# Drawing Electrical Circuits

> The standard symbols and rules for drawing circuit diagrams that anyone in the world can read.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/circuit-diagrams/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The electrician's note
Olivia's uncle Sam is an electrician. One evening he got a call from a builder: "Can you wire up the new classroom? I'll text you the plan."

The plan arrived. It was not a photo. It was a neat drawing of straight lines, little circles with crosses, and short and long bars. No pictures of real wires at all.

"Why not just send a photo?" asked Olivia.

"Because a photo is a tangle," said Sam. "This drawing uses symbols that every electrician in the world understands. I can see exactly what connects to what in five seconds."

**In short:** A circuit diagram shows how the parts of an electric circuit are connected, using standard symbols joined by straight lines. Ammeters go in series and voltmeters go in parallel across a component.

## Why use circuit diagrams?

Real circuits can look messy. A **circuit diagram** uses **standard symbols** joined by **straight lines** to show how components connect. It does not show what the parts look like or exactly where they sit. It only shows **how they are connected**.

## Standard circuit symbols

*Diagram: Circuit symbols: cell, battery, lamp, open switch, closed switch, resistor, variable resistor, ammeter, voltmeter, diode, fuse, motor*
*Common circuit symbols. In a cell, the long line is the positive (+) terminal.*

| Component | What it does |
|---|---|
| **Cell** | Provides voltage. Long line = positive, short thick line = negative. |
| **Battery** | Two or more cells joined together. |
| **Lamp** | Turns electrical energy into light. |
| **Switch** | Opens (breaks) or closes (completes) the circuit. |
| **Resistor** | Limits the current. |
| **Variable resistor** | Resistance can be changed, like a dimmer or volume knob. |
| **Ammeter** | Measures current. Connected in series. |
| **Voltmeter** | Measures voltage. Connected in parallel. |
| **Diode** | Lets current flow in one direction only (the way the arrow points). |
| **Fuse** | A thin wire that melts if the current is too big, protecting the circuit. |
| **Motor** | Turns electrical energy into kinetic energy. |

### Key idea: A battery is not a cell
Strictly, a single unit is a **cell**. A **battery** is several cells joined together. An AA "battery" is really one cell. A 9 V battery has six small cells inside.

## Rules for drawing a good circuit diagram

1. Use a **ruler**. Draw wires as **straight lines** with **square corners**.
2. Use the **standard symbols** only. Do not draw pictures.
3. Make sure there are **no gaps** in the loop, unless there is an open switch.
4. Do not draw a component on a corner. Put it on a straight side.
5. Show where wires **join** with a solid dot. Wires that cross without a dot are **not** connected.
6. Draw ammeters **in line** with the component. Draw voltmeters **beside** the component, joined on either side of it.

### Watch out: A gap means no current
If there is a gap anywhere in the loop, **no** current flows anywhere in that loop. Current needs a **complete path** from one terminal of the battery, through the components, back to the other terminal.

## A simple circuit

*Diagram: A simple circuit: a cell, a closed switch and a lamp in one loop, with an ammeter in series and a voltmeter across the lamp*
*Cell, switch, lamp and ammeter in one loop. The voltmeter (blue) is joined across the lamp.*

Follow the loop with your finger. From the cell, through the lamp, through the ammeter, through the switch, back to the cell. One complete loop. Close the switch and the lamp lights.

## Short circuits

A **short circuit** happens when current finds an easy path with almost **no resistance**, skipping the components. The current becomes very large. Wires get hot, batteries drain fast, and there is a risk of fire. Fuses and circuit breakers are there to cut the power if this happens.

**Mind map**

```text
Circuit diagrams
  Symbols
    Cell and battery
    Lamp, resistor
    Switch open or closed
    Ammeter, voltmeter
    Diode, fuse, motor
  Rules
    Ruler, straight lines
    Square corners
    Dot = joined
    No gaps
  Meters
    Ammeter in series
    Voltmeter in parallel
  Safety
    Short circuit
    Fuse
    Circuit breaker
```

### Year 11: Sensors and potential dividers
Some components change resistance with conditions:

- **LDR** (light dependent resistor): resistance **drops** in bright light.
- **Thermistor**: resistance **drops** as it gets hotter (most common type).

Two resistors in series share the supply voltage in proportion to their resistance. This is a **potential divider**:

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

Swap $R_2$ for an LDR and $V_{\text{out}}$ changes with light. This is how automatic street lights switch on at dusk.

### Year 12: Kirchhoff's laws
Two rules let you analyse any circuit, however complicated:

- **Junction rule** (conservation of charge): the total current into a junction equals the total current out. $\sum I_{\text{in}} = \sum I_{\text{out}}$
- **Loop rule** (conservation of energy): around any closed loop, the voltages gained equal the voltages used. $\sum V = 0$

These are the two reasons series and parallel circuits behave the way they do in the next chapter.

## Quick summary

- Circuit diagrams use standard symbols and straight lines to show connections.
- Cell: long line positive, short line negative. A battery is several cells.
- Draw with a ruler, square corners, no gaps, dots where wires join.
- Ammeters go in series. Voltmeters go in parallel across a component.
- A fuse melts to break the circuit if the current gets too high.
- A short circuit is a low-resistance path that causes dangerously large current.

## Quiz

Try these questions from the chapter quiz.

1. In the symbol for a cell, the longer line represents the:
   - A. Positive terminal
   - B. Negative terminal
   - C. Switch
   - D. Fuse
   - Answer: A. Positive terminal. Long line positive, short line negative.

2. A circle with a cross inside it is the symbol for a:
   - A. Motor
   - B. Lamp
   - C. Ammeter
   - D. Diode
   - Answer: B. Lamp. The lamp symbol is a circle with a cross.

3. Which component lets current flow in only one direction?
   - A. Resistor
   - B. Fuse
   - C. Diode
   - D. Lamp
   - Answer: C. Diode. A diode is like a one-way valve for current.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/circuit-diagrams/#quiz