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Another Magnetism Question

  1. Dec 28, 2011 #1
    1. The problem statement, all variables and given/known data

    A stationary loop of wire is placed in a magnetic field directed into the page as shown below.

    (the image shows a magnetic field like this and a circle in the middle to represent the loop)
    X X X X X X

    X X X X X X

    X X X X X X

    X X X X X X

    The current in the loop of the wire is:

    A. clockwise if the magnitude of the magnetic field is decreasing
    B. clockwise if the magnitude of the magnetic field is increasing
    C. clockwise if the magnitude of the magnetic field is increasing or decreasing
    D. No current will flow through the loop if the magnetic field is increasing or decreasing.

    2. Relevant equations

    --------

    3. The attempt at a solution

    My book says: "Lenz's law says that current will flow in a loop of wire to oppose the changes in magnetic field inside the loop. If the field strength is decreasing, then current will flow to try to increase the field strength."

    What does increasing and decreasing field strength have to do with the direction of the flow? Can't you just use right hand rule to figure it out?
     
  2. jcsd
  3. Dec 28, 2011 #2
    If the magnetic field (should say magnetic flux linkage) is changing then an emf will be induced in the coil. Lenz's law gives the direction of the induced emf and any resulting induced current in the loop.
     
  4. Dec 28, 2011 #3
    I am very confused by Lenz's law... is there any way you can dumb this down for me?
     
  5. Dec 28, 2011 #4
    Lenz's law states that the induced emf will oppose the change producing it.
    This means it tries to stop the change occurring, it tries to maintain the situation.
    In this question, because you are dealing with a loop, the induced emf will cause an induced current to flow in the loop.
    Do you know what magnetic fields produced by a current flowing in a loop look like????
    The magnetic field in the diagram is into the page so if it is decreasing then a current will be induced in the loop to try to maintain this field.
    If the field is increasing then the induced current will oppose this.
    The answer is either A or B.... can you work it out... I won't tell you but I will let you know if you get it right or wrong :smile:
     
  6. Dec 28, 2011 #5
    I know the answer is A (my book told me). I'm just trying to understand the concept. :)

    So if the magnetic field in this question is decreasing, then to help maintain it the current in the loop would create a magnetic field in the same direction to increase it? So if I used right hand rule my thumb would point into the page and the current in the loop would be clockwise?

    And if the magnetic field were to increase, then to maintain is would the loop produce a magnetic field in the opposite direction?? so my thumb would point out of the page and the current would flow counter-clockwise??
     
  7. Dec 28, 2011 #6
    You have got it..... but I do not use the right hand rule that you have described !!!!
    I grab hold of the loop in my right hand with my thumb pointing clockwise around the loop and my fingers curl into the loop to give the field pattern in the diagram.
    If the field was increasing I would grab hold of the loop in my right hand with my thumb pointing anti-clockwise to give magnetic field lines coming out of the page.

    You got it .... that is the important thing... well done
     
  8. Dec 28, 2011 #7
    Thanks so much for your help! :)
     
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