First Light

On the bench below lies a battery with a red lead and a black lead. Next to it is a small light with two legs, lying on its side. The light is called an LED.

Between the battery and the LED sits a little barrel with coloured stripes. It is called a resistor, and I will come back to what it does. One of its legs is already twisted to the LED’s long leg. The battery’s two leads are still loose.

Before you touch anything, make a guess. Electricity comes out of the battery, and the LED needs it to make light. So if only the red lead is joined, will the LED light? Or does it need the black lead too?

With one lead joined, nothing happened. You may already have known that both leads are needed, but why two? It is natural to picture electricity flowing out of the battery and being used up in the light. It does not work that way. The electricity has to go all the way round and come back to the battery. The black lead is its path back, and the moving dots on the bench show the electricity going round.

If you pulled a lead away again, as the guide asked, the light went out at once. It did not matter which lead. A path like this one, out of the battery and back into it, is called a circuit. Break it anywhere, and the light goes out.

A loose lead is the first of three mistakes that stop a first circuit from working. The other two are about the parts themselves, and you can make both of them below without harming anything.

Look closely at the LED’s legs. One is longer than the other. Here the long leg is twisted to the resistor, on the side that leads to the battery’s + end. So what happens if the LED is turned around, with its long leg on the other side? Will it still light?

It went dark. An LED lets electricity through in one direction only, in at its long leg and out at its short one. Turned around, it blocks the way, so nothing flows anywhere in the circuit and nothing makes light. Its long leg goes on the side joined to the battery’s + end. The short leg goes on the − side. If a circuit you build stays dark, this is the first thing to check.

That one-way habit is in the LED’s name. The letters stand for light-emitting diode. Emitting means giving off, and a diode is any part that lets electricity through one way only. Diodes have a lesson of their own later, The One-Way Toolkit.

A backwards LED is not always harmless, though. A small LED can hold back only a gentle push the wrong way. A 9 V battery pushes harder than that. So a real LED left backwards on one can be damaged. If you try this with real parts, use two AA cells in place of the 9 V battery. They push gently enough for the LED to be safe either way round. It still lights on them, only more dimly. With the 9 V battery, check the long leg before you connect.

That leaves the resistor, the striped barrel. It does not light up, and it does not move. The circuit would still go round and back if a plain wire took its place. So why not use a plain wire? Below, you can take the resistor out and lay one in its place. What do you think will happen to the LED?

When the resistor came out, the LED went dark, because the circuit was broken again. With the plain wire in its place, the LED flared much brighter than before, and then it burnt out. A battery can push far more electricity through a plain wire than a small LED can take. Too much makes the LED hot, and the heat melts it inside.

That is what the resistor was there to prevent. It makes the path harder to get through, so only a little electricity can flow. It is little enough for the LED to survive. That is all you need to know about it for now. Resistors have a lesson of their own later, The Resistor. There you learn to read their stripes and choose the right one for a job.

Please do not try the plain wire with real parts. A real LED burns out like this in a fraction of a second, and it never lights again. The bench above shows it slowly, so you never need to try it.

You have now built a working circuit and broken it in the three ways first circuits most often break. You could build it on a real table. The parts are worth knowing by sight before you meet them for real. On paper, people draw a circuit in a quick shorthand. Each part becomes a small sign, and each wire a straight line. Such a drawing is called a schematic. It shows what joins to what, and nothing else. Schematics also have a lesson of their own later, The Schematic. There you learn to build a circuit from one, and to draw one from a circuit.

Each part is shown twice below. The first picture is the part as it looks in your hand. The second is the small drawing that stands for it in a schematic, which is called its symbol.

Three questions, if you want to check yourself.

One thing on the bench may have caught your eye. When the circuit closed, the light came on at once. Every dot, all the way round, started moving at the same moment. None of them set off from the battery and travelled to the LED first. How can everything in a circuit start at the same moment? That question belongs to the next lesson, The Current.