# An electron is moving along positive $x$-axis. A uniform electric field exists along negative $y$-axis. What should be the direction of the magnetic field (constant) so that the force on the electron is zero?

$\begin {array} {1 1} (a)\;Positive \: y-axis & \quad (b)\;Negative\: y-axis \\ (c)\;Positive \: z-axis & \quad (d)\;Negative \: z-axis \end {array}$

The direction of the magnetic force must be opposite to the direction of the electric force.
$-\hat k \times \hat i = -\hat j$ that is if the magnetic field is in the negetive Z axis it will produce a force in th enegetive Y axis which opposes the force due to the electric field
Ans : (d)

edited Sep 11, 2014