How Does a Charged Particle Move in a Uniform Electric Field?

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Combining this with Newton's second law, $F=ma$, gives

Because $E$, $q$, and $m$ are constant, the acceleration is constant. The sign of $q$ determines its direction, while the magnitude is $|a|=|q|E/m$.

For example, a proton in a field of $2.0\times10^4\ \text{N C}^{-1}$ has acceleration

An electron accelerates in the opposite direction and, because its mass is much smaller, has a much greater acceleration in the same field.

A common misconception is that negative charges move in the field direction. They do not necessarily do so: their acceleration is opposite to the field, although their instantaneous velocity can initially point in any direction.

Exam Technique

For a calculation, first state $F=qE$, then apply $F=ma$. In trajectory questions, resolve the motion into independent components, just as for projectile motion, and clearly distinguish the directions of velocity, force, and acceleration.