# J. J. Thomson's experiment

#### Samanko

Homework Statement
The statement is below.
Homework 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?

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#### PeroK

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