clockwise
    b. zero
    c. counterclockwise

    to counter the into page

    but still how can i figure out the forces...this whole RHR technique confuses me so much and is so frustrating to visualize
  6. Jul 24, 2008 #5
    Now for the forces, for the first one, you know current is going clockwise and only the bottom part of the ring is in the magnetic field. So the net force on the ring will be the net force on the very bottom part of the ring (there is a force on the left and right sides that are also in the field but they get cancelled out by each other). So for #1, which way will the force be?
  7. Jul 24, 2008 #6
    hmm so will the net force act then against the object???

    such that for
    a. Up
    b. zero
    c. down
  8. Jul 24, 2008 #7
  9. Jul 24, 2008 #8


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    Homework Helper

    Hi physicsdawg,

    I believe one of those answers is not right for the force on the ring. Try again to apply cryptoguy's advice from post #6 carefully to all three cases. Do you see which one is incorrect?
  10. Jul 24, 2008 #9
    yeeep, thanks for the catch alphysicist, much obliged
  11. Jul 24, 2008 #10
    hmm still confused, would the force then be pushing down for the first part? and the last 2 right? if i'm wrong could you tell me why i have a test today and want to know these problem explanations in and out
  12. Jul 24, 2008 #11
    No, the first one is in fact upwards. You know that on the bottom part of the ring, current runs to the left, according to your answer in a. Apply the RHR. You'll see that the force on the bottom of the ring points upwards.

    Now take a look at part C. Now, which way does the current run in the top part (the only uncanceled part of the ring affected by the magnetic field)? Apply the RHR again and you will get the answer.
  13. Jul 24, 2008 #12
    aah so it will also be up?
  14. Jul 24, 2008 #13
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