Is the m/e Equation Valid When F = 0 in J. J. Thomson's Experiment?

[Samanko]

Homework Statement

The statement is below.

Relevant Equations

F = qv × B

The text showed if Fnet = 0 (with F from magnetic fields and electric field), then v = E/B . And we can measure m/e (last equation in the image).

My question is, by assuming F = 0, change in y should also be 0 , so the denominator of right side of the m/e equation must be zero. And the equation doesn't hold. Am I wrong?

 

[PeroK]

My question is, by assuming F = 0, change in y should also be 0 , so the denominator of right side of the m/e equation must be zero. And the equation doesn't hold. Am I wrong?

That's $y$ from the first case with $E$ only.