Is light just the kinetic energy of photons?

Question

What other "types" of energy are actually kinetic energy?

"Anything with mass that is moving has kinetic energy."

Then doesn't the relativity of velocity extend to kinetic energy? Velocity is meaningless without a reference point, and for that matter, mass is too, isn't it? I guess I just don't understand the "rest" versions of physical quantities in the first place.

Answer 1

A very good question.

Kinetic energy is simply inertial mass in motion. This holds true no matter what physics models we use: quantum, relativistic, classic. In fact, deriving the classic 1/2 mv^2 from E = mc^2 is a relatively easy thing to do if you know how to do the so-called Taylor expansion of the Lorentz Transform. [See source.]

Anyway, the point is this, to have kinetic energy, we must have both mass and motion. Light certainly has the motion; we don't get any faster than v = c light speed.

But light has no mass (m) in the classic sense of inertial mass. The classic equation for kinetic energy is KE = 1/2 mv^2; as m = 0 for light, it would appear it has no kinetic energy. [See source.]

It can also be shown that KE = (m' - m)c^2; where m' = m/sqrt(1 - (v/c)^2), m is rest mass, and the denominator is the aforementioned Lorentz Transform. In other words, the difference in E = m'c^2 and E = mc^2 is the kinetic energy it took to get the mass m up to speed. But still there is inertial mass; so this again does not apply to light.

But experiments show that light does have energy E = hf; where h is Planck's constant and f is the light frequency. And similar experiments show light also has momentum p = E/c = hf/c. We also know that Lf = c; where L is the wavelength of light. [This is why we can say, for example, 300 Mhz or 1 meter when tuning a radio. They mean the same thing.]

Thus we also have p = h/L = mc; where m is a virtual mass...not a real, inertial mass. Then E/c = mc and, ta da, E = mc^2. But, this is a virtual mass and not a real, inertial mass. And with virtual mass KE = m'c^2 - mc^2 = 0 because m' = m in that the virtual mass does not get larger with increasing velocity v. So once again, we have the answer, light energy is not kinetic energy.

As to the last part of your question, kinetic energy is kinetic energy. Any inertial mass that moves has kinetic energy. Moving atoms have kinetic energy, moving baseballs have it, too. When you walk through the mall, you have kinetic energy. And when a meteor crashes into Earth it has good old KE as well. But, and this is a big BUT, these are not types of kinetic energy, they are just objects of various kinds with KE.

There are indeed other kinds of energy, but not other kinds of kinetic energy. There is of course potential energy. Which is just a name we give to something having energy that could become other kinds of energy. For example, PE = mgh = 1/2 mv^2 = KE; where h is the height something of mass m is dropped. At h, the object with mass m has the potential to become KE just before impact at h = 0.

There is also work energy WE, which is just work done on or by an object with mass m. For example, if PE > KE in the example above, then PE - KE = WE would be true. That is, the object did some sort of work as it fell, which is why the KE did not come up to full potential. Drag force acting over the fall of h is an example of that kind of work energy.

Answer 2

Kinetic Energy Of A Photon

Answer 3

That is a simplified way of looking at it. But keep in mind that photons have no mass and kinetic energy is defined as 1 half the mass times the velocity squared of a particle. But because photons are moving at the speed of light, they do not "need" any mass.The energy of a photon can be thought of the energy stored in the electromagnetic field of the wave.
Anything with mass that is moving has kinetic energy.

Answer 4

The way it's usually defined, "kinetic energy" apply only to bodies which have a nonzero rest mass. Since photons don't fall into this category, you can't properly call their energy "kinetic." Clearly though, photons do have energy (in the sense that they are able to perform work). I would characterize that as electromagnetic energy rather than kinetic energy.

Answer 5

Relativistic dynamics tells us that energy E rest energy (0) and kinetic energy T are related by
E=E(0)+T
The photon is characterized by zero rest energy (E(0)=0) and so its energy is kinetic
E=T.
In the case of free (noninteracting) particles twe say that they have kinetic energy is E>R(0)

Answer 6

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KE = hf - hfo f =c / lambda given: hfo =6.03 X 10-19 J KE=hf-6.03 X 10-19 J f=3E8/239E-9 f=1.25E15 Hz KE = (6.626 E-34)(1.25 E15)-(6.03 E-19 J) 2.25E-19 J=kinetic energy of photoelectron

Answer 7

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