# The kinetic energy of proton-electron for a black body

• I
• sinus
In summary, the video explains that when light interacts with the surface of a body, the electrons and protons start oscillating. Due to the electron's smaller mass, it gains more transferred energy from the light, resulting in a larger kinetic energy compared to the proton. The misconception that kinetic energy is proportional to mass is addressed, as it also depends on the velocity. The analogy of a bullet and gun is used to explain the concept further.
sinus
I'm watching a video about " What is a black body?". That video said when the light interacts with the surface of a body, the electron and proton start oscillating. The electrons gain more transferred energy from the light that became its kinetic energy, rather than the proton because its mass are much less massive than protons.

First, I confused because kinetic energy is proportional with mass, the greater the mass the greater the energy, right? Well, proton's mass is 1.836 times larger than the electron, why electron gets more energy from light? My idea is because electron is freely to move rather than the proton that located at the center of the atom (it can't move easily). Is my idea right that even the proton's mass is more more greater, it still can't make its kinetic energy larger than the electron's?

Please tell me if my idea above is right or there's a misconception, thank you all so much.

sinus said:
The electrons gain more transferred energy from the light that became its kinetic energy, rather than the proton because its mass are much less massive than protons.

First, I confused because kinetic energy is proportional with mass, the greater the mass the greater the energy, right?
Kinetic energy is also proportional to velocity squared, the change of which depends on the mass too. You are confusing kinetic energy for a fixed velocity with kinetic energy gained in a collision/interaction.

PeroK, Gleb1964, topsquark and 2 others
A.T. said:
Kinetic energy is also proportional to velocity squared, the change of which depends on the mass too. You are confusing kinetic energy for a fixed velocity with kinetic energy gained in a collision/interaction.
Oh yeaah, I forgot to consider the velocity. Thank you so much :)

sinus said:
First, I confused because kinetic energy is proportional with mass, the greater the mass the greater the energy, right?
That assumes the same speeds!!

The Force on an electron (charge e) in a field E will be eE. Same magnitude for a proton (charge -e). If the field is applied for time t then the changes in momentum will be
eEt/me and eEt/mp. The lighter particle gets two thousand times more momentum. So the ratio of kinetic energies will be 20002. We ignore the p energy.

We're in the same neck of the woods as when a bullet gets most of the energy and the (massive) gun gets only a small amount.

sinus and topsquark
sinus said:
Well, proton's mass is 1.836 times larger than the electron, why electron gets more energy from light?
You meant 1,836 =1.836E3 times I presume. That makes gun/bullet analogy just about perfect

sinus
hutchphd said:
You meant 1,836 =1.836E3 times I presume.
Depends on where in the world you are...

SammyS
Thanks. I have seen the comma (,) used to demarcate the decimal point but not the period (.) for thousands. Very old dog new tricks!

You must NEVER use the comma (in German the point) in this way ever! Even worse are words like "billion" (is it ##10^9## as in the US or ##10^{12}## as in Germany?). We have a concise scientific notation, we should use in the natural sciences, and nothing else!

Like the kilogram to denote one unit of mass...

sophiecentaur, Vanadium 50, hutchphd and 1 other person

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