Circuit Diagrams & Circuit Components (Cambridge (CIE) O Level Physics): Revision Note

Circuit Components

• The diagram below shows the circuit symbols that could be used in circuit diagrams

  • You will be expected to know what each component is and how it behaves in a circuit

Circuit Symbols

Common circuit symbols used in circuit diagrams

Power supplies

• Cells, batteries, power supplies and generators all supply current to the circuit

Resistors

• Fixed and variable resistors, thermistors and light-dependent resistors (LDRs) are all used to control current

• Potential dividers use resistors in series to divide the potential difference

Meters

• Ammeters and voltmeters are used to measure the current and potential difference

  • Ammeters are always connected in series whilst voltmeters are always connected in parallel

Electromagnetic Components

• Magnetising coils, relays and transformers use electromagnetic effects

  • Relays use a small current in one circuit to switch on a much larger current in another

  • Transformers 'step up' and 'step down' an alternating potential difference

Fuses

• Melt and break the circuit if the current becomes too large, which stops the cables overheating and acts as a safety measure against fire 

Diodes

• A light-emitting diode (LED) is a type of semiconductor diode that emits light when current flows in a specific direction

• In the reverse direction an LED has a very high resistance, so almost no current flows and it does not emit light

• LEDs only require a current of about 20 mA and a potential difference of about 2 V to produce a light output

Circuit Symbol for a Light Emitting Diode (LED)

For light to be emitted, current will be flowing from left to right in the direction the triangle is pointing; the arrows pointing away from the diode represent light rays being emitted

• They are an efficient way of indicating the state of an output in electronic circuits

  • For example, which path the current is flowing through when switches and relays are used

• LEDs are likely to be connected to series resistors to limit the current flow so they don’t become overloaded

• Remember that LEDs will emit light when they are forward biased in the same direction as conventional current flow (positive to negative) and will not emit light when in the opposite direction to the current flow