Problem 3

What must the charge (sign and magnitude) of a particle of mass 5 g be for it to remain stationary when placed in a downward-directed electric field of magnitude 800 N/C?

Solution

We have equilibrium. It means that the net force is 0. Then the gravitational force should be equal to electric force:

From this equation we can find the charge of the partice

The direction of electric force should be upward (since the gravitational force has downward direction). Since the direction of electric field is downward then the electric charge should be negative: