# Homework Help: Light as a particle/ compton effect, conservation of energy and momentum

1. Dec 5, 2012

### skg94

1. The problem statement, all variables and given/known data

1. A photon with an initial energy of 33eV collides with an electron. If the speed of the electron after the collision is 4.56*10^5m/s the final energy of the photon is? [32.4eV]

2. Photons that compose a monochromatic beam of light each have a momentum of 1.50*10^-28 kgm/s. The period of this light [1.47*10^-14s]

3. an xray photon of wavelength .010nm strikes a helium nucleus (alpha particle) and bounces straight back. If the helium nucleus was originally at rest, calculate its velocity after interacting with the x-ray [2*104m/s]

4. A photon moving in an easterly direction with an energy of 6.21 *10^-15 J collides with an electron and scatters at an angle of 30 W of S. What is the velocity of the electron after the collision? [3.74 *10^7m/s [28 N of E]]

5. Calculate the frequency of an electron that has a speed of 9.2*10^5m/s [1.16*10^15hz]
2. Relevant equations

e=hf
E=1/2mv^2
conservation of momentum/energy
p=mv or h/lambda

3. The attempt at a solution

1.Energy of photon before = energy of electron after + energy of photon after.
energy of photon after = energy of photon before - energy of electron after

Ea = 33eV - (1/2 (9.11*10^-31) (4.56*10^5)^2)

i get 33eV seeing as how the energy of the electron is 9.47*10^-20 very small i can see why, what did i do wrong? this is my attempt maybe i did the whole thing wrong?

2. I have no idea how to relate momentum of a photon (p=h/lambda) to time, no velocity some help here?

3. Is this conservation of energy? I tried hf=1/2mv^2 , but it didnt make sense and didnt work when you solve for v.

4. Am i not missing something in this question for conservation of energy i believe i dont have enough information, or at least i cant get it.

5. lambda = h/mv
lambda =(6.63*10^-34)/(9.11*10^-31)(9.2*10^5) =7.910 *10^-10 m

f = c/lambda = 3.79*10^17

i dont where else to get frequncy.. cant use Ek=hf

Last edited: Dec 6, 2012
2. Dec 6, 2012

### skg94

solved 1 and 2.