Students should be able to draw and interpret circuit diagrams using standard symbols for power sources, switches, lamps, resistors, measuring instruments, fuses, bells, LEDs, thermistors and light-dependent resistors.
A circuit diagram is a simplified drawing of an electrical circuit. It does not show the real shape or position of the components. Instead, it uses standard symbols so that the connections and function of the circuit can be understood clearly.
| Component | How to recognise or use it in a diagram |
|---|---|
| Cell | One long and one short parallel line. The long line is the positive terminal. |
| Battery | Two or more cells joined together. |
| d.c. supply | A source with fixed positive and negative terminals. |
| a.c. supply | A source whose terminals change polarity repeatedly. |
| Switch | A break in the circuit that can be opened or closed. |
| Lamp | An output component that lights up when current flows. |
| Fixed resistor | Limits current or shares potential difference in a circuit. |
| Variable resistor | A resistor with an arrow through it, showing that its resistance can be changed. |
| Potentiometer | A three-terminal variable resistor used as a variable potential divider. |
| Fuse | A safety component connected in series with the circuit it protects. |
| Ammeter | Drawn as a circle labelled A. It must be connected in series so the circuit current passes through it. |
| Voltmeter | Drawn as a circle labelled V. It must be connected in parallel across the component being measured. |
| Bell | An output component that produces sound when current flows. |
| LED | A diode symbol with arrows showing light emitted. It must be connected in the forward direction to light up. |
| NTC thermistor | A resistor symbol with a temperature marker. Its resistance decreases when temperature increases. |
| LDR | A resistor symbol with arrows shining onto it. Its resistance decreases when light intensity increases. |
An ammeter measures current. It is connected in series with the component because it must measure the charge flow passing through that part of the circuit. A good ammeter has very low resistance so it does not significantly change the current.
A voltmeter measures potential difference. It is connected in parallel across a component because potential difference is the energy transferred per unit charge between two points. A good voltmeter has very high resistance so only a very small current flows through it.
When reading a circuit diagram, start at one terminal of the power source and trace every complete path back to the other terminal. Components along the same single path are in series. Components on separate branches that connect across the same two points are in parallel.
For example, if two lamps are in series, the same current passes through both lamps. If the circuit is broken at either lamp, both lamps go out. If two lamps are in parallel, each lamp has its own branch. One lamp can still light even if the other branch is broken.