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Homework Help: Radiation pressure of a light bulb

  1. Jan 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Estimate the radiation pressure due to a 100 W bulb at a distance of 8.4 cm from the center of the bulb.

    2. Relevant equations
    Power= Work/Time W=Force(distance)

    Pressure = Force/Area = Poynting vector/speed of light

    3. The attempt at a solution
    I found the cross sectional area by doing 4∏ r^2, so I have that. Then, seeing as I was given power and distance, I tried to figure out the Force in terms of Power and distance, but in doing that, I got the formula F= Power (Time)/distance, and I wasn't given the time. Is there some formula that I'm missing?
  2. jcsd
  3. Jan 18, 2012 #2


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    Homework Helper

    distance/Time is velocity ... so your "(Time)/distance" = 1/c .
    This means that your F= Power/c ... Pressure = Intensity/c .

    Note that this is only true if the 100W of light is absorbed
    (light being reflected would double the pressure)
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