Your first circuit

This is the first page of the course, so it assumes you know nothing about electronics yet. By the end you will know what a circuit is, what current is, and the difference between an open circuit and a short circuit. Those three ideas come back on every other page.

What a circuit needs

Every circuit has the same three ingredients. There is a source that pushes electric charge around, such as a battery. There is a path for the charge to move along, usually copper wire. And there is at least one load, something that uses the energy on the way, like a lamp, an LED or a motor.

A battery has two ends, called terminals, marked plus (+) and minus (−). The path has to start at one terminal, go through the load and come back to the other terminal. That complete loop is called a closed circuit. Charge can only keep moving if it has a way back to the battery.

The flow of charge through the loop is called current. It is measured in amperes, usually shortened to amps (A). Small currents are given in milliamps (mA), and 1 mA is a thousandth of an amp. By convention, current is drawn as flowing out of the plus terminal, through the circuit and back into the minus terminal. The electrons in the wire actually move the other way, but every diagram and calculation uses the conventional direction, and so do these guides.

The tool below is three AA batteries (4.5 V together), a switch and a small lamp. Turn the switch off and on, then try the two other options.

Open circuits

When the switch is off, there is a gap in the loop. The charge has no way back to the battery, so no current flows anywhere, not even in the parts of the wire that are still connected. That is an open circuit. A switch is simply a controlled way of opening and closing the loop.

Cutting the wire back to the battery has exactly the same effect. It doesn't matter where the gap is. One gap anywhere in a simple loop stops the whole circuit, which is why a single loose wire or a bad solder joint is enough to make a project do nothing at all.

Short circuits

A short circuit is the opposite problem. It happens when current finds a path that skips the load. In the tool, the extra wire across the lamp is that path. Wire has almost no resistance, so nearly all the current goes through it instead of through the lamp, and the lamp goes dark.

The bigger issue is what happens to the battery. With the lamp out of the way, the only thing limiting the current is the battery's own small internal resistance, so it has to deliver several amps. The battery heats up and drains quickly, and thin wires can get hot enough to melt their insulation. With larger batteries, a short circuit can cause burns or a fire.

Drawing circuits

When people draw circuits, they use simple symbols instead of pictures: two lines of different lengths for a battery (the long one is plus), a zigzag or a small rectangle for a resistor, a circle with a cross for a lamp, and a gap with a lever for a switch. The wires are straight lines. A drawing like that is called a schematic, and the tools on these pages use the same idea.

What comes next

You now know that current only flows in a closed loop. The next question is how much current flows, and that depends on two things: how hard the source pushes, called voltage, and how much the path resists, called resistance. The relation between the three is Ohm's law, and it is the subject of the next page.