IB Physics: Electric Fields & Coulomb's Law — FAQ

You need Coulomb's law (F = kq1q2/r²) for point charges, the definition of electric field strength (E = F/q), field patterns around point and parallel-plate charges, and — at HL — the relationship between field and potential (E = −dV/dr). SL stops at qualitative field lines and simple calculations; HL adds the calculus link.

Electric field (E) is a vector — force per unit charge, measured in N C⁻¹ or V m⁻¹, and it tells you direction and magnitude of force on a charge. Electric potential (V) is a scalar — energy per unit charge, measured in volts, and tells you work done moving a charge from infinity.

Electric field strength at a point is the force per unit charge a small positive test charge would feel there: E = kQ/r², derived directly from Coulomb's law F = kQq/r² divided by q. Direction is radially outward from a positive charge, inward toward a negative one.

Both follow an inverse-square law and share almost identical formula structures, but electric fields can attract or repel (charges come in two signs) while gravitational fields only attract (mass is always positive). The IB data booklet lists them side by side because the maths is deliberately parallel.

Yes — Coulomb's law, electric field strength, and field patterns around point charges are common to both SL and HL. What's HL-only is the deeper treatment: field-potential relationships via calculus, combined electric and gravitational field problems, and more demanding multi-charge superposition questions.

The data booklet gives you F = kq1q2/r² (Coulomb's law), E = F/q (field strength), E = kQ/r² (field from a point charge), and V = kQ/r (potential). HL adds W = qV for work done and the field-potential gradient relationship in the electricity and magnetism section.

The three I see every year: forgetting field is a vector and adding magnitudes instead of components; confusing field strength with potential in graph questions; and dropping the negative sign when a charge is negative, which flips the direction of force entirely.

Grade 7 students on this topic can move fluently between field, force and potential without re-deriving formulas each time, and they always sketch a diagram before calculating — field direction errors cost more marks than arithmetic errors. Practising multi-charge superposition problems is what separates a 6 from a 7.

Expect a mix: a straightforward point-charge calculation (Paper 1 or short Paper 2), a two-charge superposition problem where you resolve vectors, a field-line sketching question, and — at HL — a combined gravitational/electric field comparison or a graph interpreting E versus r.

It's moderately demanding rather than brutally hard — the maths itself (inverse-square law, vector addition) is manageable, but the conceptual overlap with gravitational fields and electric potential trips up students who memorise formulas without understanding what each variable represents.

IB Physics covers similar core content to A-level Physics and AP Physics 2, but the IB's internal assessment and data-based Paper 3 questions ask students to apply Coulomb's law in unfamiliar experimental contexts, not just recall and calculate — which tends to demand more transferable understanding than a pure recall exam.

The most effective combination is targeted topical practice, concise revision notes that separate SL from HL content clearly, and full mock papers under timed conditions closer to exams. On RevisionPrep, students can work through Topical Worksheets on fields specifically, then check understanding against structured Revision Notes.