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Electrostatics

The largest chapter on the recent papers. Coulomb's law, field and potential, Gauss's law and capacitors; capacitors alone are a large share, so learn the slab and combination rules cold.

Questions in the bank
251
q/paper in 2025–26
2.21
Numeric answer
28%
Notes pages
9

Tier: Cornerstone

When you’ll see it

Charges at rest: a force, field or potential from point charges, rods, rings or sheets, flux through a surface, a dipole in a field, or capacitors with slabs and batteries.

How this chapter is tested

Electrostatics is the largest chapter on the recent papers, and many of its questions ask for a number. Capacitors are its biggest single part: reducing networks, placing dielectric slabs and sharing charge between two capacitors. The rest moves from force to field, flux and potential.

Most field and potential questions are solved with one idea used carefully: add the effect of each charge, or let a symmetry cancel what it can. Fields add as vectors, potentials as signed numbers. Gauss's law gives flux by counting the enclosed charge and, for symmetric charge, the field itself.

In capacitor questions, decide series or parallel by the nodes, not by the drawing, and decide what stays fixed, V with the battery connected or Q once it is removed, before any energy step. The capacitor circuits lead into Current Electricity, and the dipole and orbit pages return in Moving Charges and Magnetism.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Coulomb's law and equilibrium

    F = kq₁q₂/r² added as vectors; charge shared by contact, signs first; hanging charged balls with tan θ = F/mg.

  • Electric field

    Add kq/r² as vectors and find null points; arcs and rings after symmetry cancels; a single sheet gives σ/2ε₀ at any distance.

  • Flux and Gauss's law

    Flux = E·A through each face; the net flux through a closed surface is q_enc/ε₀; fields of spheres, shells and cylinders by symmetry.

  • Potential and work

    Potentials add as signed numbers; joined spheres share potential, with charge in proportion to radius; E is minus the slope of V; work = q times the potential difference.

  • Dipoles

    p = q × separation, from − to +; the axial field is twice the equatorial; torque p × E and energy −p·E in a uniform field.

  • Charges moving in fields

    Acceleration qE/m gives a projectile path between plates; qE balances mg for a drop at rest; every orbit round a line charge has the same speed.

  • Capacitance and networks

    C = Q/V from geometry and medium alone; series and parallel decided by the nodes; in steady state a capacitor branch carries no current.

  • Dielectric slabs

    A slab across the gap acts in series, one side by side in parallel; a slab of thickness t shortens the gap by t(1 − 1/K), a metal sheet by t.

  • Energy and charge sharing

    ½CV² with the battery connected, Q²/2C once it is removed; joined capacitors keep their charge, reach a common potential and lose energy.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Field sizes added as numbers

    Fields from charges in different places point in different directions. Draw each arrow, then add them as vectors.

  • Axial for equatorial

    At the same distance a dipole's axial field is twice its equatorial field, and the equatorial field points opposite to p.

  • Series for side by side

    Dielectrics that each span the whole gap share one voltage and add as capacitances. Adding reciprocals treats them as layers.

  • Energy before asking what is fixed

    Battery connected keeps V fixed; battery removed keeps Q fixed. Using the other form gives the change in the wrong direction.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Electrostatics notes

Drill every Electrostatics question

251 questions from the bank, across 9 subtopics.

Drill the 251 questions

Drill one subtopic at a time

The 9 subtopics, in teaching order.

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