Unit 3: How Do Fields Explain Motion and Electricity?
Core concepts
Unit 3 is about fields and their effects. You must describe gravitational, electric and magnetic fields using field lines and vector quantities, and apply Newton's laws and momentum conservation. Key relationships include F = ma, p = mv, impulse FΔt = Δp, and the work–energy theorem. For gravity, understand weight as mg and analyse circular orbits using gravitational force as the centripetal force.
In electricity, know electric field strength E = F/q, potential difference, and how charged particles move in uniform fields. For magnetism, apply F = qvB sin θ and the right-hand rule for force on a current-carrying wire. Understand electromagnetic induction: a changing magnetic flux induces an emf, described by Faraday's law, with Lenz's law giving the direction opposing the change.
Traps examiners set
- Confusing electric field direction (force on a positive charge) with electron motion.
- Forgetting momentum is a vector and must be treated with signs in collisions.
- Assuming kinetic energy is conserved in all collisions - it is only conserved in elastic ones.
- Mixing up magnetic flux (Φ = BA cos θ) with flux density B.
- Applying Lenz's law incorrectly: the induced current opposes the change in flux, not the flux itself.
Worked example
A 1200 kg car travelling at 15 m s⁻¹ brakes to rest in 3.0 s. Find the average braking force.
Impulse equals change in momentum: FΔt = mΔv = 1200 × (0 − 15) = −18 000 kg m s⁻¹. So F = −18 000 / 3.0 = −6000 N. The negative sign indicates the force opposes motion; the magnitude is 6.0 × 10³ N.
Rule of thumb
Choose a positive direction, draw a free-body diagram, then apply the relevant conservation law or field equation before substituting numbers.