Fields: Gravitational, Electric, Magnetic Fields and Electromagnetic Induction
Fields covers gravitational, electric and magnetic fields and electromagnetic induction. The exam tests field strength and potential, inverse-square laws, motion of charged particles, magnetic force direction, flux, Faraday’s law and Lenz’s law. HL questions often combine electric and magnetic effects or ask for the direction of an induced current.
Core rules
- Field strength: g = F/m and E = F/q. Point sources: g = GM/r² and E = kQ/r².
- Potential is scalar: V = W/q; for a point charge V = kQ/r, and for a point mass V = −GM/r.
- Uniform field between plates: E = V/d. Equipotentials are perpendicular to field lines.
- Magnetic force: F = qvB sinθ and F = BIL sinθ; direction from the right-hand rule.
- A charge moving perpendicular to B follows a circle with r = mv/(qB).
- Flux Φ = BA cosθ; induced emf ε = −NΔΦ/Δt. Lenz’s law gives the direction opposing the change.
Worked example
A 200-turn coil of area 4.0 × 10⁻⁴ m² experiences a field change from 0.20 T to 0.50 T in 0.10 s, perpendicular to the coil. ΔΦ = AΔB = (4.0 × 10⁻⁴)(0.30) = 1.2 × 10⁻⁴ Wb. Then ε = NΔΦ/Δt = 200(1.2 × 10⁻⁴)/0.10 = 0.24 V. The induced current direction is set by Lenz’s law.
Traps
The magnetic force is perpendicular to velocity, so it does no work and cannot change speed. Do not confuse magnetic flux density B with flux Φ. Field strength and potential are different quantities with different units. The minus sign in Faraday’s law expresses Lenz’s law; it is not optional bookkeeping.