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Homework Help: Turns of a Wire

  1. Mar 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A generator is designed to produce a maximum emf of 180 V while rotating with an angular speed of 3900 rpm. Each coil of the generator has an area of 2.0×10−2 m^2.

    a) If the magnetic field used in the generator has a magnitude of 4.1×10−2 T, how many turns of wire are needed?

    2. Relevant equations
    emf = NABw ???

    3. The attempt at a solution
    I'm not sure if this is even the correct equation, but my main problem is that I'm not really sure what the 3900 rpm is (i.e. what symbol it is and what it is in general). Any help would be appreciated.
  2. jcsd
  3. Mar 2, 2010 #2


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    the 'w' in your relevant equation is the angular velocity in rad/s. So you need to convert rpm to rad/s
  4. Mar 2, 2010 #3
    how exactly would i do that? but that is the correct equation though?
  5. Mar 2, 2010 #4


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    yes it is the correct equation.

    1 rpm = 1 revolution per minute.

    1 rev = 2π radians

    1 rpm = 2π radians per minute.

    How many per second now?

    Can you now convert 3900 rpm now?
  6. Mar 2, 2010 #5
    im not sure if i did it right, but i got 234,000. the radians thing is throwing me off
  7. Mar 2, 2010 #6


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    No, that's not right. How many radians is 3900 rpm? How many seconds is one minute? How many radians does the thing cover per second if it covers that many radians in that many seconds?
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