# Pleas PLease help me

1. Jan 25, 2005

### stunner5000pt

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 travelling in the +X direction with beam diameter of 0.01m is abosorbed by the atoms travelling 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

$$M_{Li} V_{Li} = Np_{Photon}$$ but his yeidls a non integer

so is this wrong or is that the lithium atom cannot abosrb a photn of this wavelength??

2. Jan 25, 2005

### Gokul43201

Staff Emeritus
Could you write down the question EXACTLY as it appears in your homework ? It looks like there's some data missing.

3. Jan 26, 2005

### stunner5000pt

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?

4. Jan 26, 2005

### stunner5000pt

5. Jan 26, 2005

### Gokul43201

Staff Emeritus
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, $\mathbf {E} \times \mathbf {H}$ ?

Last edited: Jan 26, 2005
6. Jan 26, 2005

### stunner5000pt

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 travelling in the +X direction with beam diameter of 0.01m is abosorbed by the atoms travelling 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?

7. Jan 26, 2005

### stunner5000pt

you can ignore the Penning trap one i got it just help me with this and the other one

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