The effect of an electric field (voltage) on a negatively charged oil droplet.

Question

In the Millikan oil droplet experiment, the oil is sprayed from an atomizer into a chamber. The droplets are allowed to pass through the hole into the chamber so that their fall can be observed. The top and bottom of the chamber consist of electrically charged plates. The upper plate is positively charged, and the lower plate is negatively charged. X rays are introduced into the chamber so that when they strike the oil droplets, the droplets will acquire one or more negative charges. The electric field (voltage) is applied to the metal plates.Watch the animation and identify the effects of an electric field on the motion of a negatively charged oil droplet. Consider the gravitational force as Fg and the electric force as Fe. All the other forces acting on the oil droplet can be ignored as their effect on the motion of the oil droplet is negligible.

If Fe is increased until it is equal to Fg, the negatively charged oil droplet will remain stationary.

If Fe is greater than Fg, the negatively charged oil droplet will move freely toward the negatively charged plate.

In the absence of an electric field, the oil droplet falls freely due to the gravitational force.

In the presence of an electric field, the negatively charged oil droplet moves freely toward the negatively charged plate.

Answer 1

I assume that the question here is to select one or more correct answers from the choices.

First let's recall that opposite charges attract and that like charges repel. Since the drops have a negative charge, answers that suggest attraction to the negative electrode must be incorrect. So answers 2 and 4 are clearly not correct.

The problem with the first answer is more subtle. With the electric force and the gravitational force balanced out, the charge will not accelerate. However, the charge may still be moving at a constant velocity without accelerating. So the first answer is not necessarily true, and thus must not be chosen.

In the absence of the electric field, the oil drop will fall under the influence of gravity. That principle is clearly stated in answer 3, so that answer is correct.