Build a Circuit That Lights: Closed, Open and Short

A circuit is only ever in one of three states: closed (current follows the path you designed and the component works), open (the path is broken and no current flows), or short (current finds a route with almost no resistance and bypasses the component). The first two are safe to build on the bench. The third belongs in a simulation only — done for real it burns fingers and wrecks batteries.

The point of this experiment is not getting a bulb to light. It is building a habit: draw it before you wire it, and when it doesn't light, work out which state you are in before you touch anything.

Safety first (read this before you start)

• Use dry cells only — 1.5 V, two at most. Never run wiring experiments from a household wall outlet.
• Never join the battery terminals with a wire, and never lay a wire across the bulb's two contacts. Both are short circuits: within seconds the wire is too hot to hold, the cell swells and leaks, and a lithium cell can catch fire. Do the short-circuit part in the simulation below and nowhere else.
• Open the switch or lift out a cell before changing any connection.
• Take the cells out of the holder when you finish. A circuit packed away with its switch still closed is a common cause of hot batteries in school bags.

Materials

• 1–2 dry cells and a battery holder;
• A small bulb (2.5 V) and a lamp holder;
• A switch;
• 4–5 leads with crocodile clips — bring one spare, you will need it to hunt for the break;
• Paper and a pencil. Not filler: drawing the diagram is step one.

Procedure

1. Draw the circuit diagram first. Sketch the cell, the switch and the bulb, joined by straight lines into a closed loop. Then trace it with a finger from the positive terminal and check that you get back to the negative terminal, passing through both the switch and the bulb.
2. Build from your drawing. Lay the components out in the order shown, connect them one segment at a time, then trace the real thing with a finger and check it against the paper.
3. Close the switch. The bulb should light. That is a closed circuit.
4. Make an open circuit. Open the switch and the bulb goes out. Close it again, then have a partner quietly loosen one connection while you look away. Now the bulb is dark and you do not know where the break is.
5. Find the break by bridging. Take the spare lead and connect it across one section at a time, effectively building a bridge over that section. If the bulb lights, the fault is in the section you just bypassed; if not, move to the next one.
6. Leave the short circuit to the simulation. In the simulator, run a wire straight between the battery terminals and watch what happens; then lay a wire across the bulb and see whether the bulb lights and what the current does.

What you should see

Set the three states side by side and the differences are obvious:

The bridging hunt is worth recording too. Say the loop divides into three sections — cell to switch, switch to bulb, bulb back to cell. Bridge each in turn, and the bulb only lights when you bypass the faulty one. Three tries at most. This is standard practice for tracing an open circuit, and students never forget it once they have done it.

Laying a wire across the bulb in the simulation produces a result most students guess wrong: the bulb goes dark, yet the current in the circuit is far larger than before. All of it takes the near-resistance-free wire and none of it bothers squeezing through the filament.

The science: why the path must be closed

Current is charge in ordered motion. Charge is neither created out of nothing nor lost along the way, so it has to leave the positive terminal, pass through the component and return to the negative terminal, making a closed loop. Break the loop anywhere and every charge in it stops — which is why loosening one connection halts the entire circuit rather than only the part beyond the break.

Short circuits are clearest through Ohm's law. Which path the current takes depends on which path has less resistance:

I = U / R

A lead has a few tenths of an ohm; a small bulb has a dozen or more. Offer the current both routes side by side and nearly all of it takes the low-resistance one. Lay a wire straight across the cell and the loop's entire resistance is that fraction of an ohm, so the current becomes tens of times its normal value, and all the electrical energy turns into heat in the wire and inside the cell.

So testing a circuit for a short reduces to one question: is there a route from the positive terminal to the negative terminal that passes through no component at all? If yes, it is a short. That test is far more reliable than "don't let the wires touch", because it still works on complicated circuits.

The three states around the house

• A switch is an open circuit on purpose. Turning off a light does not "hold the electricity back"; it breaks the path.
• Fuses and circuit breakers exist for short circuits. When the current climbs abnormally, the fuse melts or the breaker trips first, cutting the path before the wiring can catch fire.
• Damaged cable insulation that lets two conductors touch is a short circuit, and among the most common causes of household electrical fires.
• Water in a socket can also short it, because tap water conducts. That is why wet hands and sockets do not mix.
• A poor contact is the everyday open circuit: a plug not pushed home, a bulb not screwed down, a terminal not tightened. All of them look like "I wired it correctly and it still won't light".

Five things students get wrong

• "It won't light, so the bulb is dead." A dead bulb is one possibility among many, and not the likeliest. Work through it in order: is the switch closed, does the cell still have charge, is every connection tight, is the bulb screwed fully in? Suspect the filament last. Bridging turns that checklist into something you can actually do.
• "A short circuit burns out the bulb." The opposite: during a short the bulb is dark, because the current went around it. What burns is the wire and the cell. The confusion comes from not asking which path the current took.
• "Current flows out of the battery and gets used up." Current is not consumed. Exactly as much charge returns to the negative terminal as left the positive one; what gets used up is energy. Left uncorrected, this idea will get in the way again when series circuits come up.
• Building from the components instead of from a diagram. It feels faster and produces far more errors, which are then hard to find because there is no correct version to compare against. The value of the drawing is not the drawing — it is that it forces you to settle where the current goes before you pick up a wire.
• "It doesn't matter how the wires are arranged as long as everything is connected." The shape of the wiring genuinely does not matter, but which two points each wire joins matters completely. Connect both of the bulb's leads to the same terminal and the layout looks tidy while the bulb has been shorted out.

Teaching notes

• Time and grouping: 30 minutes, pairs — one wiring, one checking against the diagram, then swap.
• Say this before they start: short circuits are simulated, never built. Give that its own moment; do not bury it in a list of other cautions.
• Prep that pays off: keep a few nearly flat cells aside to create the "everything is wired right but the bulb is faint" case. Students remember that one.

The step most often skipped. Drawing the diagram tends to get pushed into homework, when it belongs before the wiring. Have students draw on paper, glance over the drawings yourself, and you will catch most of the mistakes there — much faster than debugging finished circuits one bench at a time.

How to tell they have it. Hand them a circuit whose bulb won't light and ask for a plan; you want an ordered search, not random poking. Then show a diagram with a wire straight across the terminals and ask what is wrong. "That's a short — there's no component on that path" is the answer you are listening for.

Take it further

• In the simulator, build a circuit whose bulb still lights when one switch fails. (Hint: add a second branch with its own switch.) You have just invented the two-way stairway switch.
• With two switches and one bulb, build "the bulb lights only when both switches are closed" and "the bulb lights when either one is closed", then compare the layouts. The first is series, the second parallel — logical AND and OR.
• Make a conductivity tester. Wire the cell and bulb into a loop that is deliberately left open, with two bare wire ends. Touch the ends to a key, a pencil lead, an eraser, paper, tap water. If the bulb lights, that object is a conductor and has completed the loop for you.
• Estimate the short-circuit current. A dry cell's internal resistance is roughly 0.3 Ω. Use I = U/R for a shorted 1.5 V cell, then set that number beside a small bulb's normal 0.3 A. The reason the wire gets hot stops being abstract.
• Take apart a torch that has stopped working — cells out first — and trace the current path: spring, casing, switch contacts. Look for the section that might be making poor contact. Very often it is the little gap between the spring and the case.

When a student looks at a dark circuit and says "first, is it open or shorted?", this experiment has done its job.