# Homework Help: Stuck on a few magnetic field questions

1. Feb 26, 2006

### NutriGrainKiller

Problem 1:
here is what i did:

knew to find the acceleration I needed to use a=QE/M, but since I didn't have E I had to find it. Used F(mag field)=Q*V*B*Sin(theta). I found theta from drawing the ijz axis and using trig, I got 59 degrees.

After I solved this I plugged it in to F=QE to get E. Then plugged that into a=QE/M, and got an incorrect answer..not sure where I went wrong.

Problem 2:
for a:
equations I used:

P=MV
R=MV/QB -> V= RQB/M

plugged 2nd into 1st, M cancelled, answer wasn't correct. eh?

for b:

not sure i know the equation for this problem...same as in classical physics?

thanks guys!

2. Feb 26, 2006

### topsquark

1) There's nothing wrong with your first problem except that you seem to be insisting there's an electric field. There isn't. The Lorentz force says:
$$\mathbf{F}=q\mathbf{E}+q\mathbf{v}X\mathbf{B}$$. Since there is no E, the first term drops out. Then you calculated F, as you did. Now simply apply F=ma.

2) Part a) looks right, I'm not sure why things aren't working. What answer did you get and what does your book say? For part b) I would assume that you aren't being relativistic or quantum mechanical, so yes use the Classical formula.

-Dan

Last edited: Feb 26, 2006
3. Feb 26, 2006

### NutriGrainKiller

thanks, figured the first one out!

for 2a i got 4.94×10−18