What if a switch were left open?

Press a switch and the light comes on. Press it again and the light goes off.

What does a switch really do? What happens when it is left open?

You flick switches every day: for lights, fans, toys and flashlights. But what is actually happening inside when you do?

Electricity can only flow when it has a complete loop to travel around, from the battery, through the wires and bulbs, and back again. In this lab, a battery lights two bulbs, with a switch in the wire. What do you think happens to the bulbs if the switch is left open?

A circuit carries a steady current only around a closed conducting path. A switch is a device that opens or closes that path on purpose: two metal contacts that either touch or are pulled apart.

This lab places a switch in the wire next to the battery and asks what opening it does to two bulbs wired in series, and then to the same bulbs wired in parallel, where each bulb has its own path back to the battery.

The switch is closed. That means the loop has no gaps.

The battery pushes electricity around the loop. Both bulbs glow.

With the switch closed, the loop is complete. In the model, a 3-volt battery pushes a current of half an ampere around it, through the first bulb and then the second.

The two bulbs share the battery's push, 1.5 volts each, so each one glows at about a quarter of the brightness one bulb alone would have. Over the model's 12-second run, the pair uses 18 joules of energy.

Closed, the switch adds no resistance in the model. Two 3 Ω bulbs in series give 6 Ω, so Ohm's law gives I = 3 V ÷ 6 Ω = 0.50 A. Each bulb drops 1.50 V and dissipates 0.75 W, which is 25% of the 3 W one bulb alone would draw from the same battery.

Total power is 1.5 W, and over the model's 12-second run the circuit uses 18 J. The model treats each bulb as a fixed resistance and compares brightness by electrical power.

Now Olly opens the switch. There is a gap in the loop.

Both bulbs go dark at the same time. Even the bulb far from the switch!

Open the switch and both bulbs go out together. The current meter drops to zero, and the energy counter stops climbing. The bulb on the far side of the circuit goes dark just as fast as the one next to the switch.

Now choose 'parallel', so each bulb has its own path. With the switch closed they glow brightly, 1 ampere each. Open the switch again: both still go dark. In this model the switch sits on the wire that every path shares.

Open, the switch removes the connection between the battery and the rest of the network, so no closed path remains. The model's battery current falls from 0.50 A to 0 A, the bulbs lit from 2 to 0, and the power from 1.5 W to zero.

In parallel, the closed circuit draws 2 A in total (1 A per bulb at the full 3 V, 6 W). Opening the switch still gives 0 A, because both branches return through the switch on the battery's line. To control one bulb alone, a switch would have to sit on that bulb's own branch; this model does not offer that layout.

Electricity needs a full loop, like a race car needs a full track.

An open switch is like a bridge that has been lifted. Nothing can get across the gap.

Inside a switch are two pieces of metal. Metals let electric charge move through them easily; scientists call them conductors. When the switch is closed, the metal pieces touch and the current flows across.

When the switch opens, the pieces pull apart and leave a gap, and the current cannot cross it. Because the loop is broken in one place, the current stops everywhere in the loop at once, not just near the switch. Our model is simple: its battery never runs down and its bulbs never burn out.

Metals are conductors: some of their electrons are free to move through the material. Insulators such as glass, rubber and plastic bind their charges so tightly that they move as much as 10²³ times more slowly (OpenStax). An open switch leaves a gap the current cannot cross, so its resistance is effectively infinite.

In a single loop there are no junctions, so Kirchhoff's current law makes the current the same everywhere: if it is zero at the switch, it is zero in every bulb. The model's limits: fixed bulb resistance, a battery that never runs down, and an ideal switch with no resistance when closed.

Lights come on the moment you press the switch.

Homes also have special switches that turn off by themselves if too much electricity flows. They help keep wires safe.

When you flick a switch, the light comes on straight away. That is not because electrons race from the switch to the bulb. In a copper house wire, the electrons themselves drift along at less than half a millimeter per second. The push travels down the wire far faster, at a big fraction of the speed of light.

Homes also have automatic switches called circuit breakers. If too much current keeps flowing, the breaker opens the circuit by itself, so the wires do not overheat. Only grown-ups should ever touch a home's breaker box.

OpenStax works out that electrons in a 12-gauge copper household wire carrying 20 A drift at about 4.5 × 10⁻⁴ m/s, while the electrical signal moves at around 10⁸ m/s: closing a switch sends a change in electric field along the wire, which is why lights come on as soon as a switch is flicked.

Fuses and circuit breakers open a circuit automatically when a sustained current exceeds safe limits; a breaker is a restorable switch, and the one OpenStax shows trips when a bimetallic strip bends as it overheats. At high voltages a spark can ionize the air in a breaker's gap and keep current flowing; large breakers in power-distribution systems use insulating gas, even jets of gas, to blow the spark out.