Quick questions on Electric field strength and parallel plates: HSC Physics Module 6

Two flat, conducting plates held at different potentials produce a characteristic field pattern.

For a uniform field, the potential difference $V$ between two points separated by a distance $d$ along the field direction is:

This often catches students out. The field is uniform because each plate, if it were infinite, would produce a uniform field of magnitude $\sigma / (2 \varepsilon_0)$ everywhere on either side (where $\sigma$ is the surface charge density). Between two oppositely charged plates the fields from both add; outside, they cancel. The result is a constant field in the gap that does not depend on how close you are to either plate.

Markers love a clean labelled diagram. Reserve space for one even in a short answer.

By convention, field lines point in the direction of the force on a positive test charge, so they leave the positive plate and end on the negative plate. Get this wrong and you can lose every direction mark in the question.

In the central region the field magnitude is constant. The field is weaker, not stronger, near the edges of the plates.

$E = V/d$ in SI units only.

$V$ is measured in volts (J/C) and is the work per unit charge to move between two points; $E$ is measured in V/m (N/C) and is the force per unit charge at a point. They are related by $V = E d$ for a uniform field.

This formula is for a uniform field between two parallel plates. It does not apply to a point charge or a charged sphere.