Mass spectrometer problem -- Need help

[Ryan Gibbons]

Homework Statement

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) ?

Homework Equations

Fm=Fe
BQV=MV^2/R
VQ=MV^2/2

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

 

[ehild]

Homework Statement

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) ?

Homework Equations

Fm=Fe
BQV=MV^2/R
VQ=MV^2/2

The Attempt at a Solution

VQ=MV^2/2
V=2Q/M= 12030075.19 m/s

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

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

 

[Ryan Gibbons]

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

 

[ehild]

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

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