# Capacitors

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Electronics are becoming ever more prevalent in my uni course (I’m doing sound engineering). Knowing how your own equipment works and stuff is integral but what is a capacitor. I failed physics in school almost. What does it do. Help.

In: 31

A capacitor is two very thin sheets of metal film (plates) separated by a very thin layer of dielectric (non-conducting material), rolled up into a convenient size.

When you energize one side of a capacitor, the voltage creates a potential between the plates, which generates an electrostatic charge in and around the dielectric. Electrical potential in the capacitor builds at a decreasing rate until the capacitor is full.

When you disconnect the capacitor, the accumulated electrostatic charge remains (sometimes for weeks or months if left alone). When a path for the potential to flow toward opens, the capacitor discharges.

Capacitors are useful in applications where briefly storing electrical potential is useful.

As well, since resistance and capacitance (Farads, unit of capacitance) determine a capacitor’s charge/discharge time, capacitors are essential in circuits that generate oscillating waveforms.

Think of capacitors as electrical springs that can compress and rebound voltage and current.

A capacitor is a component that is able to store electric charge.

If you have a battery just connected to a loop of wire, it will push electrons around the circuit.

If you break the loop of wire completely, the battery can only push a small amount of electrons up towards the break before their repellent forces, pushing each other and resisting being pushed together, overcome the electromotive force of the battery. So not very many electrons get stuck at the break.

What if you break the loop, but leave the two tips of the wire very close together, and also add two wide plates at the tips? Then electrons build up on the plate at one tip, and the plate at the other tip is being depleted of electrons and becoming positively charged. That positive charge helps attract more electrons to the negatively charged plate than the battery would be able to push there on its own.

The wider the plates and the thinner the gap, the more charge a capacitor can store. It also matters what is in the gap: if it’s air, there’s a greater risk that the electrons will arc across as lightning, compared to if a stronger insulator is inserted into the gap between plates.

Basically a capacitor stores an electrical charge. However unlike a typical battery, which stores electric charge through a chemical reaction, a capacitor stores its charge as a build up of static charge. The exact same kind of static charge you get from rubbing a balloon on your hair, or rubbing your socks on a carpet. A capacitor can build or release its energy really fast, but it doesn’t last very long. This is perfect for a lot of applications. An easy example is the flash bulb on a camera. The bulb is powered by a capacitor that can dump all of its energy in a fraction of a second. You couldn’t get that fast of a release out of a chemical battery. With old camera you can sometimes hear a high pitched whine as the capacitor recharges after a flash. Big electric motors also use capacitors to help kick-start themselves.

Capacitors are also really useful in tons of other electronics because they act like a little reservoir. In converting AC to DC, capacitors help to smooth out the flow of electricity between the ups and down of the AC current. They are also all over audio equipment, both in power delivery and signal processing.

So I’ll leave construction part short, a capacitor is two plates separated by paper, sometimes rolled up, and soaked in some fluid.

It has three main functions,

1. it can store a charge, and when you combine it with inductors and resistors you can build simple timers with them.

2. Next it can smooth signals, you can use this to take a sine wave and turn it into other functions like saw waves and triangle waves. This is also part of how AC to DC converters work

3. And maybe the most important, is that it can filter wavelengths. To a DC signal, a capacitor is an open circuit, but the higher the frequency, the easier it passes thru.

Also, if it helps, a capacitor and and inductor are similar. A capacitor will charge when exposed to voltage, and discharge when the voltage is lowered or stops, so you can think of it as a component that resists changes in voltage, just like an inductor resists changes in current.

A capacitor stores electric charge.

– A capacitor near the + and – terminals of a component helps smooth voltage drops or spikes when that component changes its electrical demand (e.g. switching parts of itself on or off).
– A capacitor can be used to filter out certain frequencies, either by itself or as part of a larger filter with multiple components.

Capacitance is how “big” a capacitor’s “bucket” for storing charge is. Capacitance is measured in Farads (F) (named after Michael Faraday). But for most uses that’s a pretty huge unit, so you more commonly see capacitors rated in uF, nF or pF (microfarad, nanofarad, or picofarad, meaning millionth, billionth or trillionth of a farad).

Capacitors can charge or discharge very quickly. This makes them useful, but it also means they have a bit more risk of shock than batteries. If you’re actually physically building an electronic device with capacitors, for anything with a capacitance of uF or larger you should probably do some basic safety precautions:

– Add a resistor between the capacitor’s terminals to drain its charge over time
– Touch both terminals with a screwdriver to discharge it before putting your fingers anywhere near it
– Only work with low voltages

As ELI5 as I can manage:

A capacitor is a tiny battery. But it can only hold its energy for a very small amount of time due to its special construction.

They’re used in basically every electronic device out there.

