What if one bulb in a string broke?

A battery, some wire and three bulbs make a loop. The bulbs glow.

Now Olly takes one bulb out. Will the other two still glow?

Electricity only flows when it has a complete loop to travel around, from the battery, through the wires and bulbs, and back to the battery. That loop is called a circuit.

In this lab, three bulbs sit one after another in a single loop. What do you think happens to the other two bulbs if one of them breaks or is taken out? Make a guess, then test it in the model.

A circuit carries a current only around a closed path. When resistors share one path, they are in series: the same current must pass through each of them in sequence, and their resistances add.

This lab removes one bulb from a three-bulb series string and asks what the other two do. Then it compares the same change in a parallel circuit and in a mixed one, where the answer depends on which bulb goes.

The battery pushes electricity around the loop. It goes through the first bulb, then the second, then the third.

All three bulbs share the push, so each one glows a little.

In the model, a 3-volt battery pushes current around one loop with three identical bulbs. The same current, about a third of an amp, flows through every bulb, one after another.

The bulbs share the battery's push, so each gets just 1 volt. That is why they glow dimly: each one shines at about 11 percent of the power one bulb alone would get from the same battery.

With three 3 Ω bulbs in series, the total resistance is 9 Ω, so Ohm's law gives I = 3 V ÷ 9 Ω ≈ 0.33 A. Each bulb drops 1.00 V and dissipates P = I²R ≈ 0.333 W, which is 11 percent of the 3 W a single bulb would draw from the same battery (P = V²/R).

The model treats each bulb as a fixed resistance and compares brightness by electrical power. Over the model's 12-second run, the string uses 12 J of energy.

In the model, Olly takes out bulb number one.

All the bulbs go dark! Even the two bulbs that are still in place.

Switch 'Take out bulb 1' on. In the series circuit, every bulb goes out at once, and the current falls to zero. The other two bulbs are not broken. They are dark because the gap left by the missing bulb breaks the only path.

Now choose 'parallel', where each bulb has its own path to the battery. Take bulb 1 out again: the other two stay bright. Each still gets the full 3 volts and its own 1 amp.

Series: with bulb 1 open, the loop has no closed path, so the model's battery current drops from 0.33 A to 0 A and the count of glowing bulbs from 3 to 0.

Parallel: each branch sees the full 3 V and draws 1 A, so the battery supplies 3 A in all. Removing bulb 1 opens only its own branch; the battery current falls to 2 A and two bulbs stay lit at full power.

Mixed: bulb 1 carries the whole current (0.67 A, 2 V) before it splits between the others. Remove it and all three go dark, just as in series.

Electricity needs a loop with no gaps, like a train needs a track with no gaps.

In one loop, a gap stops everything. With many loops, the other loops still work.

Think of the current like water flowing through one pipe that runs past three water wheels. If you take one piece of pipe away, the water stops for all the wheels, not just one.

In a parallel circuit, each bulb is on its own pipe. Removing one pipe leaves the others flowing. That is why scientists say series bulbs share one path and parallel bulbs each have their own.

Kirchhoff's current law says the current into any junction equals the current out. In a series loop there are no junctions to split the current, so it is the same everywhere; an open circuit (infinite resistance) anywhere forces it to zero everywhere.

In parallel, each branch is connected directly across the source, so each gets the full voltage and draws I = V/R independently. The model's limits: real filaments change resistance as they heat, so series bulbs are not exactly as dim as the model says, and its bulbs never burn out.

Many strings of tiny holiday lights are joined in one long line.

Inside each tiny bulb there is a hidden helper wire. If the bulb burns out, the helper lets the electricity go around it.

Many strings of tiny decorative lights really are wired in series. A typical string has 50 mini bulbs on a 120-volt household supply, and each bulb is designed for only about 2.5 volts of that.

To stop one burned-out bulb from darkening the whole string, inventors added a shunt: a tiny wire inside each bulb that lets the current go around it if its filament breaks. It does not always work, especially when a bulb is pulled right out of its socket. Homes and cars use parallel wiring, so each device gets the full voltage and works on its own.

A patent for holiday light strings describes a typical 50-light series string on 120 V AC with bulbs designed for about 2.5 V, and explains that mini-lights usually have a shunt in parallel with the filament. When the filament opens, the full supply voltage (peaking near 170 V) appears across it, breaking down an oxide coating on the shunt so current flows again; the patent notes such schemes are not always effective, particularly when a bulb is removed.

OpenStax notes that a car's headlights and radio, and a building's circuits, are wired in parallel so that each gets the full voltage and runs independently.