Magnetic field, energy, and mass

[physgrl]

Homework Statement

A 4.0 µC particle with a kinetic energy of 0.50 J enters a uniform magnetic field of magnitude 0.20 T. In the field, the particle moves in a circular path of radius 2.0 m. Find the mass of the particle.

a. 2.2*10-12kg

*b. 5.12*10-12kg

c. 6.1*10-12kg

d. 4.7*10-12kg

e. 10.2*10-12kg

Homework Equations

E=1/2mv^2
r=mv/qB

The Attempt at a Solution

.5=1/2mv^2
1/m=v^2
v=1/√m

r=mv/qB
qBr=m/√m (substituting from above)
qBr=√m
(qBr)^2=m
m=(4x10^-6C*2m*.2T)^2
m=2.56x10^-12

The answer key says it it 5.12x10^-12...what am I doing wrong?

 

[gneill]

Your calculation and result look okay. Could be the answer key is wrong.

 

[physgrl]

Thanks!