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Homework Help: Increasing the speed of Barlow's wheel

  1. Jun 20, 2015 #1
    1. The problem statement, all variables and given/known data


    I have a problem about Barlow's wheel experiment with several questions. One of them is how to increase the angular velocity for the wheel.

    I can either:

    1. Increase the current
    2. Increase the radius
    3. Increase the strength of the magnetic field
    4. Decrease friction

    Are there any other ways to increase the speed?

    Also, Wikipedia says that using an electro-magnet instead of a permanent magnet will cause more rapid revolution, I don't see why, can someone explain this?

    "The points of the wheel, R, dip into mercury contained in a groove hollowed in the stand. A more rapid revolution will be obtained if a small electro-magnet be substituted for a steel magnet, as is shown in the cut. The electro-magnet is fixed to the stand, and included in the circuit with the spur-wheel, so that the current flows through them in succession. Hence the direction of the rotation will not be changed by reversing that of the current; since the polarity of the electromagnet will also be reversed."


    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 20, 2015 #2


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    Staff: Mentor

    Are you sure increasing the radius will increase angular velocity? Also, how do you increase the radius without increasing mass and therefore friction?
    Maybe the electromagnet is stronger.
  4. Jun 20, 2015 #3
    Actually I don't know if it'll increase the angular velocity. It'll increase the force acting on the wheel, but I guess it'll also increase friction, I don't know which one would be stronger.

    It's been sometime since I solved anything related to rotation, so any help is appreciated.
  5. Jun 20, 2015 #4
    Also, dumb question.

    F = I*r*B suggests that increasing r, would increase the force and thus the rotation, right?

    But w =v/r suggests that increasing r, would decrease the angular velocity?

    What am I doing wrong?
  6. Jun 20, 2015 #5


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    Staff: Mentor

    Torque is force multiplied with distance from the center (an exact treatment would need an integral), so doubling the radius gives four times the torque. The relation between force and angular velocity can be complicated.
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