WBJEE 2019 Physics Solved Paper

According to the question,
Charge on particle is =q
Velocity of particle =v
Due to uniform electric field.
electric force on particle
F‌electric ‌=qE....(i)
Due to a uniform magnetic field,
magnetic force on particle is given by,
F‌magnetic ‌=q(v×B)...(ii)
When v is perpendicular to E and B, which are mutually perpendicular to each other,
F‌electric ‌=F‌magnetic ‌
From Eqs. (i) and (ii)
qE=qvB
∴‌v=‌

E

B

So, v=

E

B

is the condition for which the particle moves undeflected in its original trajectory.