This is entry #0 of my 52 Project 2012: Foundational Electronic Components Crash Course series.
Since pretty much every one of these explanations of electronic components will refer to “voltage” and/or “current” as part of the description, it is important to start by having a good working understanding of what those two things are. Since this is a bonus article of sorts, I’ll be (a bit) lazy and directly point you to this brilliantly simple article on voltage and current, by John Hughes of The Electronics Club, if you’d like a simple technical overview with diagrams.
Don’t go away though, because I’ll still give my own simple explanation here.
Voltage and Current as Volume and Pressure
Although it doesn’t paint a comprehensive picture of everything that’s going on, my favorite way to think about these two critical parts of electronic theory simply is by using a plumbing analogy.
Consider a metal pipe with water running through it. The size of the pipe is voltage, and the flow rate of the water is current. Voltage is measured in volts, and current is measured in amperes, or just amps. If you push more water through the same pipe, the voltage remains the same, but the current increases. If you push the same water through a larger pipe, the current is the same, but the voltage increases.
This illustration helps explain the rule of thumb that “It’s volts that jolts, but mills that kills.” (Mills in this case refers to milliamps, a different unit for measuring current.) This is why you can have a million volts running through your body, and if the current is tiny enough, you will be completely unharmed. Or, conversely, you can have only a few volts running through your body, and if the current is large enough, it can still kill you.
Returning to the plumbing analogy, you can go swimming in a large body of water (a.k.a. “high voltage”), but as long as that water isn’t going anywhere or is barely moving (a.k.a. “low current”), you’ll be fine. Or, you can push a very small stream of water (a.k.a. “low voltage”) through a microscopic hole at extremely high pressure (a.k.a. “high current”) and use the stream to cut through solid objects. The overall power all depends on how much and how fast.
Voltage and Current, Technically
It is useful to understand voltage and current without an analogy as well, so here’s my best shot at a short explanation.
Voltage in a circuit is measured as the difference in electrical potential between two points. Wires by themselves typically don’t affect the electrical potential, but other electrical components do. The difference in electrical potential between the positive and negative terminals of a 9V battery is 9V (assuming the battery is charged). Electrical components “use up” voltage.
Current in a circuit is measured as the rate of electron flow between two points. Current is constant throughout all components connected in series, since electrons that go into a component have to go out as well. Electrical components do not “use up” current like they do to voltage.
Alternating Current vs. Direct Current
There are actually two different types of current in electrical circuits. One is direct current (DC), and the other is alternating current (AC). There is a lot that can be said to explain the difference, but I’ll try to keep it simple.
Direct current flows constantly in one direction. The voltage between two points in a circuit powered by direct current is constant (barring some modification by the circuit). Batteries produce this kind of current; it is simple to measure and generally easy to work with.
Alternating current moves backward and forward rapidly at a certain frequency. The voltage between two points in a circuit powered by alternating current varies in a waveform pattern, alternating between positive and negative peaks and typically measured using a root-mean-square (RMS) calculation. This is the kind of current that comes from the outlets in your house. Alternating current is generally much more efficient to transmit over long distances, such as from the power company to your home, and it is easy to convert into different forms using a transformer.
Many electronic devices, including computers, phones, and MP3 players, require a DC power supply. Since electrical outlets provide AC power, a conversion is necessary to change AC power into DC power. This is what those big ugly “power bricks” do. If your device has a heavy box that plugs into the wall and has a small, thin connector on the other end to supply power, it’s usually an AC/DC transformer. (The exception is when it’s an AC/AC transformer, converting one kind of AC power into a different level of AC power.)
Power is Just Voltage and Current
Since it’s directly related, what about power? You may know that typical US households run on 110V or 120V systems. But power is usually measured in watts or kilowatts, right? What’s a watt, exactly?
Power is just a simple way to refer to a quantity of voltage multiplied by current. A high-powered device might have a rating of “500W” to indicate that it uses a maximum of 500 Watts of power. But that same device might actually say “500VA” instead of “500W,” and this would be exactly the same thing. Remember that volts and amps (abbreviated “V” and “A” respectively) are units of voltage and current. One watt (abbreviated “W”) is equivalent to 1 “volt-amp.” Consider:
1V * 1A = 1W
5V * 10A = 50W
120V * 5A = 600W
120V * 0.1A = 12W
Power is just voltage and current put together.
That’s it for now!
For a description of this 52 Project series and links to the all existing entries, go to the 52 Project 2012: Foundational Electronic Components Crash Course page.