# Mangnetic Force

1. Sep 12, 2009

### howt

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

An alpha particle is moving at constant speed in a circle, perpendicular to a uniform magnetic field, making a revolution every 60 nanoseconds.
Its speed is non-relativistic

What is the numerical magnitude of the magnetic field in T ?

2. Relevant equations
alpha particle (2 protons + 2 neutrons)

We have here T= 2(pi)m / qB

in which
m=Mass (alpha particle= 6.64*10^-27) kg
q=Charge = 2(1.6*10^-19) C
T= (60*10^-9) s

3. The attempt at a solution

This should be just plugging in, I got 2.1729, yet it is wrong...
Why?

2. Sep 12, 2009

### rl.bhat

What is the radius of the circle?

3. Sep 12, 2009

### howt

it is not given....

I dont think we need it to solve the question
The formula of it doesn't involve any radius....

4. Sep 12, 2009

### rl.bhat

When the alpha particle is moving ina circle, what is the force acting on it?
What is the force on alpha particle due to the magnetic field?

5. Sep 12, 2009

### howt

The Centripetal force is going to equal the magnetic force.

Note that w = v/r

Centripetal force , F=m v^2 /r..
=m r w^2

And the magnetic force on the alpha particle is, F = B q v
= B q w r

m r w^2 = B q w r
m w = B q

in which w= 2(pi) / T

Thus T= 2(pi)m / qB

6. Sep 12, 2009