Simply stated, electricity is a flow of charged particles (electrons). When charged particles are allowed to flow from one place to another, they transfer energy that can be used to power everything from a light bulb to an entire city. All of our everyday electronic gadgets – from cell phones to computers to microwave ovens – are powered by the flow of these charged particles.
A circuit is a closed loop that allows charged particles to flow through it. The parts of a circuit that charged particles flow through are called conductors -- conductors are the building blocks of circuits, and wires are one example of a conductor. Wires and other conductors allow charged particles to flow through them and when you string together conductors in a closed loop, you have created an electric circuit.
A typical circuit has a power source, conductors, and various other components. The power source forces charged particles to flow through the circuit. The conductors (wires) connect together the components in the circuit. And, the components are the parts of the circuit that are affected by the electricity as it passes through them.
For example, a circuit may contain a battery and a lightbulb, attached together with wires. The battery serves as the power source, which forces charged particles through the wires. When the charged particles flow through the lightbulb, it illuminates.
Some circuits are very simple (like our battery and lightbulb example), while other circuits are very complicated, containing many power sources, wires and components.
When discussing electricity and circuits, there are three major concepts that are important to understand:
Voltage
Current
Resistance
These three concepts define how and when electric charge flows, and controlling these things allows us to harness the power that electricity provides. While we can’t actually see voltage, current or resistance, there are some analogies we can use to help understand what each of these concepts are and how they interact with each other.
Recall that electricity is a flow of charged particles. Well, electricity through wire (or any conductor) is very similar, in principle, to how water flows through pipes filled with water. In fact, to understand the basic components of electricity – voltage, current and resistance – it's common to use water flowing through pipes as an analogy.
Voltage is essentially the difference in the amount of charge from one point in a circuit to another. An important concept of electricity is that charge will typically only flow between two points that have a difference in charge (a voltage). Specifically, charged particles will flow from a point of higher charge to a point of lower charge. If there is the same amount of charge at two points in a circuit, no electricity will flow.
Using our water analogy, voltage is analogous to the amount of pressure there is pushing water through a pipe filled with water. Imagine a water system that contains a water pump that is pushing water around a loop of a water-filled pipe (a 'circuit') -- water enters the pump at a low pressure and is ejected from the pump at a high pressure. It is this pressure difference that generates the flow of water. Likewise in an electric circuit, the electric power source is analogous to the water pump -- a power source takes in charge at a low voltage and pushes it out at a high voltage. It is this difference in voltage on the opposite sides of of the power source that generates the flow of charged particles.
Before we go any further, let’s take a quick look at how a battery works. In a battery, there are two “terminals” – a positive terminal and a negative terminal. The positive terminal contains more positively charged particles than the negative terminal. This difference in charge is created by a chemical reaction that occurs in the battery:
The difference in charge between the positive and negative terminals creates a voltage, but there is a separator between the two terminals which keeps the positively charged particles and the negatively charged particles from mixing together. But, if we were to place a conductor (like a wire) between the positive terminal and the negative terminal, charged particles would flow from one side to the other:
But don't do this! If you do, you'll have created a short circuit, which we discuss at the bottom of this page - and you will quickly drain your battery.
What you see above is what a typical battery-operated electric circuit will look like before you insert various components into the circuit loop. We could easily insert a lightbulb into the circuit above and you would see the lightbulb illuminate:
Keep this particular circuit in mind, as we’ll be coming back to it often over the next few projects.
Current is a measure of the amount of electric charge that flows through a circuit. Using our water analogy, if voltage is the amount of water pressure in a pipe, current is the amount of water actually flowing through the pipe.
Take a look at these two water pipes:
Each pipe has the same amount of pressure pushing the water through, but notice that the second pipe is smaller than the first. Because the second pipe is smaller, less water will flow through it, despite having the same amount of pressure.
What if we wanted to get the same amount of water through the second pipe as the first, but we couldn’t change the size of the pipes? The obvious solution would be to increase the water pressure in the second pipe:
In an electric circuit, this would be equivalent to having to increase the amount of voltage (charge difference) in the circuit in order to get the same amount of charge to flow.
But, what does it mean to have a “smaller pipe” in a circuit? That’s where “resistance” comes in…
If voltage is analogous to the amount of water pressure in a pipe, and current is analogous to the amount of water actually moving through the pipe, then resistance is analogous to the size (diameter) of the pipe being used. The smaller the pipe, the harder you have to push to get the same amount of water through the pipe. In a circuit, that equates to the higher the resistance of the circuit, the more voltage you need to generate the same amount of current.
