# Homework Help: Radius of curvature R of alpha particle beam

1. May 5, 2014

### asdf12312

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

2. Relevant equations
r=mv/qb
mv=sqrt(2*KE*m(alpha))
m(alpha)=6.64e-27 kg

3. The attempt at a solution
i was just wondering how to get the answer (7.6e-4 m). i get path curving down, and do
r=sqrt(2*1e3eV*6.64e-27kg*1.6e-19J/eV)/(q*B)
=15.18e-4m

so to get the right answer i would have to divide this by 2..why is this?? i used the right equation but am I doing something wrong?

2. May 5, 2014

### TSny

What did you use for the charge of the alpha particle?

3. May 5, 2014

### asdf12312

charge of alpha particle is +2. so if its only alpha particle I have to divide by 2? if it was electron (-1) I would not need to divide by anything??

4. May 5, 2014

### TSny

You didn't show what number you used for q. It should be in Coulombs. What is the charge in Coulombs of an alpha particle?

5. May 5, 2014

### asdf12312

I'm guessing its +2 times charge of electron. q=1.6e-19C

6. May 6, 2014

### TSny

Right. An alpha particle has a charge q of twice the magnitude of the charge of an electron.

But, the magnitude of the charge on an electron is 1.6 x 10-19 C.

So, if q is 2 times the magnitude of the charge on an electron, then q ≠ 1.6 x 10-19 C

What value did you use for q when you got the wrong result of 15.18 x 10-4 m?

7. May 6, 2014

### asdf12312

I was using q=1.6e-19C for electron. I guess it should've been 3.2e-19C but I forgot to multiply by the +2 charge. thanks.

8. May 6, 2014

### TSny

Good. When using r = mv/qB, q is the charge of the particle that is moving in the magnetic field.