ELECTRICITY · CHAPTER 2 OF 13
Why must a circuit be complete?
A circuit can contain all the correct components and still not work. Just one small gap is enough to stop it—but why?
By the end, you’ll be able to…
- Explain why a circuit must form a complete loop.
- Distinguish between an open circuit and a closed circuit.
- Predict whether a component will operate in a given circuit.
A complete path
For an electric circuit to operate, all its components must be connected in a complete, unbroken loop.
The conducting path must travel from one terminal of the cell, through the connecting wires and components, and back to the other terminal of the cell.
If there is a gap anywhere in this path, charge cannot move all the way around the circuit. There is no electric current, so the lamp does not light.
Can you light the lamp?
Use the switch to close the gap. Watch what changes in the circuit.
What’s happening?
A complete loop allows current to flow
When the switch is closed, it completes the conducting path. Electric charge can move around the circuit and there is an electric current. The lamp lights.
When the switch is open, there is a gap in the path. Charge cannot move all the way around the circuit, so there is no current and the lamp remains off.
It does not matter where the gap occurs. A break anywhere in the loop stops the whole circuit from operating.
Open circuit
The conducting path contains a gap. Charge cannot move all the way around the circuit, so there is no current.
Closed circuit
The conducting path forms a complete loop. Charge can move around the circuit, so there is a current.
Remember it like this: a circuit is more like a circular running track than a one-way road. If a section of track is missing, nobody can complete a lap.In the same way, a gap anywhere in a circuit prevents charge from moving around the complete path.
What does a switch do?
A switch controls a circuit by opening or closing part of its conducting path.
- An open switch creates a gap, producing an open circuit.
- A closed switch completes the conducting path, producing a closed circuit.
The switch does not have to be beside the lamp or cell. In a circuit with one loop, opening the switch anywhere breaks the conducting path around the entire circuit.
Check your understanding
Choose an answer for each question. You will see feedback straight away.
Mia replaces a lamp with a motor. Explain what stays the same and what changes in the circuit.
The cell still provides energy, the wires still connect the components and the switch still controls the circuit.
The output changes. The lamp transfers energy by light and heating, whereas the motor transfers energy by movement and heating.
CHAPTER 2
Chapter summary
- A circuit must contain a complete, unbroken conducting path.
- A closed circuit has a complete path, so charge can move around it and there is an electric current.
- An open circuit contains a gap, so there is no current.
- A gap anywhere in a single loop stops the entire circuit from operating.
- A switch controls a circuit by opening or closing the conducting path.