Fire a charged particle between two charged plates and it does not go straight. The field pushes it sideways the whole time it is in there, exactly the way gravity pushes a thrown ball sideways to its motion. Enter along the field and it speeds up or slows down. Enter across the field and it traces a parabola.
Four situations along the top: Along Field, Across Field, Angled Entry, and With Gravity. Pick a particle — electron, positron, proton, alpha, or a custom one where you set the sign and mass yourself. Set the field strength and the entry speed, and in Angled Entry the entry angle too. Turn on the velocity arrow, the force arrow, or both. Drag the particle to change where it starts. Then Launch.
The plates are marked with their signs, and flipping the field swaps them. Once launched, the velocity arrow follows the motion; in the across and angled cases it splits into its horizontal and vertical parts so you can see one stay constant while the other grows. The panel shows position and speed. In Along Field you can add a second particle and race them.
The field between the plates is uniform. The two particles do not push on each other. The deflection is visually exaggerated so you can see it — the q/m values shown are real, but the trajectory is drawn at teaching scale, not to physical scale.
You should be able to say which way a charge bends in a field, and why the horizontal speed never changes in the across-field case.