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Homework Help: Force or dp/dt for a photon?

  1. Jan 31, 2009 #1
    If p=E/c for a photon, and I'm given a bunch of other equations such as E=hc/λ and p=h/λ and E=hf and more...

    Note: h=planck's constant, f=frequency, c=speed of light, E=energy, p=momentum.

    I'm wondering how to find dp/dt (=Force)?

    Original question:
    Photons carry momentum, hence they exert pressure on the surface they strike. In this question, we we'll computer the approximate pressure at a distance r exerted by radiation from a star, using the relativistic equation between energy, momentum, and mass. Pressure is force exerted per unit surface area. The force exerted per particle can be written dp/dt where p is the momentum of the particle (from F ~ v^2 and p ~ v). Derive an expression for pressure P at a radius r from the radiation originating isotropically from a point source (i.e. a star), in terms of the momentum of the photons and radius r. (This is most easily derived by imagining that the photons are striking a spherical shell of radius r centered on the star).
    Last edited: Jan 31, 2009
  2. jcsd
  3. Feb 3, 2009 #2


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    Science Advisor
    Homework Helper

    it depends on if the photon is absorbed or totally reflected.

    Are you looking for radiation pressure? Use stefan boltzmans law, or rather, derive it.
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