Question
A battery, a wire, a bulb, and a switch are connected together. When the switch is closed, the bulb glows. When the switch is open, it doesn’t. Why?
Draw and explain a simple electric circuit showing how current flows.
Solution — Step by Step
Every circuit has four basic jobs to fill: a source of energy (battery), a path for current (wire), a device that uses the energy (bulb), and a way to control the flow (switch). Without any one of these, the circuit can’t do its job properly.
Electric current is the flow of tiny charged particles called electrons through a conductor (the wire). Think of it like water flowing through a pipe — the battery is the pump, the wire is the pipe, and the bulb is a water wheel that spins (and glows) when water flows through it.
When the switch is closed, the circuit is complete — there’s an unbroken path from one terminal of the battery, through the wire, through the bulb, and back to the other terminal. Current flows continuously around this loop. The bulb glows because the current heats the thin wire (filament) inside it.
When we open the switch, we create a gap in the path. Current cannot jump across a gap in air (under normal conditions). The loop is broken, so no current flows — the bulb goes dark. This is called an open circuit.
We use standard symbols in circuit diagrams. The bulb glows only when the path is complete — this is called a closed circuit.
| Component | Symbol |
|---|---|
| Battery | Two parallel lines (long = positive, short = negative) |
| Wire | Straight lines connecting components |
| Bulb | Circle with a cross inside |
| Switch (open) | Gap in the line |
| Switch (closed) | Line completing the path |
Why This Works
Current needs a closed loop to flow — this is the single most important idea in all of electricity. There’s no “one-way ticket” for electrons; they need a complete path from the battery’s negative terminal, through the circuit, and back to the positive terminal.
The battery does work by pushing electrons through this loop using chemical energy stored inside it. As electrons pass through the filament of the bulb, they face resistance — this resistance converts electrical energy into heat and light.
A good memory trick: “Current flows only in a closed ring.” Open = ring broken = no current. Closed = ring complete = current flows. This simple rule will carry you through Class 6, Class 10, and even beyond.
Alternative Method
Instead of tracing current flow, think of it as a water analogy:
- Battery = Water pump (provides pressure)
- Wire = Pipe (carries water)
- Bulb = Water wheel (does work when water flows)
- Switch = Valve (open valve = water flows; closed valve = water stops)
When the valve (switch) is open, the pipe has a gap — no water (current) can flow, so the wheel (bulb) doesn’t spin (glow). When you close the valve, water rushes through and the wheel turns.
This analogy works well for Class 6. By Class 10, we’ll replace it with more precise language — but the core idea stays the same.
Common Mistake
Confusing “open circuit” and “closed circuit” — this trips up almost every Class 6 student at least once.
Students often think “open” means the circuit is open and ready to use (like an open shop), so current should flow. But in physics, “open circuit” means the path is open (broken) — current CANNOT flow.
Correct way to remember it: A closed circuit = a closed ring = current can go around. An open circuit = a broken ring = current is stuck.
Key Answer: A bulb glows only in a closed circuit — when there is a complete, unbroken path from the battery through the wire and bulb and back. Opening the switch breaks this path, creating an open circuit where no current can flow.