# Magnetic fields ahhh!

1. Feb 26, 2004

### eku_girl83

Here's my problem:

A particle with charge 4.5E-19 C travels in a circular orbit with radius .465 m due to the force exerted on it by a magnetic field with magnitude 1.7 T and perpendicular to the orbit.

a) What is the magnitude of the momentum p of the particle?
I used the equation R=(mv)/(qB) and calculated mv (linear momentum) to be 3.6E-19 kg*m/s.

b) What is the magnitude of the angular momentum L of the particle?
Herein lies the rub! Angular momentum = v/R = qB/m
I don't know velocity or mass independently, only their product (mv). So how do I calculate angular momentum?

Thanks for any help!

2. Feb 26, 2004

### gnome

angular speed: &omega; = v/r

angular momentum: L = r x p

Since your particle is moving in a circle, it's linear momentum vector p is always perpendicular to the position vector (radius) r so the angular momentum is simply the product:
L = rp = rmv

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