Magnetic Fields · revision map · A-Level 9702, Topic 20

TOPIC 20 · MAGNETIC FIELDS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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MAGNETIC FIELDS20
The Field & Its Patterns

A region where a magnet or a current feels a force. Field lines run N to S; closer lines mean a stronger field.

  • A straight wire: concentric circles (right-hand grip rule).
  • A solenoid: a near-uniform field inside, like a bar magnet.
  • Flux density B is measured in tesla (T).
grip rule: thumb = I, curled fingers = B N S B inside the solenoid coil field resembles a bar magnet
Force on a Current

A current across a field feels a sideways push, greatest when I is perpendicular to B.

F = BIL sinθ
θ is the angle
between I and B
  • Direction: Fleming's left-hand rule (thuMb = F, First = B, seCond = I).
  • This defines B: force per unit current per unit length.
  • One tesla = 1 N A−1 m−1; parallel wires attract.
B out of page I F F is perpendicular to both I and B
Force on a Moving Charge

A current is moving charge, so each charge is pushed.

F = BQv
  • F is perpendicular to v, so it does no work: speed stays constant.
  • In a uniform B the path is a circle: BQv = mv²/r, so r = mv/(BQ).
  • A velocity selector passes only v = E/B (crossed E and B fields).
B into page v F circle of radius r = mv/(BQ)
Electromagnetic Induction

A changing flux linkage drives an induced e.m.f.

Φ = BA E = −d(NΦ)/dt
  • Magnetic flux Φ = BA; flux linkage = NΦ for N turns.
  • Faraday's law: e.m.f. equals the rate of change of flux linkage.
  • Lenz's law: the induced current opposes the change (energy is conserved), giving the minus sign.
N S push in I induced I opposes the motion
BUILDS ON · Forces · Electricity · IGCSE Electromagnetism → MAGNETIC FIELDS → LEADS TO · Alternating currents · Quantum physics
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