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E=MC^2 Here,say we want to find the energy of a single

  1. Jan 1, 2012 #1
    E=MC^2

    Here,
    say we want to find the energy of a single photon.

    that means,
    mass of photon x speed of light squared = energy of photon

    but a photon is mass less. isn't it?
    but how can a photon have no energy?
     
  2. jcsd
  3. Jan 1, 2012 #2
    Re: E=mc^2

    E = mc^2 only applies to objects with mass.

    The equation for the energy of a photon is E = hc/w where h is Planck's constant, c is the speed of light, and w is the wavelength of the photon.
     
  4. Jan 2, 2012 #3
    Re: E=mc^2

    [itex] E_0 =mc^2[/itex] is the rest energy of a mass, i.e. this is the energy that a mass has when it is at rest. A photon is never at rest, so applying this equation makes no sense.


    E=mc2 is actually a specific case of a more general equation which gives the energy of a body at any velocity:

    [itex]E^2 = p^2c^2+m^2c^4[/itex]

    As you can see, when the body is at rest (i.e. the momentum, p, is zero) it reduces into the familiar E=mc2.

    This equation can be applied to massless particles as well. Setting m=0 gives us:

    [itex]E=pc[/itex]

    So a photon has energy which is proportional to its momentum.
     
  5. Jan 2, 2012 #4

    Dale

    Staff: Mentor

    Re: E=mc^2

    Please see the FAQ:
    https://www.physicsforums.com/showthread.php?t=511175 [Broken]
     
    Last edited by a moderator: May 5, 2017
  6. Jan 3, 2012 #5
    Re: E=mc^2

    e=pc...you said for calculating energy of mass less particle.
    but p=mv......so this too will eventually be 0.
     
  7. Jan 3, 2012 #6

    russ_watters

    User Avatar

    Staff: Mentor

    Re: E=mc^2

    If a fire burns at 10,000 watts, why do I get burned instead of shocked when I put my hand in it?
     
  8. Jan 3, 2012 #7
    Re: E=mc^2

    i dont know
     
  9. Jan 3, 2012 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Re: E=mc^2

    p = mv only holds for particles moving at speeds much less than the speed of light; for massive particles at high speeds you must use the relativistic momentum expression.

    The energy of a photon is given by E = hf. You can relate energy and momentum for a photon using E = pc.
     
  10. Jan 3, 2012 #9

    Dale

    Staff: Mentor

    Re: E=mc^2

    Huh? I know you are trying to make a point that is relevant to the thread. And based on history I probably agree with it, but I have no clue what you are getting at.
     
  11. Jan 6, 2012 #10

    jedishrfu

    Staff: Mentor

    Re: E=mc^2

    That happened to me when I was 5. I plugged in a cutoff extension cord my father had just removed from an old lamp and ran my hand down the cord. Zap, instant burn. My mother using the remedy of the day rubbed butter on the burn which we know nowadays not to do. The shock convinced me to study math and physics go figure.
     
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