Faraday's law and Lenz's law

Discussion in 'Homework Help' started by mo2015mo, Jul 17, 2013.

  1. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    Hello my friends,,,

    I am confused about determine the sign of current . Help Me Please,,
    Why is my question 1 (as attached fig1) ,, Assume the direction I is positive though it should be is negative by table summarizing (as attached I-table) ??????????
    and I have the same problem in Q2..

    Help me to get rid of this problem,Please..

    Is Vemf <0,, I must CCW??
    Is Vemf >0,, I must CW??
     
    Last edited: Jul 18, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    Use the Right Hand Rule to determine the direction of the magnetic field when a current flows through the wire.

    Then use Lenz's Law. The current induced in the wire must produce a magnetic flux that opposes the inducing magnetic flux. If it didn't you would be able to create a perpetual generator which would violate the laws of thermodynamics.
     
  3. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    i know that BUT in my problems above reverse the direction of I at the end of soltuion ?? why??
    and How i can determine the higher potential end if Not mentioned in the question??
     
  4. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    .................. :eek:
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    Maybe this will help you get your mind around it.

    Imagine a circular loop of wire in the plane of a piece of paper with a current flowing CW in it. Use a right-hand coordinate system with the z-axis coming out of the paper?

    1) What direction is the B field? In to or out of the paper.

    2) What is the sign of the B field?

    Now imagine that we increase the current so that the B field gets stronger.

    3) Is the change in flux positive or negative?

    4) Now imagine a second loop of wire sitting on top of the first, but not touching it electrically. What direction would a current have to flow in that loop in order to cancel out the effect of changing the current in the first loop?

    Now take away the first loop and let the B field be the result of some source somewhere else and let that source establish the same B field as we did originally and then make the same change to the B field as we did next.

    5) Can the second loop tell the difference between these two situations?
     
  6. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Good example.
    I have a question about the sign of the voltage produced in the second loop. Suppose we place a resistor (so we can measure the voltage) at (0,-y). What sign would the voltage be (V12) if point 1 (-x,-y) is to the left of point 2 (x,-y)?
     
  7. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    i answered this Q's as the attached fig.
     
  8. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    V12 is negative because the direction in second loop is CCW
     
  9. mo2015mo

    Thread Starter Member

    May 9, 2013
    157
    1
    ....................... :mad:
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    You are doing the analysis as if the magnetic field in the second case is created solely by the current in the second loop. The point was to assume that the magnetic field was being provided externally in the same way as the original loop did. Since the only way the second loop "knew" the first loop was there was through the magnetic field, then it cannot tell the difference between the two situations.

    A change in the original B field will induce a current in the loop that, ideally, counteracts the change in the B field. If the B field increases in intensity, the field created by the second loop is in the opposite direction. But if the B field decreases in intensity, the field created by the second loop is in the same direction.
     
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