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Chapter 4: Electromagnetism

Form 5 Physics Bab 4: Electromagnetism

4.1 Force on a Current-carrying Conductor in a Magnetic Field

  • A current-carrying conductor in a magnetic field experiences a force; its direction depends on the current direction and the magnetic field direction. (p.137)
  • Catapult field: the resultant field from the interaction of the conductor's field and the permanent magnet's field. (p.138)
  • Fleming's left-hand rule: index finger = magnetic field (N to S), middle finger = current, thumb = force/motion. (p.139)
  • Force increases with larger current and stronger magnetic field. (p.141)
  • A current-carrying coil in a magnetic field rotates due to a pair of equal, opposite forces. (p.142)
  • The commutator and carbon brushes of a d.c. motor reverse the current every half rotation so the coil keeps turning one way. (p.144)
  • Motor speed increases with greater current (voltage), stronger magnetic field and more coil turns. (p.146)
  • Brushless motor: coil stationary, magnet rotates; no brush friction, no sparking, quieter. (p.147)

4.2 Electromagnetic Induction

  • Electromagnetic induction is the production of an induced e.m.f. in a conductor when there is relative motion between conductor and magnetic field, or the conductor is in a changing magnetic field. (p.153)
  • Induced e.m.f. increases with faster relative motion, more solenoid turns and stronger magnetic field. (p.155)
  • Faraday's law: the magnitude of induced e.m.f. is directly proportional to the rate of cutting of magnetic flux. (p.155)
  • Fleming's right-hand rule (straight wire): thumb = motion, fore finger = magnetic field, middle finger = induced current. (p.157)
  • Lenz's law: the induced current flows in a direction that opposes the change of magnetic flux that causes it. (p.159)
  • D.c. generator uses a split-ring commutator; a.c. generator uses two slip rings. (p.160)

4.3 Transformer

  • A transformer works with a.c.; the changing flux in the soft iron core induces an a.c. voltage in the secondary coil. (p.163)
  • VS/VP = NS/NP. Step-up: NS > NP; step-down: NS < NP. (p.163)
  • Efficiency η = (output power / input power) × 100%. (p.164)
  • Ideal transformer: η = 100%, VPIP = VSIS. (p.164)
  • Energy losses: coil resistance, eddy currents, hysteresis, flux leakage. (p.164)
  • Reducing losses: thicker copper wire, laminated iron core, soft iron core, secondary wound over primary. (p.165)
  • Induction cooker uses eddy currents in the pan base to heat it. (p.165)
  • Step-up transformers raise transmission voltage to reduce current and power loss; step-down transformers lower it for consumers. (p.167)
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