What if bulbs were wired side by side?

A battery can make a bulb light up. The electricity goes around a loop of wire.

What if we have two bulbs? We can put them in one long loop, or side by side. Which way makes them brighter?

Electricity needs a complete loop, called a circuit, to flow from a battery, through a bulb and back again. With two bulbs, there are two main ways to wire them.

In a series circuit, the bulbs sit one after the other in a single loop, so the electricity flows through both in turn. In a parallel circuit, the bulbs sit side by side, each on its own path to the battery. In this lab, two bulbs start in series. What happens when you wire them side by side?

Circuit topology decides how current and voltage are shared between components. Two identical resistive loads connected to one source can be placed in series, carrying the same current, or in parallel, sharing the same voltage.

This lab uses a 3 V battery with no internal resistance and two identical 3 Ω bulbs. It compares the current, voltage and power of each bulb, and the total drawn from the battery, in the two arrangements.

First, the two bulbs are in one long loop. The electricity goes through one bulb, then the other.

Both bulbs glow, but they are dim.

In series, the electricity has only one path, so it must push through both bulbs, one after the other. Each bulb gets half of the battery's push: 1.5 volts each from the 3-volt battery.

Two bulbs in a row also resist the flow twice as much, so less current flows: 0.5 amperes. In the model, each bulb glows at only about a quarter of the brightness one bulb would have alone.

In series the resistances add to 6 Ω, so I = 3 V ÷ 6 Ω = 0.5 A flows through both bulbs. Each drops 1.5 V and dissipates P = I²R = 0.75 W, 25% of the 3 W a single bulb would take from the same battery. The battery delivers 1.5 W in total.

The current is the same everywhere in the loop, and the bulb voltages add up to the battery's.

Now Olly wires the bulbs side by side. Each bulb has its own path to the battery.

Both bulbs glow much brighter! But the battery has to work much harder.

Wire the bulbs in parallel. Now each bulb has its own path, straight across the battery, so each one gets the full 3 volts. Each bulb carries 1 ampere and glows as brightly as a bulb on its own: four times as bright as in series.

But look at the battery. It now pushes 2 amperes, one for each path, and supplies 6 watts instead of 1.5. That's four times as much power. A real battery might run down much sooner.

In parallel each bulb sees the full 3 V, so each carries 1 A and dissipates 3 W: 100% of a lone bulb, four times its series power. The equivalent resistance is 1.5 Ω and the battery supplies 2 A and 6 W, four times the series total.

With three bulbs the contrast grows: 11% brightness each in series, 100% in parallel (9 W total). Add 0.5 Ω of internal resistance and the parallel bulbs fall to 56%, because the larger current drops more voltage inside the battery.

In one long loop, the bulbs share the battery's push. Each gets only a bit.

Side by side, each bulb gets the whole push for itself. So each one shines brighter.

A battery's voltage is its push. In series, the bulbs share that push, so each gets only part of it. In parallel, every path connects straight across the battery, so every bulb gets the whole push.

More push through the same bulb means more current, and both together make more power, so the bulb glows brighter. Our model is simple: its bulbs have a fixed resistance, its battery never runs down, and its bulbs never burn out. Real bulb filaments are different: OpenStax shows that a hot filament resists the flow many times more than a cold one.

Kirchhoff's laws give the result: in series, the loop voltage is shared, V = V₁ + V₂, and the current is common; in parallel, each branch has V across it and the currents add, I = I₁ + I₂. With P = V²/R, halving the voltage across a bulb quarters its power.

Model limits: ohmic bulbs with a fixed resistance (a real tungsten filament's resistance rises many times as it heats; OpenStax works an example that climbs from 0.35 Ω cold to 4.8 Ω at 2,850 °C, so real bulbs do not follow these exact numbers, and the model compares brightness by power, not by light); an ideal battery that never runs down unless internal resistance is set; bulbs that never burn out; resistance-free wires.

The lights and plugs in a house are wired side by side.

Each one gets the full push. You can switch one off, and the others stay on.

OpenStax, a free physics textbook, explains that parallel wiring is used to give many devices the same voltage and let each one work on its own. A car's headlights and radio are wired in parallel, and the same is true in your house or any building.

That's why you can turn off a lamp without the fridge stopping. OpenStax also explains why the lights can dim for a moment when a big appliance starts: its large current lowers the voltage reaching the lights through the wires.

OpenStax notes that power distribution systems mostly use parallel connections so that many devices receive the same voltage and operate independently; an automobile's headlights and radio are wired in parallel, as is a house. Each parallel resistor gets the full source voltage, so the power dissipated in each is considerably higher than in series on the same source.

It also explains why lights dim when a large appliance switches on: the appliance's large current causes a significant IR drop in the wires, reducing the voltage across the lights. The model's internal-resistance control shows the same effect at the battery.