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Solenoids and Lenz's Law

  1. Jan 13, 2012 #1
    1. The problem statement, all variables and given/known data
    Hi. My questions are on Lenz's Law.
    We have a bar magnet indicated by the [N | S]. It is moving to the left of a solenoid that is connected to an ammeter. Two points on the circuit are labelled X and Y. ( A ) is just an ammeter. The question is.. What direction does the current flow when the magnet moves?
    (i.e. X to Y or Y to X)

    [N | S] {||||||||||}
    <----- |__( A )___|
    ..........X ............Y

    (ignore the periods, they were necessary to keep the formatting for the diagram)

    2. Relevant equations
    Lenz's Law - coil acts to oppose change in B

    3. The attempt at a solution
    Here's my crack at it. As the magnet moves away, there is less flux cutting the solenoid. The magnet's field goes from N -> S so the solenoid is getting a reduction of flux going <-. Then, using the Right Hand Screw/Grip Rule, the direction of the field is <- so current will flow going in to the page. I don't quite understand which way the current is supposed to flow from here (X to Y or Y to X) and the diagram given to me was pretty shoddy.

    Also, for these type of problems involving loops, wires, solenoids, magnets being moved around and Lenz's Law, what type of Right-Hand Rule is used? My textbook instructs me to use the Screw/Grip Rule, while some websites say different things. The Grip Rule is also changed many when working with solenoids etc. Thumb changes from direction of B to direction of current. It is quite confusing.

    1. The problem statement, all variables and given/known data
    Same sort of problem. We have a cardboard tube, and on it are wound 2 circuits. One is labelled A, and consists of a solenoid in series with a switch labelled P. The secondary circuit is labelled B. Two points on B are chosen to be X and Y. When the switch P is closed, what direction will the current flow in B? (X to Y or Y to X)
    [...|||||||||||||............|||||||||....]<- The cardboard tube
    ....|_ \_(+/-)__|............|__(A)__|

    (Again, the periods have no significance, they are present for formatting)

    2. Relevant equations
    Same as above. Lenz's Law etc.

    3. The attempt at a solution
    Once the switch P is closed, the current in A will travel from the +ve terminal to the -ve terminal. Using the Grip Rule for solenoids, North is in -> for the solenoid in A. This means that the change in magnetic field for the solenoid in B is an increase in flux in ->. To oppose this, the induced magnetic field will have to be going <-. Using the Grip Rule again, this time thumb pointing <-, current will travel in to the page and once again, I'm stuck. Which way does current flow (X to Y or Y to X)?

    Answers from the book are 1. X to Y and 2. Y to X, but these have little significance to me if I don't know the reason why.
    Thank you for your help.
    Last edited: Jan 13, 2012
  2. jcsd
  3. Jan 13, 2012 #2
    Re Q1: you have the right idea about Lenz's law.
    If the magnet is moving to the left then any current induced in the coil will try to stop the magnet moving.
    What magnetic polarity would the coil need to have?
    You need to know how the coil is wound to determine the direction of any induced current and your diagram cannot show that detail.
    Hope this helps
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