# Homework Help: What am I doing wrong?

1. Dec 6, 2012

### Schmetterling

Hello! I'm solving problems which come with their answers... but I get different results! Can you help me, please?
The problem statement and equations are in the attached images.

1. The problem statement, all variables and given/known data
(Image)
Also:
U=energy in Joules
β=v/c≈1 (relativ. particles)
μ0=4π×10-7
2. Relevant equations
(Image)
uB=B2/(2μ0), B=magnetic field in Teslas

3. The attempt at a solution
(a) Only substitute the values. I obtain -4.05×10-16 W= -2531 eV/s.

(b) $\frac{(dU/dt)proton}{(dU/dt)electron}$, only the term γ2proton2electron remains and it is equal to (mc2)2proton/(mc2)2electron=2.96×10-7!
And it yields 1.86×10-8 for α particles. :-s

(c) Like process in (b), but equating instead of dividing. This gives γproelect, and Uprot/Uelect= (mc2)prot/(mc2)elect = 1.84×103...

I have spent several hours in problem solving, now I'm tired and sleepy and can't see where is the mistakes(s), please, help!

File size:
3.1 KB
Views:
138
File size:
8.8 KB
Views:
135
2. Dec 6, 2012

### Staff: Mentor

If you use the classical electron radius re for electrons, you should substitute this for other particles. This should give two additional factors of $\frac{m_e}{m_p}$.

3. Dec 6, 2012

### Schmetterling

Ohhh!!! You're right! I didn't consider that, only took the given value for σT...
Thanks!!!!!!!!

4. Dec 6, 2012

### Schmetterling

Wait... How to obtain m_p from r_p (or viceversa) if don't know density?
I found already values for r_p and r_α, ~10^-16,-15 m, but I would like how to deduce them. Substitute p & α parameters into expression for r_e gives me radius ~10^-37!

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

### Staff: Mentor

The classical radius (as calculated with the electromagnetic field energy) is proportional to the inverse mass.

6. Dec 6, 2012

### Schmetterling

Thanks, I have the value of class. e- radius, r_e= 2.818e-15, rather I need right values for radii of proton and alpha particles, because those I found are... odd:
r_p=2.1e-16
r_α=2.1e-15
How can be possible r_p<r_e?!
Another founded value for r_p is 8.6e-16, and r_alpha =4r_p...

I've tried to calculate them from formulae in http://en.wikipedia.org/wiki/Thomson_scattering and http://en.wikipedia.org/wiki/Classical_electron_radius:

http://upload.wikimedia.org/math/f/1/9/f196c5b29b4e0fed8f4710bedc522016.png & http://upload.wikimedia.org/math/5/3/d/53dc80a0e75009917114d545c93fc0a4.png=mc^2 without the factor 1/2, as it is said there... and I got 4.51E-034 for r_e & 8.72E-061 for sigma_T! :'(

I am increasingly confused!:( HEEEEEEELP!!!!

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

### Schmetterling

Oh, I have it!, I have it! other factors are eliminated leaving only masses quotint^4!