4.3.1 Circuit diagrams and circuit components
- Syllabus
- 0625–2026–2027
- Topic
- 4.3.1
- Level
- —
Learning objectives
4.3.1.1Circuits with cells, batteries, power• Draw/interpret circuits with cells, batteries, power supplies, generators, potential dividers, switches, resistors, heaters, thermistors (NTC), LDRs, lamps, motors, bells, ammeters, voltmeters, magnetising coils, transformers, fuses and relays; know each component behaviour4.3.1.2Circuit diagrams containing diodes and• Draw and interpret circuit diagrams containing diodes and light-emitting diodes (LEDs) and know how these components behave in the circuit
Read and draw standard circuit diagrams
A circuit diagram uses standard symbols to show what each component is and lines to show how the components are connected. The exact position or shape of the drawn path does not matter; the same junctions, branches and component order must be preserved.
| Component | Behaviour in a circuit |
|---|---|
| cell / battery | a cell is one source; a battery is two or more cells and provides a potential difference |
| power supply / generator | supplies electrical energy and a potential difference to drive current |
| switch | closed completes a conducting path; open breaks it |
| fixed / variable resistor | opposes current; a variable resistor lets the resistance be changed |
| potential divider | two or more series resistors with an output taken across part of the arrangement |
| NTC thermistor | resistance decreases as temperature increases |
| LDR | resistance decreases as light intensity increases |
| fuse | melts and breaks the circuit if the current becomes too large |
| Component | Behaviour or correct connection |
|---|---|
| heater / lamp | transfers electrical energy mainly to heating / to light and heating |
| motor / bell | transfers electrical energy to movement / sound |
| magnetising coil | produces a magnetic field when current flows |
| transformer | changes an alternating potential difference |
| relay | an electromagnetically operated switch lets one circuit control another |
| ammeter | measures current and is connected in series in the path being measured |
| voltmeter | measures potential difference and is connected in parallel across a component |
To convert a physical circuit into a diagram: identify every component, replace it with the correct standard symbol, trace each conducting path, and reproduce every branch and junction. A crossing is a connection only when the diagram marks it as a junction; label source polarity and meter positions when they matter.
Connectivity carries the meaning: moving a symbol around the page does not change the circuit, but moving it to a different branch does. Detailed calculations for potential dividers and combined resistors belong to later objectives.
Use diodes and LEDs in circuit diagrams
A diode is a component that allows conventional current in one direction and blocks it in the opposite direction. Its circuit symbol therefore has a direction: the bar marks the cathode side, and forward current is from anode to cathode.
| Connection | Behaviour |
|---|---|
| forward biased | the anode is at a higher potential than the cathode, so the diode conducts |
| reverse biased | the diode blocks current, so the branch behaves approximately like a break in the circuit |
| LED forward biased | the LED conducts and emits light |
| LED reverse biased | it does not conduct or emit light |
An LED is drawn as a diode symbol with two small arrows pointing outwards to represent emitted light. It is normally connected in series with a resistor that limits the current. Reversing the LED reverses its bias, so a complete-looking circuit may still carry no current through that branch.
When interpreting a diode circuit, first mark the source polarity, then compare it with the diode direction, and only then decide which branches can conduct. A single diode in an alternating-current circuit conducts during one half-cycle and blocks during the other, producing current in only one direction through the load.
The two arrows on an LED point away from the symbol; arrows pointing towards a resistor identify an LDR. A diode controls direction because of its orientation—it is not a source and it is not an externally operated switch.