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Induced Emf due to changing magnetic flux

  1. Jul 12, 2015 #1
    If we have a conducting loop, and a magnet (say North pole first) is pushed into it; that induces and emf and hence a current in the coil due to the changing magnetic flux, right? According to Lenz' law, the direction of the current is such that it opposes the change. So from the side the magnet was pushed in, it would be anticlockwise?

    My question is, if we push the magnet completely through, after a point, won't the increasing flux actually start to decrease? And if that's right, then does the current change direction? I'm referring to a case where the pushinin of the magnet is done in a single motion/step. This feels intuitevely wrong to me, and all the resources I found only talk until the first bit i mentioned.

    I'm thinking that my understanding of the concept of magnetic flux is wrong. It can be iterpreted as the number of field lines passing through an area, right?

    Thanks for your time!
  2. jcsd
  3. Jul 12, 2015 #2


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    Science Advisor

    Yes. I have a hairy anecdote to illustrate. A long time ago, some colleagues of mine were engaged to connect some medical instrumentation to a data center (at the same floor, but about 30m along a corridor). What happened was that they got intermittent high frequency high amplitude ground current (which interfered with the data transfer). After several days of head scratching, they noted:
    • Between the instrumentation and the data center, there was a door to the stairwell.
    • The stairwell contained a lift (elevator)
    • The mains in the two rooms were separately connected to a mains distributor in the basement
    • The protective ground were also connected in the basement
    So - when the data cable set up a connection between the protective ground in both rooms, you had a ground loop - and a lift (with a current-carrying cable) going up and down through the loop, inducing all kinds of EMF in the ground loop!
  4. Jul 12, 2015 #3
    Yes, you are right. As you push the magnet, the magnetic field flux is increasing, causing a current through the wire loop. As the magnet gets pushed all the way through, and thus the magnetic field flux goes from increasing to decreasing, you will reach a point where the induced current switches directions in the loop.
  5. Jul 12, 2015 #4


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  6. Jul 12, 2015 #5
    2 more cents

    Remember, the EMF is the from the change in magnetic flux.

    Pushed part way in the flux increases, and the EMF is positive in a particular direction. Then pulled out the flux decrease, so the EMF must change sign.

    This last bit is the same as pulling it out the other side.
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