Calculate Momentum of Photon and Number of Photons Needed to Stop Li Atom

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Homework Help Overview

The discussion revolves around calculating the momentum of photons and determining the number of photons required to stop a lithium atom moving at a specific speed. The context involves a laser beam interacting with lithium atoms in a charge exchange production scenario.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to calculate the momentum of a photon using power and energy relationships, and raises questions about the number of photons needed to stop a lithium atom. Some participants question the completeness of the provided data and suggest that additional information may be necessary for clarity.

Discussion Status

Participants are actively discussing the problem, with some providing hints and suggestions for further exploration. There is an acknowledgment of missing data, and multiple interpretations of the problem are being explored without a clear consensus.

Contextual Notes

Constraints include the limited information provided in the homework prompt, specifically regarding the power, area, and assumptions about the absorption of photons by lithium atoms. Participants are also considering the implications of the wavelength of the photons in relation to the lithium atom's ability to absorb them.

stunner5000pt
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Pleas PLease help me!

FDor charge exchange production of postironium a beam of lithium atoms will be directed through a positron cloud by a lasser beam. A 0.01W laser beam traveling in the +X direction with beam diameter of 0.01m is abosorbed by the atoms traveling in the -X direction.

What ithe momentum vector of a photon?

Power = Energy/Time and Energy/ c = momentum of photon

so Pt / c = momentum of photon (YES??)

Over one second the momentum of the photon is 0.01W/ 3e+8 = 3.33 x 10^-11 kg m/s

What is the number of photons required to stop Li atom at 500m/s

Assume all the Li atom will abosrod al the photns of the same wavelength and stop it

[tex]M_{Li} V_{Li} = Np_{Photon}[/tex] but his yeidls a non integer

so is this wrong or is that the lithium atom cannot abosrb a photn of this wavelength??
 
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Could you write down the question EXACTLY as it appears in your homework ? It looks like there's some data missing.
 
Gokul43201 said:
Could you write down the question EXACTLY as it appears in your homework ? It looks like there's some data missing.

The only thing i skipped on in the other sub questions within this question

A) what is the Pynting Vector of the EM wave
B) What are th average ELectric and magnetic field amplitude of the wave
C) Write an expression for the radiation pressure on the atoms as a function of the electric and magnetic fields
D)What tieh momentum vectory of each phton
E)What is the change in momentum of an LI atom when it absorbs 1 photon
F) What is the number of potons required to stop an Li atom from inital thermal speed of 500 m/s?
 
stunner5000pt said:
The only thing i skipped on in the other sub questions within this question

A) what is the Pynting Vector of the EM wave
B) What are th average ELectric and magnetic field amplitude of the wave
C) Write an expression for the radiation pressure on the atoms as a function of the electric and magnetic fields
D)What tieh momentum vectory of each phton
E)What is the change in momentum of an LI atom when it absorbs 1 photon
F) What is the number of potons required to stop an Li atom from inital thermal speed of 500 m/s?

And the only data given is the power and area ? Can you write this part of the question down too, so you may be better helped. I've got to go now, but I'll check back tomorrow.

PS : Quick hint : what are the dimensions of the Poynting vector, [itex]\mathbf {E} \times \mathbf {H}[/itex] ?
 
Last edited:
Let me post exactly what was gienn to me, rmember this is my prof who wrote this and not a textbook

FDor charge exchange production of postironium a beam of lithium atoms will be directed through a positron cloud by a lasser beam. A 0.01W laser beam traveling in the +X direction with beam diameter of 0.01m is abosorbed by the atoms traveling in the -X direction.
A) what is the Pynting Vector of the EM wave
B) What are th average ELectric and magnetic field amplitude of the wave
C) Write an expression for the radiation pressure on the atoms as a function of the electric and magnetic fields
D)What tieh momentum vectory of each phton
E)What is the change in momentum of an LI atom when it absorbs 1 photon
F) What is the number of potons required to stop an Li atom from inital thermal speed of 500 m/s?
 
you can ignore the Penning trap one i got it just help me with this and the other one
 

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