4.3Electricity and Magnetism

Electric Circuits and Electrical Safety — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

15 min read 26 parts Lesson 30 of 65 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

4.3 Electric Circuits and Electrical Safety (Part 1) — IGCSE PHYSICS notes: Recognise circuit components and apply the rules for current in series and parallel circuits.Power supplies — cells, batteries, power supplies and generators all supply current to a circuit.Meters — ammet…

Functional components
Fixed and variable resistors
Thermistor
Light-dependent resistor (LDR)
Magnetising coils, relays and transformers
Diode (Extended)

Part 1 of 8Power supplies, meters and switches

Power supplies, meters and switches

Power supplies

Power supplies cells, batteries, power supplies and generators all supply current to a circuit.

Meters

Meters ammeters measure current and are connected in series;
- voltmeters measure potential difference and are connected in parallel.

Switches

Switches open and close a circuit and so decide whether current can flow.

Functional components

Functional components motors rotate, lamps emit light, heaters transfer thermal energy and bells emit sound.

Part 2 of 8Resistors that change with conditions

Resistors that change with conditions

Fixed and variable resistors

Fixed and variable resistors used to control current;
- a potential divider is a resistor arrangement that controls p.d.

Thermistor

Thermistor a non-ohmic, temperature-dependent resistor whose resistance decreases as the temperature rises.

Light-dependent resistor (LDR)

Light-dependent resistor (LDR) a non-ohmic sensory resistor whose resistance decreases as the light intensity increases.

Part 3 of 8Electromagnetic components, fuses and diodes

Electromagnetic components, fuses and diodes

Magnetising coils, relays and transformers

Magnetising coils, relays and transformers use electromagnetic effects.
A relay uses a small current in one circuit to switch on a much larger current in another.
A transformer steps potential difference up or down.

Fuse

Fuse protects components from current surges and acts as a safety measure against fire.

Diode (Extended)

Diode (Extended) allows current in one direction only, the way the triangle points;
- reversed, it blocks current.
A light-emitting diode (LED) behaves in the same way and also emits light when current flows.

Part 4 of 8Current in a series circuit

Series circuit

Series circuit a single complete loop containing all the components.

Same current everywhere

Same current everywhere the number of electrons per second passing any point is the same.

Factors

Factors current rises if the source p.d. rises and falls if more components are added because the total resistance goes up.

Part 5 of 8Current in a parallel circuit

Parallel circuit

Parallel circuit several loops, with components on separate branches.

Splitting

Splitting the current from the source divides at a junction, so each branch current is smaller than the source current.

Lighting circuits

Lighting circuits parallel wiring gives each bulb the same p.d. and brightness, lets other bulbs keep working if one breaks,
- and allows each lamp to be switched separately.

Junction rule (Extended)

Junction rule (Extended) current into a junction equals current out of it, because charge and electrons are conserved.
The branch currents are equal only if the branch resistances are equal.

Part 6 of 8Meters and the effect of components

Meters and the effect of components

Adding a lamp in series

Adding a lamp in series the current falls in every part of the loop because the total resistance increases.

Adding a lamp in parallel

Adding a lamp in parallel the other lamps are unaffected and the supply current increases.

Dark and light

Dark and light in a circuit with an LDR, the p.d. across the LDR rises in the dark because its resistance is large.

Switches

Switches a switch in a parallel branch controls only that branch, while a switch in the main wire controls the whole circuit.

Part 7 of 8Comparing series and parallel circuits

Feature	Series	Parallel
Current	Same everywhere	Splits at junctions
If one lamp breaks	All go out	Others stay on
Total resistance	Increases when components are added	Decreases when branches are added

Part 8 of 8More from the specification

Key terms

Term	Meaning
Series circuit	A circuit with one loop where the same current flows through all components.
Parallel circuit	A circuit with multiple loops, so the current splits at junctions.
Thermistor	A resistor whose resistance changes with temperature.
LDR	A resistor whose resistance decreases as light intensity increases.
Relay	A switch that uses a small current to switch a larger current.
Fuse	A safety device that melts and breaks the circuit when the current is too high.
Diode	A component that allows current in one direction only.

Avoid these mistakes

Connecting an ammeter in parallel — ammeters are in series.

Thinking current is used up round a series circuit — it is the same at every point.

Saying branch currents are always equal — they are equal only if the branch resistances are equal.

Mixing up thermistor and LDR — one responds to temperature and the other to light.

Last updated: 15 October 2026