Circuit symbols
- Syllabus
- 9702–2028–2029
- Topic
- —
- Level
- A2
Learning objectives
—C.1-Circuit symbols used in examination papersRecognise, use and interpret the circuit symbols that may appear in examination papers, including cells, batteries, power supplies, switches, conductors, lamps, resistors, thermistors, LDRs, meters, diodes, capacitors, oscilloscopes and transducers.—C.2-Drawing and interpreting circuit diagramsDraw and interpret circuit diagrams using the official symbol set, connecting symbol recognition to current, potential difference, resistance, component behaviour and measurement choices.
Recognise circuit symbols by distinctive feature, direction and function
Source/control family: a cell is one long and one short parallel line; a battery repeats cells; a d.c. supply has marked polarity; switches show open/closed contacts; earth is the reference-potential connection.
Passive family: fixed/variable resistors, thermistor and LDR share resistive behaviour but have distinct modifier marks; a capacitor has two plates (one curved for a polarised form where specified); an inductor/transformer uses coil symbols.
A diode conducts in its forward direction and blocks reverse current. An LED is a diode with arrows pointing away (light emitted); a photodiode has arrows pointing toward it (light received). Preserve the diode bar/orientation when copying.
Meters are circles labelled A or V; an oscilloscope/CRO symbol marks waveform display. Output/transducer symbols include lamp, heater, motor, loudspeaker, microphone, relay and sensors—identify whether each converts electrical energy to another form or the reverse.
Recognition routine: name the family, identify the distinguishing mark, note polarity/direction, then state the component's circuit role. Use the exact official appendix form when drawing in an examination.
Similar outline is not enough: arrows away versus toward distinguish LED and photodiode, and a repeated-cell battery is not the same as a single cell. Exact symbol geometry must follow the official paper appendix.
Build and read circuit diagrams by tracing nodes, polarity and component role
A continuous conductor is one node at one potential. A filled junction dot joins branches; crossing lines without a junction mark are not automatically connected. Redraw complicated layouts by preserving nodes, not page position.
Connect an ammeter in series with the branch whose current is measured; connect a voltmeter in parallel across the two nodes of the component. Put the positive meter terminal at the higher-potential side for a positive reading.
FortwoseriesresistorsacrossVin:Vout=VinRoutput/(R1+R2)Identifywhichresistorliesbetweentheoutputnodeandreference/earthbeforesubstituting.
For a diode/LED output, first predict which node is at higher potential, then orient the diode for forward current in the required state. Opposite-parallel LEDs can indicate positive versus negative output; label each only after tracing polarity.
A relay separates the low-power sensing/control coil from the switched load circuit. Connect the coil to the intended output and use the relay contact in the separate load supply; include required protective/directional components as shown by the task.
Diagram audit: source present, switch/control sensible, no unintended short circuit, meter topology correct, junctions unambiguous, polarised components oriented, output path complete. Explain function as input change → component change → voltage/current change → output response.