# Gravitational, electric and magnetic fields

> Newton's law of gravitation and Coulomb's law, field strength and potential, orbits, the motion of charges in magnetic fields, electromagnetic induction and Faraday's and Lenz's laws.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.fields
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 3.4–3.6
- Difficulty: 5/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 3.4(a) | state Newton law of gravitation and Coulomb law and compare their forms | AO1 |
| Unit 3.4(b) | define gravitational and electric field strength as force per unit mass or charge | AO1 |
| Unit 3.4(c) | define electric and gravitational potential and relate field strength to potential gradient | AO1 |
| Unit 3.4(d) | apply inverse-square laws to calculate field strength potential and orbital motion | AO2 |
| Unit 3.5(a) | describe the force on a current carrying conductor and on a moving charge in a magnetic field | AO1 |
| Unit 3.5(b) | apply F equals B I l sin theta and F equals B Q v to determine trajectories | AO2 |
| Unit 3.6(a) | state Faraday law and Lenz law for electromagnetic induction | AO1 |
| Unit 3.6(b) | calculate induced emf as rate of change of flux linkage and predict direction | AO2 |

## Summary

Newton's law of gravitation and Coulomb's law, field strength and potential, orbits, the motion of charges in magnetic fields, electromagnetic induction and Faraday's and Lenz's laws.

## What earns marks

- Gravitational and electric fields are both inverse-square, but gravity is always attractive while electric forces can repel.
- Field strength is a vector (force per unit mass/charge); potential is a scalar and is zero at infinity.
- F = BIL sinθ for a current-carrying conductor; F = BQv for a moving charge, giving circular motion.
- Faraday: induced EMF = rate of change of flux linkage; Lenz: the induced effect opposes the change causing it.

## Where marks are lost

- Forgetting that gravitational potential is negative everywhere.
- Using r as height above the surface instead of distance from the centre.
- Omitting the minus sign or the opposition statement when quoting Lenz's law.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Gravitational, electric and magnetic fields lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields)