For the lamp to glow, current must finish the whole loop
A battery, a bulb, two wires. The moment they connect, it lights—but it is not just “connected”: current leaves one battery terminal, runs through a wire, passes the bulb, and returns along the other wire. It must be one complete loop.
Open any joint and the lamp goes out quickly. Charges are already in the wires; the lamp does not wait for one to travel from the battery. The violet ring helps trace the route: follow it and check whether the whole loop is connected.
A switch is simply a deliberate gap. Close it and the loop is whole; open it and everything goes dark—no matter where on the loop it sits.
Build a loop with dry cells and matching small bulbs
Ask an adult for a low-voltage circuit kit: cells in a holder, filament bulbs rated for that supply, bulb holders, insulated leads and a switch. An LED is a different component and cannot simply replace the bulb.
- Start with the switch open. Connect battery → switch → bulb → battery, joining the metal contacts.
- Close and observe. After it lights, open any one joint. Disconnect the battery before moving wires.
- Move the switch. Place it on the other side of the bulb. In the same series loop it still controls the lamp.
- Compare two lamps. First explore series and parallel in the animation, then let an adult follow the kit instructions with matching bulbs. Real battery voltage can sag and a filament’s resistance changes as it warms, so actual brightness need not match the ideal animation exactly.
Predict: if one parallel branch opens, does the other still have a complete loop? Trace that loop before testing.
Use only the dry-cell supply specified by the kit, never a wall socket. Do not join the battery terminals directly: a short circuit can heat wires and cells.
Ohm’s law, and the arithmetic of series and parallel
More: filament bulbs and LEDs
The hands-on activity uses filament bulbs matched to the supply. LEDs require a suitable supply and current limiting; parallel LED branches each need appropriate limiting. A single 1.5 V cell may not light an LED at all.
Expand: voltage, current and resistance as formulas
Picture the water circuit: the battery is a pump lifting charge to a “high level”; wires are pipes; the bulb is a waterwheel spun by the flow. The higher the lift (voltage), the stronger the flow (current), the brighter the lamp. Ohm wrote it as a formula (OpenStax College Physics 20.2):
I is current in amperes, V is voltage in volts, R is resistance in ohms. Here the supply voltage and lamp resistance are held constant as teaching approximations: raise the voltage from 1.5 V to 3 V and the current exactly doubles.
Two identical lamps in series form one loop: resistances add, current halves, the voltage is shared:
Each bulb gets V / 2, and its power becomes P = V²/4R—one quarter of a lone bulb, so both lamps are dim. Old-style fairy lights dying all at once is the price of one chain.
In parallel, each bulb loops with the battery alone, keeps the full voltage and its full brightness; the cost lands on the battery, whose main-line current is the sum of the branches (OpenStax College Physics 20.3):
Why home wiring is parallel
Home appliances are wired exactly in parallel: each forms its own loop with the supply, so switching off one lamp does not darken its neighbours. The building’s main breaker sits on the main line—open it and the whole flat goes dark; a room’s light switch sits on a branch and governs only that lamp. That is precisely the on/off matrix of the demo’s sixth chapter.
Two footnotes: the schematic shows conventional current (out of the “+”); in metal wires the electrons actually drift the opposite way, which changes none of the arithmetic. And the meter bills accumulated energy W = P t—the more parallel appliances, the harder the battery (or the grid) works.
Next time you flip a switch, hold three questions at once: is the loop closed? how large is the current? and how many lamps does that switch govern?