Electric field between oppositely charged plates

Electric field between oppositely charged plates.

The behavior of charged particles within the uniform electric field between charged plates is a favorite MCAT theme. One of the favorite types of questions derives from traditional projectile motion problems, where instead of the typical uniform gravitational field, we have a uniform electric field. The similarities and differences with traditional projectile motion leads to interesting questions that assess conceptual insight.

Like projectile motion, the motion of a charged particle through a uniform electric field, such as between the plates of a capacitor, is characterized by a uniform acceleration parallel to direction of the field.

With gravity, however, the acceleration is the same for any projectile, as long as there is no air friction. This differs from the situation within a uniform electric field, where the acceleration produced on a test particle depends on its charge to mass ratio. For this reason, questions can often occur where like charged particles are launched into the field with the same initial velocity, but because the particles have different mass, their motion is not identical. Unlike charges may be launched. The magnitude of acceleration increases with charge but decreases with mass (inertia), unlike the situation with projectile motion where the acceleration is the same for any projectile.