IGCSE Physics Circuit Diagrams and Circuit Components Questions and Worked Solutions (4.3.1) | 0625

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IGCSE Physics 0625 · Topic 4.3.1

IGCSE Physics Circuit Diagrams and Circuit Components Questions and Worked Solutions

50 topic-wise practice questions on Circuit Diagrams and Circuit Components (Unit 4: Electricity and Magnetism), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions29 Core11 Extended10 Challenge108 marksTextbook §4.3 · pp. 209–221
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Circuit Diagrams and Circuit Components

Circuit diagrams use standard symbols. You must draw and interpret circuits containing cells, switches, resistors, sensors, lamps, motors, meters, coils, transformers, fuses and relays, and (Extended) diodes and LEDs.

Key formulas

No equations to learn in this subtopic — learn symbols and behaviour.

Prerequisite knowledge

  • Current, p.d. and resistance (4.2)

Exam tips & common mistakes

  • Draw circuits with straight lines and no gaps
  • Ammeter in series, voltmeter in parallel
  • LEDs usually need a series resistor to limit current

Key facts from the syllabus (4.3.1)

  • Symbols: cell, battery, power supply, generator, potential divider, switch, fixed and variable resistor, heater, thermistor (NTC), LDR, lamp, motor, bell, ammeter, voltmeter, magnetising coil, transformer, fuse, relay
  • NTC thermistor: resistance decreases as temperature increases
  • LDR: resistance decreases as light intensity increases
  • Variable resistor: adjusts the current
  • Extended diode allows current in one direction only; LED emits light when forward biased

Textbook: §4.3 Electric circuits, pp. 209–221

1
Core[1 mark]

Describe the circuit symbol for a cell.

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Method: Long and short lines.

A long thin line (positive) and a short thick line (negative)
2
Core[1 mark]

State the difference between a cell and a battery.

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Method: Battery = multiple cells.

A battery is two or more cells connected together
3
Core[2 marks]

State how the resistance of an NTC thermistor changes with temperature.

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Method: Decreases as T increases.

Its resistance decreases as temperature increases
4
Core[2 marks]

State how the resistance of an LDR changes with light intensity.

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Method: Decreases with more light.

Its resistance decreases as light intensity increases
5
Core[1 mark]

State the function of a variable resistor.

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Method: Adjust current.

To change the resistance and so adjust the current in a circuit
6
Core[1 mark]

State the function of a fuse.

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Method: Melts when current too high.

It melts and breaks the circuit if the current becomes too large
7
Core[1 mark]

State the function of a relay.

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Method: Switch using electromagnet.

A switch operated by an electromagnet, letting a small current control a larger one
8
Core[2 marks]

Describe the circuit symbol for an ammeter and a voltmeter.

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Method: Circles.

A circle containing A (ammeter) and a circle containing V (voltmeter)
9
Core[2 marks]

Describe how to draw a potential divider in a circuit diagram.

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Method: Two resistors in series.

Two resistors (or a resistor and a variable resistor/sensor) in series across a supply, with the output taken across one of them
10
Core[2 marks]

A thermistor is placed in a warm room and then in a cold fridge. State what happens to the current through it from a constant supply.

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Method: Resistance rises in cold.

Current decreases in the fridge, because the thermistor's resistance increases
11
Core[2 marks]

An LDR is moved from bright light into darkness. State what happens to its resistance and the current.

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Method: Dark → high R.

Resistance increases; current decreases
12
Core[1 mark]

State the function of a magnetising coil (solenoid) in a circuit.

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Method: Electromagnet.

To produce a magnetic field when a current flows
13
Core[1 mark]

Name the component whose symbol is a circle with a cross inside.

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Method: Lamp.

Lamp
14
Core[1 mark]

Name the component whose symbol is a circle containing M.

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Method: Motor.

Motor
15
Core[2 marks]

Describe the circuit symbol for a fixed resistor.

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Method: Rectangle.

A small rectangle
16
Core[2 marks]

Describe the circuit symbol for a variable resistor.

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Method: Rectangle with arrow.

A rectangle with an arrow through it
17
Core[2 marks]

Describe the circuit symbol for a fuse.

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Method: Rectangle with line.

A rectangle with a line running through it lengthwise
18
Core[2 marks]

Describe the circuit symbol for an LDR.

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Method: Resistor in circle with arrows.

A resistor symbol (often in a circle) with arrows pointing in towards it
19
Core[2 marks]

Describe the circuit symbol for a thermistor.

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Method: Resistor with bent line.

A resistor symbol with a diagonal line with a short horizontal tail
20
Core[2 marks]

Describe the symbol for a heater.

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Method: Rectangle divided.

A rectangle divided into sections (heating element)
21
Core[2 marks]

Describe the circuit symbol for a power supply (a.c.).

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Method: Circles with ~.

Two terminals/circles with a wavy line (~) indicating a.c.
22
Core[2 marks]

State the function of a switch in a circuit.

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Method: Make/break.

To make or break the circuit (turn current on or off)
23
Core[2 marks]

State the function of a potential divider.

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Method: Divide p.d.

To provide a fraction of the supply p.d. as an output
24
Core[2 marks]

State one use of an NTC thermistor.

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Method: Temperature sensor.

