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= So,
v= is the condition for which the particle moves undeflected in its original trajectory.