Condition(i)For directions of $\overline{E}, \overline{B}, \vec{v}$ (ii)For magnitudes of $\vec{E}, \vec{B}, \vec{v}$ (i)The velocity $\vec{v}$ , of the charged particles, and the $\vec{E}$ and $\vec{B}$ vectors, should be mutually perpendicular. Also the forces on $q$ , due to $\vec{E}$ and $\vec{B}$, must be oppositely directed. (Also accept if the student draws a diagram to show the directions.) (ii) $qE = qvB$ $\text{ or } v = \frac{E}{B}$ [Alternatively, The student may write : Force due to electric field $= q \vec{E}$ Force due to magnetic field $= q(\vec{v} \times \vec{B})$ The required condition is $q \vec{E} = -q(\vec{v} \times \vec{B})$ $\text{ [or } \vec{E} = -(\vec{v} \times \vec{B}) = (\vec{B} \times \vec{v})$