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: Induced Electric Fields of metal ring

  1. May 23, 2008 #1
    1. The problem statement, all variables and given/known data

    A metal ring 4.50 cm in diameter is placed between the north and south poles of large magnets with the plane of its area perpendicular to the magnetic field. These magnets produce an initial uniform field of 1.12 T between them but are gradually pulled apart, causing this field to remain uniform but decrease steadily at 0.250 T/s.

    (a) What is the magnitude of the electric field induced in the ring?

    (b) In which direction (clockwise or counterclockwise) does the current flow as viewed by someone on the south pole of the magnet?

    2. Relevant equations

    Do i need to do anything with the initial B value?

    Change in flux dB/dt = -0.25t?

    Or is it dB/dt = 1.12 - 0.25t?

    3. The attempt at a solution

    a) d[phi]/dt = (dB/dt)*Acos(0) = (-0.250)*(pi(2.25*10^-2)^2) = -3.98*10^-4 Wb

    E = (1/2r*pi)*(d[phi]/dt) = -2.8*10^-3 N/C

    b) Clockwise?
  2. jcsd
  3. May 23, 2008 #2


    User Avatar
    Homework Helper

    Hi aznkid310,

    I don't believe your answer to part b is correct. Can you explain your reasoning for that part?
  4. May 24, 2008 #3
    induced emf = - (change in magnetic flux)

    Since magnetic flux is decreasing, an induced magnetic field opposite to that must be created to counteract this change in flux. Is my reasoning off?

    Also, is part (a) correct?
  5. May 24, 2008 #4


    User Avatar
    Homework Helper

    Your answer to part a looks right to me (except they want the magnitude of the field, so you don't need the negative sign).

    The induced magnetic field will be in the direction to oppose the change. Since the magnetic flux from the magnets is decreasing, the induced magnetic field will be in the same direction as the magnet's field. Does that make sense?
  6. May 24, 2008 #5
    Ah that makes sense. It would oppose only if the magnetic flux is incresing right?
  7. May 24, 2008 #6


    User Avatar
    Homework Helper

    It always opposes the change, so if the magnetic flux is increasing, then yes, the induced field will be in the opposite direction as the external flux.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook