Velocity Selector

[boaz]

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So I've seen this device described in some practice passages but I don't understand how it works.

If the velocity of a particle is not large enough it will be deflected? Why? Don't the magnetic and electric forces balance each other exactly?

 

[boaz]

OK, OK I see. Electric force has nothing to do with velocity, whereas for magnetic force it makes all the difference.
Since QVoB=QE
if V<Vo
then QVB<QE
so the electric force wins!