Electrical circuits
In this topic you will:
- learn how to draw and compare circuit diagrams
- learn the circuit symbols for cells, switches, lamps, buzzers and ammeters.
Key Words
ammeter, circuit diagram, circuit symbols
Circuit diagrams
Circuits can be shown in a drawing or a circuit diagram.
Advantages of circuit diagrams compared with pictures
- Circuit diagrams are easier to draw.
- The components have standard symbols.
- Wires are drawn with straight lines, which is easy to interpret.
Circuit symbols
Components in circuits can look very different.
The picture shows how different some lamps can look.
Standard circuit symbols are used in circuit diagrams so there is no confusion. The same symbols are used in all countries.
The table shows the names, symbols and functions of some components.
Common Mistake
Don’t confuse a symbol for an actual component. A real lamp may look different, but its circuit symbol is always the same.
In the circuit symbol for a cell, the positive is the longer of the two lines. The negative is the shorter line.
Take care when drawing circuit diagrams. Make sure:
- there are no gaps in the lines, especially at the corners and where wires meet components
- wires are not drawn through components.
Comparing circuit diagrams
Different circuits are used to do different jobs.
This circuit contains a cell, a switch and a buzzer. The circuit could be used in a doorbell. If the switch is pushed outside a door, it makes the buzzer sound and attracts attention inside.
This circuit contains a cell, a switch, a lamp and an ammeter. The ammeter measures the electric current. This circuit could be used to light a room and measure the current flowing through the circuit. By measuring the current, you could tell if the cell needs to be replaced – a smaller current means the cell is low on stored energy. The lamp would also become less bright, but you might not notice until too late and you find yourself sitting in the dark!
Important Concept
An ammeter is always connected in series with a component to measure the current flowing through it. It does not affect the circuit's operation if wired correctly.
Questions
Show Answer
A lamp is represented by a circle with a cross inside it.
Show Answer
A cell is shown by a long line (positive) next to a short line (negative).
Show Answer
A buzzer is drawn as a circle with a symbol resembling a small speaker or bell inside.
Show Answer
A closed switch is represented by a straight line connecting two dots or circles.
Show Answer
1. The switch is open, so the circuit is incomplete.
2. The cell terminals are reversed or doubled incorrectly — the negative is drawn at the top.
3. The lamp symbol is not connected correctly — it's missing a connecting wire.
Show Answer
Circuit D is correct. The ammeter is connected in series with the buzzer, which is necessary for measuring current accurately.
Think Like a Scientist
Work individually. Draw circuit diagrams for each of the following tasks:
- A. Two lamps that can be switched on and off together in a car.
- B. A buzzer on a front door with a lamp that comes on at the same time as the buzzer.
- C. Three lamps that operate all the time, with a way to measure current in the lamps.
Follow-Up Questions
Show Answer
They may be similar or different depending on interpretation. Circuit diagrams can vary in layout while still being functionally identical.
Show Answer
Yes. As long as the electrical function and component connections are the same, the visual layout can differ. Both can be correct.
Self-assessment
Write a number from 1 to 5 for each of the statements below. This shows how confident you are:
- 1 = not confident
- 5 = very confident
- I can remember all the circuit symbols in the table.
- I can draw circuit diagrams correctly and accurately.
- I can tell what the circuits in different circuit diagrams could be used for.