physics velocity?

Question

A velocity selector for ions uses perpendicular electric and magnetic fields. If the electric field used were 800 V/m, what magnetic field would allow the selection of a velocity of 4000 m/s??

a) 0.040 T
b) 0.080 T
c) 0.16 T
d) 0.20 T
e) 0.50 T

Answer 1

E = 800V/m, v = 4000m/s

electric force = magnetic force
qE = qvB
so
B = E/v = 800/4000 = 0.2T

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Answer 2

Eq=Bqv
B=E/v
B=800/4000
B=0.2 T