Circuit Diagrams, Components and Potential Dividers

Physics· 4.3.1, 4.3.3· 25 min read

1. Circuit Component Symbols (Core)★★☆☆☆⏱ 8 min

📘 Definition

Circuit Component Symbol

Standardised graphical representation of an electrical component used in circuit diagrams, consistent across international exam boards.

Example:

A circle with a cross inside is the official symbol for a filament lamp.

All CIE IGCSE Physics 0625 candidates must memorise core component symbols to draw and interpret circuit diagrams correctly. The table below lists the most frequently tested symbols and their functions:

Component	Symbol Description	Function
Filament lamp	Circle with cross inside	Emits light when current flows through its filament
Fixed resistor	Rectangle with leads on either end	Limits current flow to a fixed value
Ammeter	Circle with capital A inside	Measures current in a circuit, connected in series
Voltmeter	Circle with capital V inside	Measures potential difference across a component, connected in parallel
LDR	Circle with rectangle inside and two incoming arrows	Resistance falls as light intensity increases
Thermistor	Circle with rectangle inside and notched diagonal line	Resistance falls as temperature increases

📐 Worked Example

Draw the symbol for a closed switch and state its function in a circuit.

1
1. Draw a straight horizontal line, with a lever connected at the left end touching the right end of the line to form a continuous path.
2
1. Function: When closed, the switch completes the circuit, allowing current to flow through connected components.

2. Drawing and Interpreting Circuit Diagrams (Core)★★★☆☆⏱ 7 min

Circuit diagrams must follow strict marking rules to be valid: all connections are drawn as straight horizontal or vertical lines, components are placed at right angles to connecting wires, no components are placed at diagram corners, and all symbols match the 0625 specification.

📐 Worked Example

Draw a circuit diagram for a series circuit with a 2-cell battery, open switch, filament lamp, ammeter, and a voltmeter connected across the lamp.

1
1. Draw the 2-cell battery symbol (two long-short parallel line pairs connected end to end) on the left of your diagram.
2
1. Draw a horizontal line right from the positive battery terminal to the open switch symbol.
3
1. Draw a line right from the switch to the filament lamp, then to the ammeter, then back to the negative battery terminal to complete the series loop.
4
1. Draw the voltmeter in parallel across the lamp, with junction dots marking the connections to the main circuit.

3. Potential Divider Principles (Extended only)★★★★☆Extended only⏱ 5 min

📘 Definition

Potential Divider

V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}

A series circuit of two or more resistors connected across a voltage source, designed to produce an output voltage that is a fixed fraction of the source voltage.

V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}

Where $V_{o u t}$ = output voltage across $R_{2}$ , $V_{in}$ = total source voltage, $R_{1}$ = series resistor next to the source, and $R_{2}$ = resistor across which output voltage is measured.

📐 Worked Example

A potential divider uses a 10Ω fixed resistor ( $R_{1}$ ) and 20Ω fixed resistor ( $R_{2}$ ) connected across a 12V battery. Calculate the output voltage across $R_{2}$ .

1
1. Identify values: $V_{in} = 12 V$ , $R_{1} = 10Ω$ , $R_{2} = 20Ω$
2
1. Substitute into the potential divider formula:
3
$V_{out} = 12 \times \frac{20}{10 + 20}$
4
1. Simplify and calculate:
5
$V_{out} = 12 \times \frac{2}{3} = 8V$

4. Potential Divider Applications (Extended only)★★★★☆Extended only⏱ 5 min

Potential dividers are widely used in sensor circuits, where one resistor is replaced by a sensing component (LDR or thermistor). This allows the output voltage to change automatically in response to environmental conditions.

📐 Worked Example

A potential divider uses a thermistor as $R_{2}$ in series with a 10kΩ fixed resistor $R_{1}$ connected across a 9V battery. At 20°C, the thermistor resistance is 20kΩ. Calculate $V_{o u t}$ at this temperature.

1
1. Identify values: $V_{in} = 9 V$ , $R_{1} = 10 k Ω$ , $R_{2} = 20 k Ω$
2
$V_{out} = 9 \times \frac{20}{10 + 20}$
3
1. Calculate the result:
4
$V_{out} = 9 \times \frac{2}{3} = 6V$
5
If temperature rises, the thermistor resistance falls, so $V_{o u t}$ will drop below 6V.

5. Common Pitfalls

Wrong move:

Drawing voltmeters in series with components

Why:

Voltmeters have very high resistance, so connecting them in series stops current flow, giving invalid readings.

Correct move:

Always draw voltmeters in parallel across the component you are measuring potential difference for.

Wrong move:

Forgetting to mark wire junctions with a solid dot

Why:

Unmarked junctions are treated as disconnected crossing wires, leading to incorrect circuit interpretations by markers.

Correct move:

Add a small solid dot at every point where two or more connecting wires are permanently joined.

Wrong move:

Swapping $R_{1}$ and $R_{2}$ in the potential divider formula

Why:

This gives an inverted output voltage value, costing you calculation marks.

Correct move:

Confirm $R_{2}$ is the resistor across which you are measuring $V_{o u t}$ before substituting values into the formula.

Wrong move:

Drawing components at the corners of circuit diagrams

Why:

This violates CIE marking rules for valid circuit diagrams, leading to lost marks even if the rest of the diagram is correct.

Correct move:

Place all components along straight horizontal or vertical connecting lines, never at diagram corners.

Wrong move:

Assuming thermistor resistance increases with temperature

Why:

CIE 0625 only specifies negative temperature coefficient (NTC) thermistors, where resistance falls as temperature rises.

Correct move:

Always use the NTC relationship for all thermistor questions unless explicitly told otherwise.

6. Quick Reference Cheatsheet

Topic	Tier	Key Information
Component Symbols	Core	Ammeter = series, Voltmeter = parallel; memorise all standard symbols
Circuit Diagram Rules	Core	Straight wires, no components at corners, mark junctions with dots
Potential Divider Formula	Extended	$V_{o u t} = V_{in} \times \frac{R _{2}}{R _{1} + R _{2}}$
Sensor Rules	Extended	LDR resistance falls with light; thermistor resistance falls with temperature

7. Frequently Asked

Do I need to memorise all component symbols for the exam?

Yes, CIE expects you to recall and draw all standard symbols listed in the 0625 syllabus, including resistors, lamps, switches, meters, LDRs, thermistors, diodes and LEDs.

Can Core candidates skip potential divider content?

Yes, potential dividers are exclusively part of the Extended (Supplement) syllabus for 0625, so Core candidates do not need to study this section for Papers 1 and 3.

Going deeper

syllabusCIE IGCSE Physics 0625 2026-2028 SpecificationRefer to sections 4.3.1 and 4.3.3

What's Next

Now that you have mastered circuit diagrams, components and potential dividers, you are ready to progress to more advanced electricity topics in the CIE IGCSE Physics 0625 syllabus. Next, you will learn to calculate current, voltage and resistance in series and parallel circuits, followed by the function of mains electricity safety features such as fuses and circuit breakers. Extended candidates will also study electronic switches like transistors and relays, which build directly on the potential divider concepts covered in this guide. Practice drawing circuit diagrams regularly and work through past paper questions to reinforce your understanding, as this topic appears frequently in both Core and Extended exams.