Proton Beam therapy (magnetic field)

[tombroz]

Homework Statement

A beam of protons with v=1.8x10^8 are deflected 90 degrees by a constant magnetic field. The trajectory of the beam has a circular arc with a radius of 4m.

I am asked to find the magnitude of the constant magnetic field.

Homework Equations

I know that r=mv/eB. When solving for B the equation is B=mv/er. Right?
m=1.67x10^-27
v=1.8x10⁸
e=1.6x10^-19
r=4

The Attempt at a Solution

This gives me 7.515 for the magnitude of the magnetic field. The correct answer is 0.47T.
Did I start with the wrong equation?

 

[Redbelly98]

Welcome to PF.

You may have entered a number wrong. Using your values I get 0.47T as well.

 

[tombroz]

Ahh... I see.

That was easy. Thanks!