# What is the force exerted on an electron if it absorbs a photon with frequency v

1. Dec 22, 2006

### az2212

if an electron was moving with velocity V1 then it absorbed a photon with frequency v to move with velocity V2 thus, what is the force exerted on that electron ? what is the formula that describes the force as a function of frequency?

2. Dec 22, 2006

### Andrew Mason

You would have to know the time that it took for the photon to be absorbed. $\Delta p = F\Delta t$.

I am not sure how an electron can absorb a photon by itself. But lets assume that it can.

Assume the electron is a point charge. It would completely absorb the photon in the time it takes for the photon to pass it (keep in mind that the photon is always traveling at speed c relative to the electron, regardless of the electron's speed). So, how big is the photon? And how long does it take the photon to pass a point in space?

AM

3. Dec 23, 2006

### az2212

is the a physical formula describes the force as a function of frequecy

Is there a physical formula describes the force as a function of frequency
HOW can we describe the force as a function of frequency????

Last edited by a moderator: Dec 23, 2006
4. Dec 23, 2006

### cristo

Staff Emeritus
What branch of Physics are you dealing with? If you give more details about the specific question, then you are more likely to receive an answer.

5. Dec 23, 2006

### az2212

I mean the quantized force. for example, if an electron absorbed a photon with frequency v, then its velocity will change, thus what the force exerted on this elecron. I want a formula describe that, gives a force as a function of frequency.

6. Dec 23, 2006

### az2212

how can we compute the time required to the electron to absorb this photon? since according heiseberg uncertainty principle it impossible measuring the energy and time as the same time. where the photon is an energy.

7. Dec 23, 2006

### tim_lou

I wonder how one would set up the Schrödinger's equation for the interaction between an electron and a photon....

8. Dec 24, 2006

### inha

You just write down the Hamiltonian so that it includes the electromagnetic interaction.

Andrew: A free electron can not absorb a photon. An electron bound to an atom or a band electron in a solid for example can. Even then the electron does not absorb the photon but the whole atom or solid does.

9. Dec 24, 2006

### az2212

how the photon can not absorb the photon ....in the photoelectric the electron absorb a photon with frequency v to move in a kinetic energy E where E=hv-hv0

10. Dec 24, 2006

### ZapperZ

Staff Emeritus
You need to study things more carefully than this.

In a photoelectric effect, the METAL (or the cathode material) is the one that absorbs the photon. The presence of the lattice ions is necessary not only for the energy BANDS that allow for such a transition, but also to absorb and provide the RECOIL momentum. A free electron sitting there cannot do this, and that's why you do not get a "photoelectric effect" from a bunch of free electron gas.

Zz.

11. Dec 24, 2006

### az2212

thank you very much Mr ZapperZ
But how can we describe the fore as a function of frequency?
or there is no formula and there is no force in quantum

12. Dec 24, 2006

### ZapperZ

Staff Emeritus
As people here have mentioned, your question is undefined!

A "force" is a generic term for ANY action. You can't just ask something like that and expect to have a useful answer. What force are you refering to? If we are talking about the force that causes an apple to fall, then there is no "frequency".

Secondly, the term "force" is really quite secondary once you get into more complicated system. It appears that you have never encounter the Lagrangian/Hamiltonian mechanics, something that all physics majors in college have to go through and understand. Here, there is no "force" that is necessary to describe completely the dynamics of a system. This concept is carried through into quantum mechanics. That is why when we try to solve the description of a system, we write down the Hamiltonian/Schrodinger Equation. We do not deal with "force".

So if you're trying to connect "force" with the energy carried by light that has a connection to frequency, then this is a rather dubious way of trying to understand something. That is why you are not getting any answer to your question.

Zz.

13. Dec 25, 2006

### az2212

Dear Zapperz
Thank you very much for your answer.... I really trying to connect the energy carried by light that has connection to frequency. in fact i reached to a formula as: F=Hv/V,, whereas F is force, H is constant and equals to 6.6*10^-34 joule. like the value of plank's constant but defferent in dimension, and v is the frequency, and V is the velocity equivalent to the energy frequency. This formula gives us the relativistic quantized force and it can be applied for micro and macro. i hope to send me your openion.

14. Dec 25, 2006

### ZapperZ

Staff Emeritus
My opinion here is that (i) you haven't learn much physics yet and (ii) you obviously missed the "overly speculative" rule in our Guidelines, even when you were warned about it earlier.

Zz.