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

What happens when deforming a coil on a magnetic field

  1. Jun 5, 2012 #1
    1) Having in mind the Faraday-Lenz law, i'm still not sure if anything happens to it? [1) SOLVED]

    The area is always the same, the flux idem, flux density as well, so at first glance it would appear nothing happens?


    This problem was on a exam.

    2) Let's say i have a permanent magnet and a wire like in the picture, should there be an emf, in the wires if we move them into or out of the magnetic field of the magnet? Being a wire, it can only produce flux lines (2-D) and they can't oppose the change in magnetic field. The one on the right because it only produces flux perpendicular to the magnetic field of the magnet, the one on the right, because it's a circle, and if one side of the circle counteracts the change of magnetic flux the other helps it.

    So basically i think this can be resumed in: can a straight line of wire have an emf on its ends generated by the change of the flux its subject to?


    Please do correct any errors in my thinking.
    Last edited: Jun 5, 2012
  2. jcsd
  3. Jun 5, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi An1MuS! :smile:
    The area isn't the same :redface:

    if you deform a circle (keeping the perimeter the same), the area must decrease. :wink:
  4. Jun 5, 2012 #3
    Ups, i was thinking of perimeter, good thing you mentioned it -__- 1) solved then. Thanks.

    What about 2) ?
  5. Jun 5, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper

    a straight wire moving through a magnetic field will receive an emf

    a square wire moving through a uniform magnetic field will receive opposite emfs in opposite sides, so the total emf will be zero

    this is because opposite sides are moving in the same direction

    if you deform the square so that opposite sides move in opposite directions, the emfs will add :wink:

    (and the total emf will of course be proportional to the change in area)
  6. Jun 5, 2012 #5
    But if the question include the posibility of stretching the wires so when you pull the coil to the elliptical shape AND still maintain the same area inside the coil. AND IF the B field is uniform, the induced EMF should be the same, isn't it?

    Sure the exam question don't mean this?
  7. Jun 5, 2012 #6


    User Avatar
    Science Advisor
    Homework Helper

    you can do that for a rectangle, but not for a circle

    for a fixed perimeter, the circle has a strictly greater area than any other shape

    (the ancient greeks knew that!)
  8. Jun 5, 2012 #7
    tiny-tim you said a straight wire would have an emf induced


    did you mean like this?

    (Sorry for my paint skills)
  9. Jun 5, 2012 #8


    User Avatar
    Science Advisor
    Homework Helper

  10. Jun 5, 2012 #9
    I meant IF the wire is stretchable ( big if) and you manage to pull stretch the wires it so that the area inside the ellipse is still equal to the circle it is stretched from. I am worry it is a trick question!!!

    No, if wire is not stretchable, when you pull it like in the diagram, the area decrease and all bets are off.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook