Circuit Symbols
- All students in the WJEC GCSE examination will be expected to recognise the following standard symbols and be able to construct circuits using them:
Collection of Common Circuit Symbols
Included in the diagram are the most common circuit symbols needed for the exam
- The function of the most common components are:
- Cell / battery: Provides the circuit with a source of voltage. A battery is two or more cells
- Switch: Turn the circuit on (closed), or off (open)
- Fixed resistor: A resistor limits the flow of current. A fixed resistor has a resistance that cannot be changed
- Variable resistor: A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls
- Thermistor: The resistance of a thermistor depends on its temperature. As temperature increases, the resistance of the thermistor decreases and vice versa
- Light-dependent resistor (LDR): The resistance of an LDR depends on the light intensity. As the light intensity increases, the resistance of the LDR decreases and vice versa
- Diode: A diode allows current to flow in one direction only (towards the point of the triangle). They are used to convert alternating to direct current
- Light-emitting diode (LED): This is equivalent to a diode that emits light when a current passes through it. LEDs are used for lighting and displays (TVs, road signs, household bulbs)
- Ammeter: Used to measure the current in a circuit. Connected in series with other components
- Voltmeter: Use to measure the voltage of an electrical component. Connected in parallel with the relevant component
- Each of these components have an electrical resistance that may impact the current in the circuit
- However, the resistance of the ammeter and voltmeter are taken as negligible in exam questions
Drawing & Interpreting Circuit Diagrams
- Being able to draw and interpret circuit diagrams using circuit symbols is an essential skill in the electricity topic
- Electric circuit diagrams require the following to work effectively:
- An energy source – This is a source of voltage so that a current can flow. This can be a cell, battery, or a power supply
- A closed path or a complete circuit – Electrons need to flow in a complete loop for a current to flow. A circuit can be open and closed using a switch
- Electrical components – These could act as sensors that respond to the environment (LDR, thermistor), or measure a value (ammeter, voltmeter), or transfer energy (LED, lamp). These must be drawn with the correct circuit symbol
- The key rules to remember are:
- An ammeter is always connected in series
- A voltmeter is always connected in parallel to the component the voltage is being measured
- The direction of conventional current is always from the positive to the negative terminal of the power supply
Worked example
Which circuit diagram correctly represents a circuit with current flowing through?
Answer:
- For a circuit to be connected, the switch must be closed
- This discounts circuits A and C
- The other circuit symbol is a diode
- Diodes only allow current to flow in one direction (towards the point of the triangle)
- Since conventional current flows from positive to negative, a forward-biased diode must point in this direction in order for the current to flow
- This is seen in circuit B
Exam Tip
When asked to draw a circuit diagram, make sure to draw the wires as straight lines with a straight edge or a ruler and make it as neat as possible, especially the circuit symbols.
If the diagram is too small or there is ambiguity as to what a symbol represents, the examiner may not award you full marks!
Make sure the lines of your wires meet. There should be no gaps or it is not a complete circuit!
Think of the triangle diode symbol as an arrow pointing in the direction that the current can flow.
