# Unit 3 – Circuit

Current always flows from plus pole to minus pole. You already know that from batteries. They’ve got a plus and a minus side.

If you put batteries for instance into a flashlight it lights up. But that will only happen if you turn on the switch. This is called: closing the circuit.

Since the word circuit comes from circle. In the image above the components are arranged in a circle but this is only for a better understanding. Current flows always from plus to minus.

The batteries in the circuit are called current source. There are other current sources like power adaptors, generators or solar cells.

### Current and voltage? What’s that?

Well, that is a very good question and not so easy to explain. Let’s compare electricity with a water pipe. The water in the pipe is under pressure. The more pressure in the pipe, the more powerful water squirts out of the faucet. If there wouldn’t be any pressure, no water would flow. The amount of water is like the electrical current, the pressure is the voltage.

Of course, there is no water in a socket. But there are little loaded parts inside. They are called electrons. The higher the voltage, the more electrons want to move. The movement of the electrons is the current.

Warning: Don’t ever connect something to a wall socket, what is not made for it! The voltage of wall sockets is really dangerous!

So, now we want to build our own circuit, right? We will do that with the 5V which are provided by the Arduino board.