Circuit diagrams: standard symbols make every circuit readable
Definition
Circuit diagram
A circuit diagram is a drawing of an electric circuit that uses standard symbols for the components and straight lines for the connecting wires.
- Standard symbols let anyone read a circuit the same way, so a component is drawn with the same symbol whatever the real thing looks like.
- A circuit diagram shows how the components are electrically connected, not their real positions, sizes or appearance.
- Draw the wires as straight lines with right-angled corners; a filled dot shows wires are joined at a junction, and wires that cross without a dot are not connected.
The symbols you need to recognise and draw
- You should be able to recognise and draw each of these standard symbols:
- cell: one long line beside one short parallel line, with the long line as the positive terminal
- battery: two or more cell symbols in a row
- open switch: two contacts with a gap, so the circuit is incomplete
- closed switch: the two contacts joined, giving a complete path
- lamp: a circle with a cross inside
- resistor: a rectangle
- variable resistor: a resistor crossed by a diagonal arrow
- fuse: a small rectangle with a line through it
- thermistor: a resistor crossed by a diagonal line ending in a short bar
- light-dependent resistor (LDR): a resistor in a circle with two arrows pointing towards it
- diode: an arrowhead pointing towards a vertical line
- light-emitting diode (LED): the diode symbol with two arrows pointing away from it
- ammeter: a circle containing the letter A
- voltmeter: a circle containing the letter V
- The small arrows tell the light-sensitive parts apart: they point towards an LDR and away from an LED.
Common Mistake
- Do not draw a realistic picture of a component: a lamp is a circle with a cross, not a sketch of a bulb.
- Do not confuse a cell with a battery: a cell is one pair of lines, a battery is two or more pairs.
- Check whether a switch is open or closed, because an open switch leaves a gap and breaks that path.
Drawing and reading a circuit diagram
- To draw a circuit diagram, work through it in order:
- identify every component in the circuit
- replace each component with its standard symbol
- join them with straight lines for the wires
- arrange the symbols so the connections are clear
- check every symbol is complete and each junction is shown correctly
- To read a circuit diagram, find the power supply first, then trace each complete path, name the components in order, and watch for branches, junctions and open switches.
Example
Question: A diagram shows a cell connected to a closed switch, a lamp and an ammeter in one loop, with a voltmeter on a separate branch across the lamp. Describe what it shows.
Answer: The cell, closed switch, lamp and ammeter form one complete loop. The switch is closed, so there is a complete path. The ammeter measures the current through the lamp, and the voltmeter across the lamp measures the potential difference across it.
Exam technique
- When you draw a circuit, marks come from the correct standard symbols and the correct connections, so use a ruler and place A and V clearly inside their circles.
- When you read a circuit, describe only what is shown, and do not assume crossing wires are connected unless a junction dot is marked.
Self review
- What is the difference between a cell symbol and a battery symbol?
- Which component is a circle with a cross inside?
- How do you tell an ammeter from a voltmeter in a diagram?
- Which way do the arrows point on an LDR, and which way on an LED?
- What does a filled dot where wires meet tell you?
