Question
Draw a simple circuit consisting of a bulb, a switch, and a cell (battery). Explain the difference between a closed circuit and an open circuit, and what happens to the bulb in each case.
Solution — Step by Step
A simple circuit has three essential components:
- Cell (battery): Source of electrical energy. Symbol: two parallel lines of unequal length (long = positive terminal, short = negative terminal).
- Bulb (lamp): Converts electrical energy to light. Symbol: circle with an X or a loop inside.
- Switch: Controls current flow. Symbol: a gap in the wire with a small opening or connecting lever.
- Connecting wires: Conduct electricity. Symbol: straight lines.
In a circuit diagram, these symbols are connected by lines representing wires.
Closed circuit: When the switch is ON (closed), there is a complete, unbroken path for current to flow.
Current path: positive terminal of cell → wire → switch (closed) → wire → bulb → wire → negative terminal of cell.
Since the circuit is complete:
- Charge flows continuously
- Current exists throughout the circuit
- The bulb glows (converts electrical energy to light and heat)
Think of it like a complete loop: water can flow around a circular pipe. If the pipe is intact throughout, water flows.
Open circuit: When the switch is OFF (open), there is a break in the conducting path.
The switch creates a gap in the wire. No charge can “jump” across this air gap (for normal household voltages).
Consequences:
- No current flows anywhere in the circuit (not even in parts of the circuit before the break)
- The bulb does not glow
- No energy is consumed from the battery
The open switch is like a cut in the water pipe — water stops flowing everywhere in the loop, not just at the cut.
A key concept: in a series circuit (like our simple circuit), if ANYWHERE the path is broken, current stops throughout. The circuit is like a chain — one broken link stops the whole chain from working.
This is different from a parallel circuit, where multiple paths exist and breaking one path doesn’t stop all current flow.
Why This Works
Electrical current (flow of charge) requires a complete conducting path — a closed loop. Electrons need somewhere to go and somewhere to come back from. A battery pushes electrons from its negative terminal, through the external circuit, and back to its positive terminal. Any break in this path stops all electron movement.
Ohm’s law: . With the switch open, (infinite resistance of the air gap), so .
Common Mistake
A common misconception: “current is used up by the bulb, so the return wire to the battery carries less current.” This is wrong. In a simple series circuit, the same current flows through every component — the bulb, the switch, the wires, and inside the battery. Current is not “consumed.” What the bulb consumes is energy (power = ), not charge. Charge is conserved and flows back to the battery.