Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Lenz's Law question

  1. Dec 17, 2009 #1
    1. The problem statement, all variables and given/known data

    I don't have the question with me, it was on a test I did. It was something like this though:

    A coil is placed vertically on a table. The magnetic field there is also vertical. If the magnetic field is suddenly switched off, in which direction would the current in the solenoid move?

    A) Clockwise
    B) Counter-clockwise
    C) There would be no current
    -and I forgot the rest of the choices.

    3. The attempt at a solution

    Well it would look something like this:

    http://img43.imageshack.us/img43/2058/emfb.jpg [Broken]

    So, current is produced when there is a change in the magnetic field, so there is obviously would be current if the field is turned off. And that current's field would try to counter the flux.

    But some things aren't clear to me;

    1) Since the field is even on all points of the solenoid, which side of the solenoid (inner or outer) would be the one that creates the "counter-"magnetic field against the flux? Would the field be countered from within or from the outside?

    2) Which direction would the flux be (that creates the current) if the "field is suddenly switched off"?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 17, 2009 #2
    Your current would be counterclockwise. Lenz's Law states that this coil will attempt to fight the change in flux that is brought about when the B field is switched off. Since that original B field was pointed up, the current must be CCW to produce a B field in that same direction in an attempt "to fight" the change in flux from some flux to zero.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook