Another basic electrical component is the capacitor. A capacitor is a device that can store energy within itself using an electric field. These devices are the second most common component after resistors, and are inside essentially every electronic device save for the most basic circuits like flashlights.

Electrolytic (Polarized) Capacitor
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What is a capacitor?

Also know as "condensers" (I like this name better but no one here in the US uses it so I'm out of luck), capacitors use a gap between the end of a circuit to store electrical energy. The most basic capacitors is just two metal plates that are close together, but not touching.

Capacitor electrical symbol
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To demonstrate, imagine you take two big metal plates and put them a centimeter apart. Then, attach a battery to both plates. One of the plates will suddenly have the same voltage as the top of the battery. An electric field forms between the two plates, causing charges to move and collect on the plates. The charges can't leave the plates, so charge continues to build up, with positive charge on one plate and negative charge on the other. An equal amount of charge will always build up on each plate (but the sign of the charge will be opposite on each plate).

Electric field lines and charges on a parallel plate capacitor
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Eventually the charge reaches a maximum and the capacitor is fully charged. If you pull the capacitor off of the battery, it will now have the same voltage across it as the battery did. The capacitor has taken energy out of the battery and stored it inside itself. This energy is in the form of potential energy in the charges: The opposite charges want to recombine and neutralize, but can't because of the air gap between the plates.

If you connect the charged up capacitor to another circuit, it will briefly act like a battery of whatever voltage you charged it to, powering said circuit as normal. However, as charge is pushed through the circuit, charge must leave the capacitor, and almost all capacitors will rapidly run out of energy, with the voltage across them dropping alongside the charge and energy.

A capacitor's ability to store electric charge is given by its capacitance. Capacitance is a property of any capacitor determined by the capacitor's physical dimensions. It's defined as:

C = Q/V

...Where Q is electric charge stored on the plates, and V is the voltage across the capacitor. Unlike inductors, capacitors can store energy in a stable manner: Disconnect a capacitor from its battery and it will retain its energy for quite a bit of time. Capacitance is basically for much charge you can store per unit voltage. C is measured in units of Farads. For example, a 1 F capacitor can store 1 coulomb of charge (6.25*10^18 electrons) when it reaches a voltage of 1 volt. In practice, most capacitors have way smaller capacitances (typical values are in the microFarad range, with is around 10^-6 farads).

Really a capacitor is just a break in an electric circuit. Cut the wires to a circuit and you've actually created an extremely tiny capacitor that will store an extremely tiny charge. The interesting part is that breaks in circuits end up storing energy, especially if the break has a ton of surface area (parallel plates!).

Types of Capacitors

Now that we've covered the basics of how these devices work, I'd like to cover a few types of capacitors. Different capacitors are made of different materials and have different properties.

The most common type is probably the Ceramic capacitor. Ceramic capacitors have pretty low capacitances but can take (usually) somewhat high voltages before breaking. These are found just about everywhere, often in tiny surface-mount form inside your computer or phone. Ceramic capacitors don't have polarity, so they can be connected in a circuit either way with no problems.

Example of a tiny surface-mount ceramic capacitor
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Another common type of capacitor are electrolytic capacitors. These devices use a chemical reaction to assist in dividing the two metal plates. Electrolytic capacitors usually have somewhat high capacitances, and as such can store a lot more energy (the energy in a capacitor is equal to E = 0.5CV^2, so higher C means higher energy). However, electrolytic capacitors are polarized, meaning that if you hook them up the wrong way around they can fail or even explode. These capacitors usually have a grey band indicating the negative terminal to avoid mistakes.

The last major kind of capacitor I'd like to cover are supercapacitors. These devices have extremely high capacitances (several farads or more) but very low voltage ratings (usually only a few volts, although you can increase overall voltage capacity by making a bank of many capacitors). These capacitors can store huge amounts of energy, rivaling even some types of batteries. But, being capacitors, they can charge and discharge much, much faster than batteries can. This makes them very useful for fast-charging applications: I've seen them in solar flashlights, wind-up generators and even as a car starter.

Other types, of course, exist, including film and tantalum capacitors, if you're interested.

What can you use capacitors for?

The answer to that is, just about anything electrical. Their ability to store energy allows capacitors to find their way into most circuits. One use is in LC circuits: Connect a capacitor across an inductor and you have an electrical seesaw, where charge will oscillate back and forth between the two devices. By carefully choosing the right capacitor, you can make very high frequency oscillators using this technique that can be used to transmit radio waves or act as a precise filter (say, for receiving radio broadcasts).

Capacitors can also be used as a standalone filter. Alternating current experiences less electrical impedance (basically how easy it is for current to pass) through a capacitor as the frequency of the signal increases. This means that a well-placed capacitor can effectively block out low frequency signals while letting the high frequency ones pass unhindered, letting you filter a noisy signal. Capacitors are also used to "smooth out" pulsed electrical signals, removing major dips.

You can even use capacitors to generate energy. The first ever nuclear battery (a device that uses radioactive decay to generate electricity) was essentially a high voltage capacitor that collected charged particles radiating off of a radioactive sample. This collected charge could then be stored and used to drive other circuits.

By building a variable capacitor you can listen to radio stations. A variable capacitor is just a capacitor with the metal plates attached to a rotating knob. Turn the knob and you change the surface area of the plates that is exposed to the other plates, reducing or increasing the capacitance. This in conjunction with a LC circuit changes the frequency of the oscillator, allowing you to select only the station (frequency) you want to hear. The tuning knob on older radios is almost always a variable capacitor.

Various capacitors. The colorful canisters are electrolytic capacitors. The white-and-grey gridded devices are variale capacitors. The flat orange ones are ceramic capacitors.
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Capacitors can also be used with diodes to multiply voltages. A single diode and capacitor can double the voltage coming in from an AC input. This is done in microwave ovens to boost 2000 volt AC output from a deadly transforme rup to 4000 volts to power the magnetron vacuum tube that generates the microwaves to heat your food.

This is one of my microwave oven capacitors I pulled out last year. These things can be charged up to over 2,000 volts, at which point they are deadly if touched wrong. I don't have a use for them but they were too big to just throw away. You'll notice they have a marking indicating that a big 10 MOhm resistor is inside, meant to discharge the capacitor and eliminate the safety risk. This image is mine and you are welcome to use it with credit.

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Essentially everything electronic that you use relies on capacitors to function. Without capacitors, the computer you are reading this on wouldn't even be close to be able to function.

Let me know if you have any questions or comments, or if I got something wrong. Now that I've got capacitors/inductors out of the way I plan to continue this series to more complicated components.

Thanks for reading!

Additional Reading:
Electrolytic Capacitors
How a supercapacitor works