# Homework Help: Mass of charged particle in magnetic field

1. Apr 6, 2009

### rinarez7

1. A 1.56024 μC charged particle with a kinetic
energy of 0.115905 J is placed in a uniform
magnetic field of magnitude 0.150267 T.
If the particle moves in a circular path of
units of kg.

2. Relevant equations
Fe= Fm
Fe= qE
KE=mv^2/2
Fm= qvB= mv^2/r

3. I decided to first find E: E =Ke (q/r^2)= 8.98e10 (1.5602e-6 C/3.13685^2)=1425.455

Then I solved for Fe= qE= 1425.455 (1.5602e-6)= 2.2239e-3= Fm

Then I used Fm= qvB and solved for v = 9503.84615 m/s
Then I used KE = mv^2/2 to sove for m = KE(2)/ v^2= (0.115905)(2)/ (9503.84615 m/s )^2=3e-9 kg=m
But this isn't correct. What am I missing here? Thank you in advance for any help!

2. Apr 6, 2009

### Nabeshin

Your work is a little tough to follow, but it is worth noting that this problem has nothing to do with electric fields, only magnetic ones. So, I'll help you out by saying that only equations 3 and 4 under your list of relevant equations are actually relevant.

3. Apr 6, 2009

### rinarez7

So would I be able to solve eqautions 3 and 4 somehow to find mass and velocity? Those are my two unknowns. I need velocity to solve for mass.
Is there another relationship I am missing to help me solve for velocity?

4. Apr 6, 2009

### Nabeshin

Well you have equations 3 and 4, and only two unknowns like you say, m and v. 2 equations 2 unknowns sounds like a solvable system to me. (Try substitution!)

5. Apr 6, 2009

### rinarez7

Substitution worked wonderfully! Thanks again!