Suppose you have one or more stationary charged particles on a plane. These stationary particles produce an electric field at all points on the plane. Let us place a moving charged particle on this plane. The moving particle will experience a force exerted by the electric field, causing it to accelerate. Will it move, and if it does, where will it move? I have always struggled to draw a mental picture of how I expect P will behave under the electric field before actually turning to my pencil and paper to do the calculations.