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LearnCambridge97022025–2027Paper 4 · A Level Structured QuestionsElectric fields

Paper 4 · Module

Electric fields

3 subtopics≈ 110 min in total

Start the module: Electric fields and uniform fields Open graphing calculator

Electric fields and uniform fields
An electric field is a region where a charge experiences a force. Field strength is the force per unit positive charge: $E=\dfrac Fq$ (N C⁻¹ or V m⁻¹). Between parallel plates with p.d. $V$ and separation $d$ , the field is uniform: $E=\dfrac Vd$ . A charged particle in a uniform field feels a constant force $qE$ , so it accelerates uniformly and, if it enters at right angles, follows a parabola like a projectile.
≈ 35 min
Coulomb's law
Two point charges exert forces on each other proportional to the product of the charges and inversely proportional to the square of their separation: $F=\dfrac{Q_1Q_2}{4\pi\varepsilon_0r^2}$ , with $\varepsilon_0=8.85\times10^{-12}$ F m⁻¹. Like charges repel; unlike attract. A point charge (or charged sphere, from outside) creates a radial field $E=\dfrac Q{4\pi\varepsilon_0r^2}$ . Fields from several charges add as vectors.
≈ 35 min
Electric potential
Electric potential at a point is the work done per unit positive charge bringing a small test charge from infinity: $V=\dfrac Q{4\pi\varepsilon_0r}$ . Unlike gravitational potential, it can be positive (near a positive charge) or negative. A charge $q$ at that point has potential energy $E_P=qV=\dfrac{Qq}{4\pi\varepsilon_0r}$ . Field strength is minus the potential gradient, $E=-\dfrac{\Delta V}{\Delta r}$ , so field lines cross equipotentials at right angles.
≈ 40 min