Why?
Because we can use the fact that it releases its energy quickly to do lots of cool things! Like keep your fan running steadily, or make a huge camera flash.

Without getting into details, a capacitor is a tiny battery used for lots of cool stuff.

It’s like a small battery that runs out of juice faster than a standard battery.

They’re used to smooth out variations in voltage in case the voltage increases or decreases, or provides power for short periods of time in case the power goes out.

More technically, the simple capacitor has two plates with a dielectric material between them. Electrons gather on one plate while the other plate has none. The electrons want to jump to the other plate creating a voltage potential, effectively “charging” the capacitor as more electrons gather on the charged plate until it can’t hold any more. When the charged plate is full, electrons begin jumping across the dielectric material generating current. When the source voltage drops, the electrons continue to jump to the uncharged plate matching the capacitor’s rated voltage and decreasing voltage over time as all the electrons jump across. This provides the circuit with a short burst of voltage and current. Capacitors are widely used as voltage filters to prevent over/under-voltage from the source that can potentially damage components further down the circuit or cause unwanted circuit behavior.

Correction to all the answers here: Capacitors do NOT store charge. They store an imbalance of charge, stated as “they store energy”. Any EE or Physicist would blanch at calling it a storage of charge.

As far as ELI5? If you think of electricity as a flow of water, where the pressure is voltage and the total volume of water flowing is the current, then a capacitor is like a bucket that saves up some of that water at various points. This is useful for when the circuit suddenly requires more “water”, it can get it from the bucket and not from the source which is a longer distance away.

Extending this analogy, you can think of inductors as flywheels. When current flows through the inductor , it at first slows the current down, but once the flywheel gets going, it’s as if it doesn’t exist anymore because it’s turning at the same rate as the water flow. If the water stops flowing, the flywheel continues to kick water in the direction of the current, keeping the current going for a bit.

There’s some great explanations here, but I’ll add one other bit to them, which may help you understand why they’re so useful and ubiquitous.

Capacitors are frequently compared to rechargeable batteries- they store energy, and while they use different principles (batteries use chemical energy, capacitors hold a charge difference on conductive plates), they can both release that charge and be charged up.

But capacitors can be charged and discharged *extremely* quickly. The flip side of this is that they can’t store very much energy compared to a battery (make no mistake, though- some capacitors can store enough power to hurt or even kill you).

A battery could be thought of as a giant reservoir of energy that can only be tapped slowly- it’ll keep providing that energy for a very long time, but you can only pull power out of it at a specific rate. A capacitor has a bigger input and output, allowing it to store and release energy extremely quickly, but has a smaller overall storage volume.

In sound engineering, you’ll frequently find capacitors being used for filtering- taking a noisy signal and helping to smooth it out- as well as in power supplies and amplifiers. You’ll learn more as you go further in your course, but just be aware that when you work with big amps, they can pack a wallop. 🙂

Here’s a graph of a square wave at the input plugged into an RC (resistor and capacitor) circuit.

[https://cdn2.webdamdb.com/md_MbQx0zCMqo01.png?1535479309](https://cdn2.webdamdb.com/md_MbQx0zCMqo01.png?1535479309)

As you can see, the high voltage takes a bit to “fill up” the capacitor, and then when the input goes low the capacitor takes some time to “empty” which slows down the edge rate of the square wave.

This on its own is kinda meaningless. Why are capacitors useful? Well here, lets look at a signal that doesn’t have perfect signal integrity like our model from above:
[https://www.researchgate.net/figure/Overshoot-and-undershoot_fig2_37931458](https://www.researchgate.net/figure/Overshoot-and-undershoot_fig2_37931458)

This is an example of the input signal having too fast of an edge rate and as the electrical waves arrive at the endpoint of the signal, extra charge is collecting at the input pin of whatever device needs to examine the signal. A physical example is if you push water towards the drain while you’re in the bathtub, the waves collect unevenly at the end before they settle out to equilibrium. The electrical solution to this overshoot/undershoot “ringing’ problem is to add capacitance to slow down how quickly the waves can collect at the input pin.

Instead of slowing down the edge rate and distorting the signal (first example), we’re filtering out noise from our system so that we’re able to measure something closer to the intended signal that had been distorted by the non-ideal aspects of our system.

The physics-ey buzzwords like “low-pass filter” apply here. The ringing from the overshoot/undershoot would add extra high frequency noise that could sound like a whine to our ears if we heard it in speakers. Properly designed sound systems have the right amount of capacitance (and some other things too) to filter out unwanted whining or buzzing noise in our sound systems.

It would be fun to make a presentation or a lab demonstration of this phenomenon where you plug in a signal that has noise and then have the students tune the capacitance of an RC filter to get rid of the noise in the signal.

Source: Am electrical engineer who plays guitar and took a class on “acoustic engineering” as an elective while in college.