Electric Fields & Field Lines
Electric Fields & Field Lines
- AS defined the electric field; A2 makes it quantitative. But first: the definition itself and two drawings are direct marks, and each has a checklist.
The definition, with its trap
- The at a point is the force per unit positive charge on a stationary charge at that point: . The word positive is not decoration — as a one-marker the whole phrase is required (9702/41/M/J/23 Q1(a)(ii)) (9702/42/O/N/25 Q6(a)), and as a two-marker “force per unit charge” and “on a positive charge” score separately (9702/41/M/J/24 Q5(a)).
- Why positive? The field's direction is defined as the direction of the force on a positive charge — a negative charge feels the opposite force. Units: or equivalently (both accepted when a unit is demanded).
The two drawings and their checklists
- Between parallel plates (three marks, one per feature): lines straight and perpendicular to the plates, evenly spaced, with arrows from + to − (9702/42/O/N/23 Q5(c)(ii)) (9702/41/M/J/24 Q5(b)(i)).
- Around a charged sphere: lines straight and radial (they must aim at the centre), evenly spaced, arrows outward for positive charge (9702/42/O/N/24 Q6(b)) (9702/42/O/N/25 Q6(b)(i)).
- Inside a conductor the field is zero. The charge spreads over the surface, so a test charge inside is pulled equally in every direction — and if the field were not zero, the charges themselves would move until it was (9702/42/O/N/24 Q6(c)(iii)). Outside, the sphere behaves as if all its charge sat at the centre — the key to every sphere calculation in this chapter.
E = force per unit positive charge. Plates: straight, evenly spaced, + to −. Sphere: radial, evenly spaced, outward if positive — and zero field inside.