# Homework Help: Electromagnetism-Radius of Curvature-Question

1. Jun 14, 2015

### RoyalFlush100

1. The problem statement, all variables and given/known data
http://postimg.org/image/gdfsdglpt/

v=2.2E5 m/s
l=0.16 m
I=5.3 A
N=820
mp=1.67E-27 kg
B=3.4E-2 T

Looking for radius of curvature in the solenoid with the above variables.

2. Relevant equations
BIl=F

3. The attempt at a solution
BIl=mv^2/r
3.4E-2*5.3*0.16=1.67E-27*(2.2E5)^2/r
r=2.8E-15 m

The correct answer however, is 6.8E-2 m, so what did I do wrong?

2. Jun 14, 2015

### haruspex

Think about what I and L refer to in BIL=F. You already used the length of the solenoid and the current in it to find the field. Does it make sense to be using those again to find the force?

3. Jun 14, 2015

### RoyalFlush100

Fm=Fc
Bqv=mv^2/r
r=mv/Bq
r=(1.67E-27*2.2E5)/(3.4E-2*1.6*10^-19)
r=6.8E-2 m

Thanks. Why didn't the other formula for force work though?

4. Jun 14, 2015

### haruspex

What you calculated previously seems to be the solenoid exerting a force on another like itself. This is the danger with learning formulae but not exactly what the variables in the formula represent. It's not enough that I is a current, it matters how that current relates to the magnetic field, etc.