4.3.3Electricity and Magnetism

Action AND USE OF Circuit Components — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 38 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

4.3.3 Action and Use of Circuit Components — IGCSE PHYSICS notes: Describe the action and uses of the variable potential divider, thermistor, LDR, diode, LED and relay.Action — it changes the current in a circuit.Uses — a dimmer or a volume control.Thermistor (NTC) — its resistan…

Vout = Vin × R2 / (R1 + R2)
Variable resistor as one part
LDR in the lower part
Rectification
Thermistor (NTC) — its resistance decreases as temperature increases.Used in fire alarms and thermostats
LDR — its resistance decreases as light intensity increases.Used in automatic street lights and light meters

Part 1 of 7Variable resistor

Action

Action it changes the current in a circuit.

Uses

Uses a dimmer or a volume control.

Part 2 of 7Thermistor and LDR

Thermistor (NTC)

Thermistor (NTC) its resistance decreases as temperature increases.
Used in fire alarms and thermostats.

LDR

LDR its resistance decreases as light intensity increases.
Used in automatic street lights and light meters.

Part 3 of 7Variable potential divider (Extended)

Variable potential divider (Extended)

Vout

Vin × R2 / (R1 + R2)

the output p.d. across a component rises when its share of the total resistance rises.

Ratio

R₁/R₂ = V₁/V₂.

Variable resistor as one part

Variable resistor as one part the p.d. across the other part, and across any component in parallel with it, can be altered.

LDR in the lower part

LDR in the lower part darkness raises R_LDR, so the p.d. across it rises and can switch on a lamp.

Thermistor

Thermistor heating lowers its resistance, so the p.d. across the fixed resistor rises and can operate an alarm.

Part 4 of 7Temperature-sensing alarm

A thermistor and a fixed resistor form a potential divider across the supply.

The temperature rises, so the thermistor's resistance decreases.

The p.d. across the fixed resistor increases.

The larger p.d. drives current through a relay coil or switching circuit.

The relay closes contacts in a second circuit and the alarm sounds.

Part 5 of 7Diode and LED

Diode

Diode allows current in one direction only (forward bias) and has very high resistance in reverse.

Rectification

Rectification a diode converts a.c. to d.c.
A single diode gives half-wave rectification, where one half of each cycle is blocked (Extended).

LED

LED a diode that emits light when current flows forwards, used as an indicator.
It needs a series resistor to limit the current.

Part 6 of 7Relay (Extended)

Action

Action a small current in the coil magnetises an iron core, which attracts an armature and closes (or opens) switch contacts in a second circuit.

Purpose

Purpose it allows a low-current sensor circuit to switch a high-current device safely.

Part 7 of 7More from the specification

1Standard electrical wiring schematics

Standard electrical wiring schematics standard circuit diagrams provide an unambiguous universal means to display component connections, sources,
- and measuring instruments.

2Circuit symbols

Standard symbols represent cells, batteries, switches, lamps, resistors, fuses and meters so circuits can be drawn clearly.

3Potential difference

Energy transferred per unit charge between two points, measured by a voltmeter connected in parallel;
- the source supplies the energy.

4Junction and branch

Current splits at a junction into branches in a parallel circuit, and the total current entering equals the total leaving.

5Power of an appliance

P = IV gives the rate of energy transfer for an appliance connected to the mains supply.

6Safety

A fuse and an earth wire protect against excess current, so a faulty live wire cannot make the metal case dangerous.

7Circuit symbols

Standard symbols represent cells, batteries, switches, lamps, resistors, fuses and meters so circuits can be drawn clearly.

8Potential difference

Energy transferred per unit charge between two points, measured by a voltmeter connected in parallel;
- the source supplies the energy.

9Junction and branch

Current splits at a junction into branches in a parallel circuit, and the total current entering equals the total leaving.

10Power of an appliance

P = IV gives the rate of energy transfer for an appliance connected to the mains supply.

11Safety

A fuse and an earth wire protect against excess current, so a faulty live wire cannot make the metal case dangerous.

12Potential difference

Work done by a unit charge passing through a component.

13Combined resistance

Single resistance that could replace several resistors.

14Mains supply

Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

15Circuit diagram

Drawing of a circuit using standard symbols.

16Circuit symbol

Standard picture for a component.

17Source

Reservoir at T_H providing heat Q_H

18Branch

Path connecting two junctions.

19Junction

Place where a pipe splits or joins; flow rates add.

20Insulation

Material that reduces thermal energy transfer.

Key terms

Term	Meaning
Thermistor	Resistor whose resistance decreases as temperature increases.
LDR	Resistor whose resistance decreases as light intensity increases.
Rectification	Converting a.c. to d.c. using a diode.
LED	Light-emitting diode.
Relay	Electromagnetic switch operated by a small current.
Potential divider	Two resistors in series that share the supply p.d.

Avoid these mistakes

Using an LDR the wrong way round — the lamp then comes on in the day instead of at night.

Saying a thermistor's resistance rises as it heats — for an NTC thermistor it falls.

Forgetting the series resistor with an LED — without it the current is too large.

Thinking a relay carries the large current in its coil — the coil carries a small current and the contacts switch the large one.

Last updated: 15 October 2026