What is the equation for finding the mass of an ion using a mass spectrometer?

[shimizua]

Homework Statement

A mass spectrometer applies a voltage of 2.00 kilovolts to accelerate a singly charged ion (+e). This ion then enters a region of uniform magnetic field (B= 0.400 T) which then bends the ion into a circular path of radius 0.305 m What is the mass of the ion?

Homework Equations

after breaking everything down from F=ma i got to m = (qB^2r^2)/2V

The Attempt at a Solution

well i am just unsure of how to find q. I know that V=2kV, B=.4 T, and r=.305 m. so if you can just help me with finding q. i can do that rest

 

[LowlyPion]

Isn't q given as +e?

http://en.wikipedia.org/wiki/Elementary_charge

 

[shimizua]

ok well so i tried doing the m= -1.602x10^-19(.4^2)(.305^2)/4 and got -5.96x10^-22. would it be in kg though?

 

[LowlyPion]

ok well so i tried doing the m= -1.602x10^-19(.4^2)(.305^2)/4 and got -5.96x10^-22. would it be in kg though?

To put it in perspective for a single proton:

1.672621637(83)×10−27 kg

 

[drastichawk]

The conundrum you got going here is that the charge is +e, not e. SOO...in taking your big step in finally realizing that q is given as the charge +e, you can now go figure that the charge is POSITIVE (1.6*10^-19 C), not negative.