Electric Fields and Field Strength

Electric field — a region in which a charged object experiences a force.

Electric field strength (E) — the force per unit positive charge on a small test charge:

E = F / Q

measured in N C⁻¹. It is a vector, and its direction is the direction of the force on a positive charge.

Field lines — drawn from positive charge towards negative charge. Their direction gives the force on a positive charge, so a negative charge feels a force in the opposite direction to the field line. Closer lines mean a stronger field.

Field patterns worth knowing

A point positive charge — lines radiate outwards, spreading with distance.

A point negative charge — lines converge inwards.

Two opposite charges — lines run from positive to negative, curving between them.

Two parallel charged plates — lines run straight from the positive plate to the negative plate, evenly spaced across the middle: a uniform field, though the lines bulge outwards at the edges.

Both attractive and repulsive — unlike gravity, electric forces come in two signs. Like charges repel, unlike charges attract. This is the single most important structural difference between electric and gravitational fields, and it has a large consequence: charge can cancel, so bulk matter is electrically neutral and electric forces do not accumulate over astronomical distances the way gravity does.

Example: the electrostatic repulsion between two protons is about 10³⁶ times stronger than their gravitational attraction. Gravity dominates the universe not because it is strong but because it never cancels.

Field lines never cross — if they did, the force at that point would have two directions at once, which is impossible. They also meet a conductor's surface at right angles, since any component along the surface would drive charge until it vanished.