A level Physics · Edexcel 9PH0 · Topic 7A

Electric fields

Key points

Electric field strength is the force per unit positive charge: $E = \frac{F}{Q}$ (N C $^{-1}$ or V m $^{-1}$ ).
Field lines point from positive to negative; closer lines mean a stronger field. Between parallel plates the field is uniform: $E = \frac{V}{d}$ .
Coulomb's law: $F = \frac{Q_1Q_2}{4\pi\varepsilon_0 r^2}$ , an inverse square law. Like charges repel, unlike attract.
Radial field of a point charge: $E = \frac{Q}{4\pi\varepsilon_0 r^2}$ ; electric potential $V = \frac{Q}{4\pi\varepsilon_0 r}$ .
Equipotentials join points of equal potential and are always perpendicular to field lines; no work is done moving along one.
Field strength is the potential gradient: $E = -\frac{\Delta V}{\Delta r}$ .
A charged particle entering a uniform field at right angles follows a parabolic path, like a projectile.

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An electric field is the region around a charge where other charges feel a force. These notes cover field strength, uniform and radial fields, Coulomb's law, electric potential and the motion of charged particles in fields.

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