A mass thrown into the air follows a parabolic path if the only thing acting on it is gravity. Applying Newton's Second Law tells us that mg = ma, so a = g.

Similarly, a charge thrown into a uniform electric field follows a parabolic path. If gravity can be neglected applying Newton's second law gives:

qE = ma,

so

a =

q E m

All the constant acceleration equations from first semester still apply.

Is it valid to neglect gravity? If qE / m is much larger than g, gravity can be ignored. Gravity is easy to account for, of course : simply add mg to the free-body diagram and go from there.

One big difference between gravity and electricity is that m, the mass, is always positive, while q, the charge, can be positive, zero, or negative.