Fields
IB Physics Guide (first exams 2025) — Theme D: Fields, sections D.1–D.3 (SL)
Every field question in this course is really the same question asked three times: what creates the field, how strong is it, and what does it do to something placed inside it? Gravity, electricity and magnetism all follow that pattern — a source (mass, charge, current) sets up a field, and a test object (mass, charge, moving charge) feels a force. Once you see and as the same equation wearing different clothes, half the topic becomes memorisation of one structure instead of three separate ones.
Overview — The Logic of Fields
What a 'field' actually is
A field is a region of space where a mass, charge, or current experiences a force without any contact. The source (mass for gravity, charge for electricity, moving charge/current for magnetism) modifies the space around it whether or not you place a test object there — the field exists first, the force appears only when something is placed inside it.
- Field strength (a vector) tells you the force per unit mass or charge at a point — it has direction, pointing the way a positive test object would accelerate.
- Potential (a scalar) tells you the energy per unit mass or charge at a point — no direction, just a number, and it's what you use for work-done and energy calculations.
- Field lines show direction of force; equipotential surfaces are always perpendicular to field lines and cost zero work to move along.
- Gravitational and electric fields share the same mathematical skeleton — the difference is that gravity is always attractive while electric force can attract or repel depending on the sign of the charges.
The shape of the chapter
Command terms that decide how you answer
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| State | Give a fact or law with no supporting working | AO1 | 1 mark for exact correct statement — vague paraphrasing of Newton's law or Coulomb's law loses the mark |
| Determine | Obtain a numerical answer using given data, showing method | AO2 | Method marks awarded even if final value is wrong due to an arithmetic slip — but working must be visible |
| Show that | Substitute given data into a known relationship to confirm a stated result | AO2 | You must show more significant figures than the quoted answer — writing just the final rounded value loses the mark even if correct |
| Derive | Produce an algebraic chain from a known relationship to a stated result | AO2/AO3 | Every intermediate step must appear — skipping the substitution step (e.g. equating to ) costs a mark |
| Sketch | Draw a diagram/graph with correct shape and key features labelled | AO2 | Axes, intercepts and asymptotic behaviour must be indicated even without exact scale |
| Compare and contrast | State both similarities and differences explicitly | AO3 | A one-sided answer (only differences, or only similarities) forfeits half the marks available |
Key point
Overview