Temperature sensor (e.g. fire alarm, thermostat)
25
Core[2 marks]

State one use of an LDR.

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Method: Light sensor.

Light sensor (e.g. automatic street lights)
26
Core[2 marks]

State the function of a transformer in a circuit diagram.

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Method: Change p.d.

To change an alternating p.d. (step up or step down)
27
Core[2 marks]

State the function of a bell in a circuit.

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Method: Sound.

To produce sound when current flows
28
Core[2 marks]

Describe what happens to the current through an NTC thermistor as it warms (constant p.d.).

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Method: R falls.

The current increases
29
Core[2 marks]

Describe what happens to the current through an LDR when a torch is shone on it (constant p.d.).

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Method: R falls.

The current increases
30
Extended[1 mark]

State the function of a diode.

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Method: One-way.

It allows current to flow in one direction only
31
Extended[1 mark]

State the function of an LED.

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Method: Emits light, one-way.

It emits light when current passes through it in the forward direction
32
Extended[2 marks]

An LED in a circuit does not light. Suggest one reason.

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Method: Reverse bias.

It may be connected the wrong way round (reverse biased)
33
Extended[2 marks]

Describe the symbol for a diode.

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Method: Triangle and bar.

A triangle pointing to a bar (current flows in the direction the triangle points)
34
Extended[2 marks]

Explain why an LED is usually connected in series with a resistor.

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Method: Limit current.

To limit the current so the LED is not damaged
35
Extended[2 marks]

Diodes can be used to convert a.c. into d.c. Explain why.

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Method: One-way flow.

The diode only allows current in one direction, so it blocks the reverse half of each a.c. cycle
36
Extended[2 marks]

Describe the symbol for an LED.

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Method: Diode with arrows out.

A diode symbol with two arrows pointing outwards
37
Extended[2 marks]

State what is meant by a forward-biased diode.

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Method: Conducts.

A diode connected so that it conducts (current in the direction of the arrow)
38
Extended[2 marks]

State one advantage of using LEDs as indicator lamps.

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Method: Efficient.

Low power use, long life, efficient
39
Extended[3 marks]

Two LEDs are connected in parallel but in opposite directions to an a.c. supply. Describe what is seen.

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Method: Alternate.

Each LED lights during alternate half cycles (both appear lit)
40
Extended[2 marks]

An LED and resistor are in series with a 6 V battery. The battery is reversed. State what happens.

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Method: Reverse bias.

The LED goes out (no current flows)

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Draw (describe) a circuit to investigate how the current through a thermistor changes with temperature. Name each component.

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Method: Series with ammeter.

Cell/battery, switch, NTC thermistor in series with an ammeter; voltmeter across the thermistor; thermistor placed in a water bath with a thermometer
42
ChallengeCore[4 marks]

Describe a circuit using an LDR and a fixed resistor as a potential divider to provide an output that rises in the dark.

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Method: Output across LDR.

LDR and fixed resistor in series across the supply; take the output across the LDR; in the dark its resistance rises, so its share of p.d. rises
43
ChallengeCore[3 marks]

Explain the role of a relay in a circuit where a small sensor circuit controls a large motor.

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Method: Electrically separate switch.

The sensor circuit energises the relay coil, which closes contacts in a separate high-current motor circuit
44
ChallengeCore[4 marks]

Explain how a variable resistor can control the brightness of a lamp and what happens to the current as resistance increases.

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Method: Series control.

In series with the lamp; increasing resistance reduces the current, making the lamp dimmer
45
ChallengeCore[3 marks]

Explain why a fuse is placed in series with the device it protects.

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Method: All current passes.

All the current to the device passes through it, so it melts if the current is too large
46
ChallengeCore[4 marks]

A transformer and a magnetising coil look similar in circuit diagrams. Describe the symbol of each and their different functions.

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Method: Coils.

Transformer: two coils with a core between (lines); changes a.c. p.d. Magnetising coil: single coil; produces a magnetic field
47
ChallengeExtended[4 marks]

An LED rated 2.0 V, 20 mA is used with a 9.0 V battery. Calculate the series resistor needed and explain why it is needed.

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Method: R = (9.0 − 2.0)/0.020.

R = 350 Ω; it limits the current so the LED is not damaged
48
ChallengeExtended[4 marks]

Explain how four diodes or a single diode can be used to convert a.c. to d.c., and why a single diode gives only half the cycle.

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Method: Rectification.

A single diode conducts only when forward biased, so it passes half of each cycle (half-wave); four diodes in a bridge pass both halves in the same direction
49
ChallengeExtended[3 marks]

An LED lights when connected one way but not when reversed. Explain why, and state one advantage of LEDs over filament lamps.

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Method: Diode behaviour.

It is a diode: conducts only in forward bias; advantage: more efficient (less energy wasted as heat) and longer life
50
ChallengeExtended[4 marks]

A diode is connected in series with an expensive circuit and a battery. Explain how it protects the circuit if the battery is connected the wrong way round.

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Method: Reverse bias blocks current.

When reversed the diode is reverse biased and has a very high resistance, so almost no current flows and the circuit is not damaged
All IGCSE Physics topics← 4.2.5 Electrical Energy and Electrical Power4.3.2 Series and Parallel Circuits →

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