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Homework Help: Which case generates emf by electromagnetic induction

  1. Aug 19, 2013 #1
    a. coil is moved up and down in the same direction of flux.
    b. coil is rotated around an axis that is parallel to the flux.
    c. coil is rotated to the direction of the flux like a dynamo.

    2. Electromagnetic inductions laws.

    3. I'm really having a hard time understanding how change in area changes the rate of flux so plz explain to me the basic rules that will help me understand and solve the problem on my own, i studied induction on my own and i have an important test coming soon. you are not solving my homework i promise you, you are helping me understand induction.

    however, first two cases no induction will occur because there is no change in the area that cuts the flux lines, third one emf generates
    Last edited: Aug 19, 2013
  2. jcsd
  3. Aug 19, 2013 #2


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    I think option (c) has a few words missing
    c. coil is rotated around an axis perpendicular to the direction of the flux like a dynamo.

    In that case / the dynamo case the turns of the coils are alternately perpendicular to the parallel to the flux, and so the area presented changes - so emf is induced.

    imagine the flux lines run up the page.

    That means the axis could be positioned across the page. (it is perpendicular to the flux)

    Lets begin with the coil flat on the page. (so no flux passes through it)

    As the coil rotates, perhaps the top of the coil gets buried into the plane of the page, while the bottom of the coil will rise out from the plane of the page.

    after 90 degrees of rotation, the plane of the coil will be perpendicular to the flux, so the amount of flux threading the loop is increased.
  4. Aug 19, 2013 #3
    thank you, that is exactly like the dynamo, but what about my answer to the first two? am i correct? will the same happen if we replaced the flux lines with an upright solenoid? what's the conventional direction for the flux of a solenoid?
  5. Aug 19, 2013 #4


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    You were right about the first two.

    I don't quite see what you're getting out with the bit I made red?
  6. Aug 19, 2013 #5
    some problems mention solenoids instead of flux lines which means i have to know the direction of the flux of a solenoid which is only regular in the center, i'm asking what is the conventional direction if a solenoid is mentioned, if it was upright the flux will be from downwards to upwards correct?
  7. Aug 19, 2013 #6


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    To find the direction of the flux inside a solenoid, you imagine grasping the solenoid with your right hand with your fingers pointing in the direction of the current in the coils of the solenoid.
    You thumb then points in the direction of the flux.
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