An electric charge in a uniform external electric field will feel a force due to that field given by, F = qE, where F is the force, q is the charge and E is the external electric field (i.e. the charge is not affected by its own electric field.). This force points in the direction of the field if the charge is positive and in the opposite direction if negative.

One of the most famous experiments using this force is the Millikan oil-drop experiment. A microscopic drop of oil was placed in an electric field between two plates. From the motion of the drop, the value of q was determined. Millikan discovered this value was always some integer times a constant, e. E was later determined to be the elementary charge of an electron. This experiment proved that charge was quantized and...