Circuit symbols (AQA GCSE Physics Combined Science): Revision Notes
Circuit symbols
What are circuit symbols?
Electric circuit diagrams use agreed symbols that everyone around the world can understand. This means that no matter where you are, you can read and draw circuit diagrams using the same symbols.
Circuit symbols are truly universal - an electrician in Japan uses the exact same symbols as an engineer in Brazil or a student in the UK. This standardisation makes electrical work and communication possible across all countries and cultures.
Learning these symbols is essential for understanding how electrical circuits work and for drawing your own circuit diagrams.
Key circuit components and their symbols
Here are the important circuit symbols you need to know:
Power sources
- Cell: Shows a single cell with positive and negative terminals. It provides electrical energy to push current around a circuit.
- Battery: Made up of two or more cells joined together. It also provides electrical energy but usually gives a higher voltage than a single cell.
Cell vs Battery: While we often use these terms interchangeably in everyday language, technically a cell is a single unit and a battery is multiple cells connected together. However, even a single cell is sometimes called a battery in common usage.
Control components
- Switches: These can be open or closed. When closed, they allow current to flow. When open, they stop the current flow completely.
Measuring devices
- Voltmeter: This measures the potential difference (voltage) across any component in a circuit. The symbol shows a 'V' inside a circle.
- Ammeter: This measures the current flowing through a component. The symbol shows an 'A' inside a circle.
Critical Difference: Remember that ammeters measure current through components (connected in series), while voltmeters measure voltage across components (connected in parallel). Getting this wrong can damage the metres!
Resistors
- Fixed resistor: This always provides the same resistance to current flow. It's shown as a simple rectangle.
- Variable resistor: This can be adjusted to change the amount of resistance. It's shown as a rectangle with an arrow through it.
Special resistors
- Thermistor: Its resistance changes when temperature changes. As temperature increases, resistance decreases.
- LDR (Light Dependent Resistor): Its resistance changes when light intensity changes. As light gets brighter, resistance decreases.
Practical Applications
Thermistor example: In a car's cooling system, a thermistor detects engine temperature. As the engine gets hotter, the thermistor's resistance drops, sending a stronger signal to turn on the cooling fan.
LDR example: Street lights often use LDRs. During the day when light is bright, the LDR has low resistance. As it gets dark, resistance increases, triggering the light to turn on automatically.
Output devices
- Lamp: Lights up when current flows through it. It's shown as a circle with a cross inside.
- LED (Light Emitting Diode): A special type of diode that gives out light when current flows through it. Current can only flow in one direction.
- Diode: Only allows current to flow in one direction. It's like a one-way valve for electricity.
LED vs Lamp: An LED will only allow current to pass through it in one direction (like all diodes), while a normal lamp allows current to flow both ways. If you connect an LED backwards in a circuit, it won't light up and could be damaged!
Safety components
- Fuse: A safety device that melts if too much current flows through a circuit. This protects other components from damage.
Safety First: Fuses are critical safety devices. When a fuse "blows" (melts), it's doing its job by preventing dangerous overcurrent that could cause fires or damage expensive equipment. Never replace a fuse with a higher rating - this defeats the safety protection!
Practical applications
Output devices are components like lamps, LEDs, buzzers, and motors. These are called output devices because they convert electrical energy into other forms of energy like light, sound, or movement.
Energy Conversion Examples
Lamp: Electrical energy → Light energy (and heat)
Buzzer: Electrical energy → Sound energy
Motor: Electrical energy → Kinetic energy (movement)
LED: Electrical energy → Light energy (very efficient, minimal heat)
Understanding how these components work together helps you design circuits that can control lights, make sounds, or create movement - the building blocks of almost all electronic devices.
Key Points to Remember:
- Circuit symbols are universal - they're the same all over the world
- Learn the difference between a cell (single) and battery (multiple cells)
- Output devices convert electrical energy into other forms like light or sound
- LEDs only work when current flows in the correct direction
- Thermistors and LDRs change their resistance based on temperature and light
- Ammeters measure current through components, voltmeters measure voltage across components
- Fuses are essential safety devices that protect circuits from overcurrent