Mass spectrometer problem -- Need help

1. Jul 27, 2016

Ryan Gibbons

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

A mass spectrometer is constructed using a velocity selector constructed using a parallel plate capacacitor in a uniform magnetic field Bin. The electric field in the selector is 1003.9 V/m and the magnetic field in that portion of the spectrometer is Bin = 0.998 T. The balanced forces only allow specific velocities through the selector.

http://misterfig.org/pluginfile.php/1936/question/questiontext/8281/18/123907/spect.jpg
The mass of the ions can be found by measuring the radius of the motion when the ions strike point P.
The magnetic field in the circular motion chamber is Bout = 0.807 T.
What is the radius (in millimeters) for an ion of mass 2.66 x 10-26 kg and a charge of +e (1.60 x 10-19 C) ?

2. Relevant equations
Fm=Fe
BQV=MV^2/R
VQ=MV^2/2

3. The attempt at a solution
VQ=MV^2/2
V=2Q/M= 12030075.19 m/s

BQV=MV^2/R
R=MV/BQ=2.48 m
This is not the correct solution but i cannot figure out how to find the correct one. Please help. Thanks

Last edited by a moderator: Jul 27, 2016
2. Jul 27, 2016

ehild

You apply the wrong formula. What is the electric force on the particle?

3. Jul 28, 2016

Ryan Gibbons

the electric force on the particle is 1.6x10^-19 C

4. Jul 28, 2016

ehild

No, it is the charge of the particle. What force acts on it due to the electric field of the velocity selector?