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Lenz's law and direction of current

  1. Jun 7, 2009 #1
    1. The problem statement, all variables and given/known data
    What is the direction of the magnetic field due to the induced current when the following changes are made to the initial field?
    a. The field is switched off.
    b. The field strength is doubled.
    c. The direction of the field is reversed.

    2. Relevant equations
    Lenz's law?
    "Any induced current in a loop will be in a direction such that the flux it creates will oppose the change in flux which produced it."

    3. The attempt at a solution
    I can find the direction that current flows in a coil when a magnet in inserted into, or removed from, a coil, using the right hand curl rule, but this question confuses me. Isn't it necessary to know the origin of the field so that you know where the lines of flux are expanding from or contracting towards? The field lines coming out of the page dont indicate the origin of field? Help understanding this concept much appreciated.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 7, 2009 #2


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    Science Advisor
    Homework Helper

    I don't think you need to know the exact source of the magnetic field.
    For example in a) you turn off the magnetic field. In a sense, this is the same as adding a flux into the page. Therefore, by Lenz's law, the current in the loop will produce a current that causes a flux out of the page to cancel it. You can also look at it this way: there is currently a flux outward through the loop. When you suddenly take that away, a current will start flowing which tries to preserve that flux.

    The direction of the current can now simply be found by - for example - the right hand rule (thumb along the field lines, fingers curl in the current direction).
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