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Name: Nikky
Status: student
Grade: 9-12
Location: NY
Country: N/A
Date: 4/18/2005


Question:
Does gamma radiation have kinetic energy?


Replies:
Yes, it does. In fact, since photons have zero rest mass, all their energy is kinetic. In fact, if you could slow them every so little from the speed of light, they would cease to exist, since they would have zero energy. Relativity is very strange, isn't it? A photon is a quantum (a little piece) of light.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


I would call that a yes.

Gamma radiation is photons. Photons are all energy, and they are moving. They have mass, velocity, and momentum, just like anything with kinetic energy. When they strike some small particle, that particle does get bumped in the direction of the gamma-ray's motion.

Suppose their energy is referred to as "E", and their speed "v" is the speed of light "c":
     v=c.

By E=mc2, photons have mass "m":
    m = E/c2.

They have momentum "M":
    M =  m  *  v
        =(E/c2)*c
        = E*c.

The classical definition of kinetic energy "Ke":
    Ke= (1/2) * m * v2
        = 1/2 * (E/c2) * c2
        = E/2

So I think it is fair to say that half the total energy of any gamma ray is kinetic energy. The other half might be considered the energy it takes to build the electric and magnetic fields in the photon's wave, but I have not thought through what it takes to evaluate that, to see whether it comes to E or E/2. This kind of intuitive. classical thinking may break down for something as fundamental and non-classical as a photon.

Jim Swenson



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