Electricity is all about moving really tiny stuff, really fast. It is contained in almost everything around you all the time. Your computer, lights, telephone, and even your own cells, are all charged with electricity at this very moment. Excited about this? Don't get too amped up just yet, there is a lot to know about this stuff, and it keeps getting more interesting as you go.
The atom is the building block of the universe. Everything is made of them, everything uses them, and everything would disappear with out them. So what is an atom made of? Electrons, protons, and neutrons, really tiny stuff, connected together with magnetism and electric fields, in a volume of space. But before this turns into a description of matter, lets stop and focus on the relevant parts of atoms to electricity. If you want more info on atoms and sub atomic particles, check out Wikipedia - "atom". The component we are interested in is the electron, the charged particle that helps define the material the atom is going to make.
Depending on how many electrons there are in an atom, how densely packed they are, and how free they are to move around, an atom and the material it makes can be hard or soft, flexible or rigid, and conductive or nonconductive (conductivity is the measure of whether or not electricity will travel through the material). Since we are interested in electricity, not material properties, we will look at what makes a material more conductive at the atomic level.
Conductivity is the measure of how easily electricity moves through a material. Materials like copper, gold, and iron are easy to pass electricity through, while materials like wood, glass, and plastic are not. Why is this? It is all about freedom of movement of electrons. From the human perspective, a grey piece of plastic is not all that different form a piece of aluminum. Both are hard, can be dented, can bend a little then break, can be shiny or not, and can come in any shape or size. So why is it that the aluminum is a good conductor, but the plastic is not? You have to look at it from an atomic level to understand this.
You can see from the image that electrons in the atomic view of the plastic are stuck in place and have no ability to move around, and the electrons of the metal can move around freely. This is the fundamental difference between something that will carry electricity, and something that wont. It works a lot like dominoes. If you line up 1000 dominoes on end back to back with no space in between (analogous to the plastic) and tap the first domino in line, what will happen? Nothing. The dominoes are not free to move and they just stand there as though nothing happened. Now if you were to line them up again on end, but leave an inch or so between each domino (analogous to the aluminum) and then tap them again, would they just stand there? No, they would obviously fall down, one after another, until the last domino has fallen over at the end of the line.
This is exactly how electricity works, and understanding this will make understanding the rest of the topic much easier. Electricity happens when a force moves the first electron on a surface where electrons are free to move, and this moving electron bumps into the next electron, and so on until the last free electron moves, and since the electrons can't fall over, they are instantly ready to repeat this process. If the last electron happens to come in contact with the initial pushing force, the cycle continues until there is no energy left.
Voltage, Current, and Table Top Games
Current and voltage are the primary descriptors of an electric circuit, but what exactly are they? Voltage is similar to a pump; it creates an electrical "pressure" that can apply a force on electrons. In the domino example, voltage is the initial force that pushes the first domino over, starting the chain reaction. Imagine if the domino chain were to have one part that went up a hill. As each domino falls over going up the hill, they will slow down since it takes extra work to go up hill (just like a car will slow down when coasting up a hill). If the hill is large, or the speed and force of the domino is low, there may not be enough power to push all of the dominoes over, and you will have to exert another force at the place they stopped falling over, or try again and make the initial force stronger. This is just like voltage. If you are simply trying to send electricity from one side of a battery to the other, there will always be enough energy regardless of the voltage to do this. However, if you put a hill (a power consuming device such as a light) in the path of electricity, you will need to have a strong enough initial push to ensure that the energy can make it up the hill.
The other aspect of dominoes you can observe and control is how many dominoes fall over in a given amount of time. If you have one column of dominoes, no matter how hard you push the initial domino, they will fall over at about the same rate as a result of how gravity works. If you want more to fall over in the same amount of time, you will need to have additional parallel columns of dominoes. This is analogous to current. Current is a measure of how many electrons pass through cross section of a material in a second, where one amp is equal to 6.24x1018 (624 followed by sixteen zeros) electrons per second. That's a lot of electrons.
Electric circuits are the exploitation of the way current and voltage work. More voltage will push you higher up an electrical hill, and more current will send more energy through a wire. A switch works like removing a domino from the path, and a motor or a light is equivalent to the electrical hill.
The basic rule of electric circuits is that energy that leaves a power source (ie a battery) must return. This is why batteries and plugs have two terminals, one positive, and one negative. If the positive and negative terminals of a battery or other power source are connected, you have made a circuit, and can use this to engage electrical devices as long as the rule is followed, negative connects to positive, and vice versa. If you install a lamp in the circuit, assuming there is the correct amount of voltage as specified by the lamp designer, the lamp will glow yellow, and if you install a motor with the correct voltage, it will spin. Want to control when it spins? Add a switch. The switch connects and disconnects power as the toggle switch is flipped from on to off.
How does electricity make a light glow and a motor spin?
Moving electrons have energy (as does any moving mass) as indicated by Einstein's law, E=MC2. Since an electron has a mass (M) of 9.10x10-31 (a decimal point followed by thirty zeros, then 91) and the speed of light (C) is 299,792,458 meters per second, the energy of an electron is 8.18x10-14 (a decimal point with thirteen zeros followed by 818) joules.
What is a joule? A measure of how much energy something has or needs. A 100 watt lamp for example, requires 100 joules of energy every second. And since we know how much energy is in an electron, we know we need to provide 1.22x1015 electrons per second. Again, that's a lot of electrons.
The rules of electricity
Like everything else in life, electricity has rules that it must follow (and always does) in order to work. These rules are:
The first rule, parallel and series voltages and currents, describes how electricity travels based on the shape of its path. If electricity travels down a wire, splits into two wires, and the becomes one wire again, and there are similar light bulbs installed on each leg of the split, each bulb will receive the same voltage as was applied to the single wire before the wire split, but only half of the current available.
If one wire does not split, but still contains two light bulbs in a row, each light bulb will receive the same amount of current, but only half of the voltage.
The rule of the completed circuit is that at the end of a circuit, there is always the same exact amount of current flowing as there was at the beginning, but the voltage drops to zero as it has been dissipated as electrical energy in the form of light, motion, or heat. That is why a wire heats up if you connect it between the positive and negative side of a battery with no other load. The voltage must be zero at the end of the circuit, and heat is the only load available to dissipate the energy.