Electromagnetic Effects · revision map · IGCSE 0625, Topic 4.5

TOPIC 4.5 · ELECTROMAGNETIC EFFECTS
CAMBRIDGE IGCSE PHYSICS 0625 · ONE-PAGE REVISION MAP
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ELECTRO-
MAGNETISM4.5
Magnetic Effect of a Current

An electric current always makes a magnetic field around it.

  • The field circles a straight wire.
  • A coil (solenoid) makes a field like a bar magnet.
  • More turns, more current, or an iron core make it stronger.
N S solenoid acts like a bar magnet
The Motor Effect

A current-carrying wire in a magnetic field feels a force.

  • Fleming's left-hand rule gives the direction.
  • Reverse the current or the field and the force reverses.
N S force on the wire
The d.c. Motor

A coil in a magnetic field carries a current, so each side feels a force. The forces act in opposite directions and turn the coil.

  • A split-ring commutator reverses the current every half turn, so the coil keeps spinning the same way.
  • More current, more turns, or a stronger field gives a bigger turning effect.
opposite forces turn the coil
Electromagnetic Induction

Moving a magnet near a coil induces an e.m.f. (a voltage) across the coil.

  • A changing magnetic field through the coil makes the voltage; faster motion gives more.
  • Reverse the motion and the induced e.m.f. reverses.
  • No movement means no change, so no induced e.m.f.
G move
The a.c. Generator

A generator turns a coil between magnets to induce an alternating voltage.

  • As the coil spins, the e.m.f. rises, falls and reverses each turn, giving a.c.
  • The current is taken off through slip rings and brushes.
  • Faster spinning gives a bigger and more frequent voltage.
e.m.f. alternates: a.c. output
Transformers & the GridEXTENDED

A changing current in the primary coil induces a voltage in the secondary coil on the same iron core.

Vp / Vs = Np / Ns Vp Ip = Vs Is
turns ratio sets voltage ratio; for an ideal transformer power in = power out.
  • Step-up raises voltage; step-down lowers it.
  • The grid transmits at high voltage, low current, so less power is wasted as heat (I²R) in the cables.
BUILDS ON · Magnetism · Electrical quantities → ELECTROMAGNETISM → LEADS TO · Motors, generators & the power grid
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