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Homework Help: Stuck on light intensity problem

  1. Mar 21, 2009 #1
    stuck on light intensity problem....

    1. The problem statement, all variables and given/known data


    A conducting rod slides down 2 frictionless copper tracks at a constant velocity.
    There is a .50 T magnetic field perpendicular to the rod. The rod maintains electrical contact at all times. The rod has a length of 1.0 m and a mass of .25kg. A 1.5 Ohm light bulb is attached between the tops of the tracks.

    What is the intensity of the light from the light bulb at a distance 3.0 away from the bulb?

    2. Relevant equations

    S = c / (magnetic permeability of free space) * B^2

    3. The attempt at a solution

    S = ?
    B = .50
    c = 3.00 *10^8 m/s
    (magnetic permeability of free space) is a constant... = 4pi *10^-7 T*m/a

    if all these variables are plugged into the equation i can solve for S and get the intensity. But "3 meters" is mentioned in the problem. I know im using the wrong formula...which one am i supposed to use?
  2. jcsd
  3. Mar 21, 2009 #2


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    Homework Helper

    Re: stuck on light intensity problem....

    The rod moving through the magnetic field acts as a generator. The generator formula is
    V = LvB (potential = length x velocity x magnetic field).
    Knowing the potential, you can use electricity formulas to find the current and power delivered to the light bulb.

    Unfortunately no efficiency is given for the light bulb - perhaps you are supposed to look up the efficiency of a typical incandescent bulb in order to estimate the light power produced. The light will spread out in every direction, illuminating an imaginary sphere 3 m in radius, and you want the number of watts of power per square meter on it.
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