But, in a circuit, wires are generally the same size. So, you might be wondering -- what causes different amounts of resistance in an electric circuit?
The answer is that lots of things can impact the resistance of a circuit, including the type of conductors being used (the material through which the charge is flowing) and the resistance of the components being used. As an example, different types of wire will have different levels of resistance, which will ultimately impact the current and voltage levels within the circuit. And some mediums – like air – have a nearly infinite resistance, meaning electricity won’t flow through them at all. In addition, there are lots of times where we need to add resistance to a circuit (using things called “resistors”) in order to control the flow of current in the circuit.
By the way, we just said above that charged particles won't flow through air, but that's not completely true. Lightning is a great example of electricity moving through air, but in order for electricity to move from the clouds to the ground, it requires a tremendously high voltage (meaning a tremendous difference in charge from the clouds to the ground). In fact, the voltages in a lightning strike are about a hundred million times greater than what we'll be working with in our circuits.
In all circuits, we can talk about three parts of the circuit:
Power: This is generally the point of highest charge in your circuit -- for example, the positive terminal of your battery. This point in a circuit is sometimes generically called "Vcc" or in the case of the projects we'll be doing, "3.3V" (since the power source we'll be using will be 3.3 volts). The power source provides the energy that makes the circuit do something useful.
Load: The part of the circuit that uses the flow of charged particles -- the components in the circuit -- is typically referred to as the Load. Above we talked about circuit resistance -- the Load is the part of the circuit that provides resistance. An example from our earlier circuit would be the lightbulb -- that is a part of the circuit Load. The Load uses the energy that the power source provides.
Ground: In an electric circuit, Ground is generally the point of lowest charge in the circuit. For example, in our lightbulb circuit above, the negative terminal of the battery is the point of lowest charge, and would be the physical ground point of this circuit. Interestingly, the reason we use the term "Ground" is because in many electrical applications (like the electricity running through your house), the earth is used as both our reference point of lowest charge and our physical point of attaching the electrical circuit. This is because the earth is able to receive nearly infinite amounts of electricity without increasing its total charge, so we can always assume that the voltage between a power source and the earth will be constant.
In our assembly guide, we mentioned that you should "ground" yourself before touching electronic components. Now that you have a conceptual idea of what ground is, we can explain this further. Have you ever gotten a static shock by touching someone or something? Sometimes, electric charge builds up in your body. This charge is then released -- in the form of a static discharge (or shock) when you touch someone or something that has a different amount of charge. In other words, if there is a voltage difference between you and some object, and you touch that object, you'll get a shock. Once your body has discharged the excess charge, you don't get any more shocks -- until, of course, you build up more charge.
If you were to touch an electrical component when you have a charge built up in your body, that resulting static shock that you feel could very well be enough to zap the component and break it (we sometimes call this "frying the component.") So, to avoid frying any electrical components, we recommend that you discharge any excess charge in your body before touching the components.
You can ground yourself by touching any piece of metal that is connected via some relatively direct path to the earth. Since all the electrical outlets in your house are connected to the earth, you can generally ground yourself by touching the metal case of a electrical device that is plugged in. On the RaspberrySTEM CREATOR Kit, you can touch the metal casing on any of the USB ports or the Ethernet port on the back of the Raspberry Pi -- assuming the RaspberrySTEM CREATOR Kit is plugged into a wall outlet.
Where we have electricity flowing from a power source (Vcc), through one or more components (Load) and to a point of lower voltage (many times 0V or Ground), we call that type of circuit a Resistive Circuit. Creating a resistive circuit is the first step in creating a useful piece of electronics.
Remember our battery and lightbulb circuit above?
This is an example of a resistive circuit.
Sometimes we don't have all the components of a resistive circuit, in which case our electronics will typically not work as expected. Two common examples are open circuits and short circuits:
Open Circuit – An open circuit is one that is not complete, and therefore electric charge can’t flow. As mentioned earlier, electric charge can't easily flow through air, so if a circuit is not complete, the electrical current is experiencing nearly infinite resistance at some point in the circuit and circuit will not function.
Short Circuit – A short circuit is what happens when the beginning of the circuit (Vcc) is attached to the end of the circuit (Ground) with nothing (no Load) in between. Electricity can flow through wire with almost no resistance, so attaching a wire from power to ground with nothing in between will cause large amounts of electricity to flow through that wire. This can dissipate your power source very quickly, or worst-case, the large flow of current can heat up the wire to the point where it breaks or causes a fire.