Bainbridge mass spectrometer

In the Bainbridge mass spectrometer (see Figure), the magnetic-field magnitude in the velocity selector is 0.650 T, and ions having a speed of 1.82 x 10^6 m/s pass through undeflected.

If the separation of the plates is 5.20 mm, what is the potential difference between plates P and P '?

http://www.physics.sjsu.edu/becker/physics51/images/28_20_Mass_spectrometer.jpg

For an ion of charge q to be undeflected, the magnitude of the magnetic force (=Bqv) must equal the magnitude of the electric force (=qE) (and the 2 forces must have opposite directions).

qE = Bqv
E = Bv

E = 0.650 x 1.82 x 10^6 = 1.183x10^5 V/m

For a uniform electric field E = V/d so V = Ed
V = 1.183x10^5 x 0.00520
= 615V