To trace a circuit, follow the path of conventional current from the positive terminal, through each component, and back to the negative terminal. If the path is broken anywhere, no current flows.
This lesson belongs to SPM subjects and learning resources. Real wiring and testing are practical work for your school workshop, so this lesson stays on paper.
How do I trace a circuit?
Use a pencil, and trace once for each state of the circuit.
- Find the positive terminal of the source.
- Move along the wire to the next component, and name it.
- Continue until you return to the negative terminal.
- If you reach a gap or a dead end, the circuit is incomplete.
Naming each component as you pass it builds the path in words, which is what an exam answer needs.
Worked example: switch, resistor and LED
An original circuit, described so you can sketch it: a 5 V battery, a switch, a 300 Ω resistor and an LED, all connected in a single loop. The LED symbol points from the resistor towards the negative terminal.
Switch open. Start at the positive terminal and move to the switch. The switch has a gap, so the path stops there. No current flows, and the LED stays off.
Switch closed. The gap is closed. Current flows from the positive terminal, through the switch, then the resistor, then the LED, and back to the negative terminal. The LED lights.
Explanation. A complete loop is needed for current to flow. The resistor limits the current so the LED is not damaged.
The mistake that costs marks
A common slip is to say the current “is used up” as it passes the LED, so less returns to the battery. The current is the same at every point in a single loop.
| Statement | Verdict | Correction |
|---|---|---|
| Current is used up by the LED | Wrong | The same current flows through every component in a single loop |
| Energy is transferred by the LED | Correct | The LED changes electrical energy into light |
| The switch is open so the LED is dim | Wrong | An open switch means no current, so the LED is off |
The fix is to separate current from energy. Current is the flow of charge, and it continues around the loop. Energy is what the components transfer.
Why trace in two states?
Exam questions often ask what happens when a switch changes state. If you trace both, you can answer “what changes and why” from the same sketch.
Write one sentence for each state, such as “the switch is open, so the path is broken and no current flows”. This shows the marker you know the condition for current.
Check yourself
A circuit has a 6 V battery, a switch and two lamps in one loop. The switch is closed but one lamp has a broken filament. What happens to the other lamp, and why?
Answer
Both lamps go out. A broken filament is a gap in the loop, so the current has no complete path from the positive terminal to the negative terminal.
The other lamp is intact but no current reaches it, so it stays off. In a single loop, one break stops the current everywhere.
What to study next
Go on to explaining the role of labelled components, then try the original mixed practice.
For a teacher to trace circuits from your own notes, see online one-to-one Electrical and Electronic Engineering Studies tuition.