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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 7

Topic 4.3.1 · Circuit diagrams and circuit components

Reading circuit diagrams

You must draw and interpret circuits using the official symbols for: cells, batteries, power supplies, generators, potential dividers, switches, fixed and variable resistors, heaters, NTC thermistors, LDRs, lamps, motors, bells, ammeters, voltmeters, magnetising coils, transformers, fuses and relays.

Extended also requires the diode and LED symbols and how they behave.

Standard IGCSE circuit symbols: cell, battery, d.c. and a.c. supplies, lamp, fixed and variable resistors, ammeter, voltmeter, switch, fuse, LDR, thermistor, heater, motor, bell, generator, transformer, relay coil, diode and LED.
Diagram 1: Standard IGCSE circuit symbols: cell, battery, d.c. and a.c. supplies, lamp, fixed and variable resistors, ammeter, voltmeter, switch, fuse, LDR, thermistor, heater, motor, bell, generator, transformer, relay coil, diode and LED.

How the components behave

ComponentBehaviour in a circuit
Cell / battery / d.c. supplySource of e.m.f.; conventional current leaves +
a.c. supply / generatorSource whose current repeatedly reverses
SwitchOpen: gap, no current. Closed: complete path
Lamp / heater / motor / bellTransfers electrical energy (light, thermal, kinetic, sound)
Ammeter / voltmeterAmmeter in series; voltmeter in parallel
FuseMelts if current is too large, breaking the live path
NTC thermistorResistance decreases as temperature increases
LDRResistance decreases as light intensity increases
Variable resistor / potential dividerLets you change current or share p.d.
Relay / magnetising coilA small current magnetises a core and can close another circuit
TransformerChanges a.c. voltage (needs a changing current)
Diode / LEDAllows current one way; LED also emits light when forward-biased
NTC thermistor: resistance falling as temperature rises. LDR: resistance falling as light intensity rises. Diode/LED: conduction in one direction only.
Diagram 2: NTC thermistor: resistance falling as temperature rises. LDR: resistance falling as light intensity rises. Diode/LED: conduction in one direction only.

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