2.5 Electric Field

What an electric field is

An electric field (E) is the region around a charge where another charge feels a force. It's a vector defined as E = F/q, pointing in the direction a positive test charge would move. Field lines point away from positive charges and toward negative charges, and their density shows field strength.

Field from a point charge

For a point charge, E = kQ/r², where Q is the source charge and r is distance from it. Field strength decreases with the square of distance. Multiple charges' fields add as vectors (superposition), so the net field at a point is the vector sum of each individual field.

Fields and forces together

Once you know E at a location, the force on any charge q placed there is F = qE. Positive charges feel force along E; negative charges feel force opposite to E. Uniform fields (like between parallel plates) have equally spaced, parallel field lines and exert constant force on charges.

Key terms

Electric field (E)

A vector quantity representing force per unit charge at a point in space; E = F/q.

Point charge field formula

E = kQ/r², where Q is the source charge and r is the distance from it.

Force on a charge in a field

F = qE; direction depends on the sign of q relative to E's direction.

Superposition of electric fields

The net electric field at a point is the vector sum of the fields from all individual charges.

Direction of field lines

Point away from positive charges and toward negative charges; density indicates field strength.

Try an AP-style question

A 2.0 μC test charge feels a force of 0.010 N. What is the field strength there?

1. 5,000 N/C
2. 2 × 10⁻⁸ N/C
3. 0.020 N/C
4. 500 N/C