Electric field lines, force direction and strength

Electric field lines, force direction and strength

An electric field assigns a force direction and strength to each location around charge. Its direction is defined as the direction a small positive test charge would move: away from positive source charge and toward negative source charge. Field lines are a representation, not physical wires. They never cross because the field at one point cannot point in two directions at once. Where field lines are drawn closer together, the field is stronger. The electric field magnitude is E = F/q for a specified test charge, measured in N C⁻¹. The test charge should be small enough not to disturb the source-charge arrangement.

Work it through

A +3.0 μC test charge experiences 0.12 N to the right. E = F/q = 0.12/(3.0 × 10⁻⁶) = 4.0 × 10⁴ N C⁻¹ to the right. A negative test charge at that point would feel a force left, but the field direction remains right because field direction uses a positive test charge convention.

Mastery target

Interpret field lines, calculate field strength and distinguish the direction of an electric field from the force on a negative charge.