# Circuit Diagrams, Components and Potential Dividers

> Physics · CIE IGCSE 0625 2026-2028
> Source: https://www.owlsprep.com/study/cie-0625-u4-circuit-diagrams-components-and-potential/

This guide covers circuit component symbols, diagram drawing rules, and component functions for CIE IGCSE Physics 0625 Core candidates, plus extended content on potential divider calculations and applications for Extended candidates.

**Prerequisites:** [Basic understanding of current, voltage and resistance in circuits](https://www.owlsprep.com/study/cie-0625-u4-current-voltage-resistance/); [Knowledge of series and parallel circuit rules](https://www.owlsprep.com/study/cie-0625-u4-series-parallel-circuits/)

## Learning objectives

- Recognise and draw standard circuit component symbols per CIE 0625 specification
- Draw and interpret valid series and parallel circuit diagrams
- Describe the function of common electrical components in circuits
- (Extended only) Calculate output voltage of a potential divider for fixed and variable resistors
- (Extended only) Explain applications of potential dividers in sensor circuits

## Circuit Component Symbols (Core)

**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. 2. Function: When closed, the switch completes the circuit, allowing current to flow through connected components.

## Drawing and Interpreting Circuit Diagrams (Core)

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.

> **Exam Marking Tip**
>
> Always mark permanent wire junctions with a small solid dot. Unmarked crossing wires are assumed to be disconnected, so missing dots will cost you marks.

**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. 2. Draw a horizontal line right from the positive battery terminal to the open switch symbol.
3. 3. 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. 4. Draw the voltmeter in parallel across the lamp, with junction dots marking the connections to the main circuit.

## Potential Divider Principles (Extended only)

**Potential Divider** — 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.

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

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

Where $V_{out}$ = 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: $V_{in} = 12V$, $R_1 = 10\Omega$, $R_2 = 20\Omega$
2. 2. Substitute into the potential divider formula:
3. $$V_{out} = 12 \times \frac{20}{10 + 20}$$
4. 3. Simplify and calculate:
5. $$V_{out} = 12 \times \frac{2}{3} = 8V$$

## Potential Divider Applications (Extended only)

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.

> **Sensor Circuit Rule**
>
> If the sensing component is $R_2$, $V_{out}$ increases when the sensor resistance increases, and falls when sensor resistance falls. If the sensor is $R_1$, the opposite relationship applies.

**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 $V_{out}$ at this temperature.

1. 1. Identify values: $V_{in}=9V$, $R_1=10k\Omega$, $R_2=20k\Omega$
2. $$V_{out} = 9 \times \frac{20}{10 + 20}$$
3. 2. Calculate the result:
4. $$V_{out} = 9 \times \frac{2}{3} = 6V$$
5. If temperature rises, the thermistor resistance falls, so $V_{out}$ will drop below 6V.

## Common pitfalls

- **Wrong:** Drawing voltmeters in series with components
  - Why it fails: Voltmeters have very high resistance, so connecting them in series stops current flow, giving invalid readings.
  - Correct: Always draw voltmeters in parallel across the component you are measuring potential difference for.
- **Wrong:** Forgetting to mark wire junctions with a solid dot
  - Why it fails: Unmarked junctions are treated as disconnected crossing wires, leading to incorrect circuit interpretations by markers.
  - Correct: Add a small solid dot at every point where two or more connecting wires are permanently joined.
- **Wrong:** Swapping $R_1$ and $R_2$ in the potential divider formula
  - Why it fails: This gives an inverted output voltage value, costing you calculation marks.
  - Correct: Confirm $R_2$ is the resistor across which you are measuring $V_{out}$ before substituting values into the formula.
- **Wrong:** Drawing components at the corners of circuit diagrams
  - Why it fails: This violates CIE marking rules for valid circuit diagrams, leading to lost marks even if the rest of the diagram is correct.
  - Correct: Place all components along straight horizontal or vertical connecting lines, never at diagram corners.
- **Wrong:** Assuming thermistor resistance increases with temperature
  - Why it fails: CIE 0625 only specifies negative temperature coefficient (NTC) thermistors, where resistance falls as temperature rises.
  - Correct: Always use the NTC relationship for all thermistor questions unless explicitly told otherwise.

## 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 | $V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$ |
| Sensor Rules | Extended | LDR resistance falls with light; thermistor resistance falls with temperature |

## 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.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0625-u4-circuit-diagrams-components-and-potential/