Downwards-when a drop is held motionless between the plates, there are two forces acting on the dropl the force of gavity acting downwards, which equals mg; and an equal but opposite force acting upwards, which is due to the electric field. the force due to the electric field is given by f=qe, where f is the force, q is the total charge on the drop, and e is the electric field. both the force f and the electric field e are vector quantities; in other words, they have both magnitude and direction. therefore, when we use this equation, we keep the sign of the charge. so for a negative charge, the force f is in the opposite direction to the electric field. since the droplet is stationary, the force due to the electric field must be acting upwards to counteract the force due to gravity. therefore, the electric field must be in the opposite direction, which is downwards, and choice b is correct.(kaplan,135)
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