Yeah that's a reasonable summary Gryd3.
You can look at a capacitor from two points of view: the time domain, and the frequency domain. The frequency domain means that you look at the behaviour of a capacitor in response to different frequencies. This tends to be quite mathematical and I don't know of any real-world analogies that can make it easier to understand. I only have a basic knowledge of the frequency domain myself.
The behaviour of a capacitor in the time domain is much easier to explain and understand. A capacitor integrates current with respect to time. That means that the voltage across a capacitor changes over a period of time according to the amount of current that is fed into (or out of) it.
The basic definition of a capacitor's behaviour is:
dV/dT = I / C
I is current, in amps, and C is capacitance, in farads.
dV/dT is the "rate of change of voltage". dT is a change, or difference, in voltage (the voltage across the capacitor), measured in volts, and dT is a change or difference in time, measured in seconds. The unit for dV/dT is volts per second (V/s).
For example, a voltage change of 1V in 1 second is a dV/dT of 1/1 or 1 volt per second (1 V/s), which you could describe as fairly gradual. But a voltage change of 1000V in 1 second is a dV/dT of 1000/1 or 1000 V/s which is a more rapid change in voltage. A moderate change in a short time is also a high rate of change - 1V in 1 µs is a dV/dT of 1/1
-6 V/s which is 1,000,000 V/s or 1 V/µs, a rapid rate of change.
A simple analogy for a capacitor is a container with an open top, with water in it. The container must have straight sides, or at least an equal cross-sectional area from the bottom to the top. This cross-sectional area corresponds to the capacitance, which I will get to later.
The depth of water in the container corresponds to the voltage, and current is represented by water flowing into, or out of, the container. You can imagine that with a constant flow of water (current) into the container, the voltage (water level) will increase steadily. With a constant flow of water out of the container, the water level will drop steadily.
The cross-sectional area determines how quickly the water level will change for a given flow. If the cross-sectional area is small, for example a chemistry test tube, the capacitance is small, and it will charge or discharge quickly, even with only a small flow. If the cross-sectional area is large, for instance a swimming pool, the level will only change slowly and it will take a lot of time, even at a high flow rate, to change the level significantly. This is like a supercapacitor.
Now you should be able to understand the formula dV/dT = I / C.
A higher current (I) will cause a greater rate of change in the voltage (dV/dT), and a lower capacitance (C) will also cause a greater rate of change. That formula just spells out the exact relationship. Quantities are defined as:
dV = change in voltage across the capacitor, in volts
dT = change in time, in seconds
I = current flow, in amps
C = capacitance, in farads.
That's a simple model of a capacitor, but it doesn't deal with negative water levels.
The direction of the voltage change corresponds to the direction of the current. In the simplest terms, if current (conventional current, which flows from positive to negative) is flowing into one plate, that plate will become more positive, or less negative, with respect to the other plate. This is always true. Sometimes talking of "charging" or "discharging" can become confusing, because capacitors (non-polarised capacitors, at least) can be "charged" in either direction (either plate can be positive with respect to the other), and "charging" is best defined as increasing the voltage between the plates without regard for its polarity. Therefore, a capacitor may be "charged" by a positive current, or by a negative current, depending on how it is used in the circuit.
One more thing I should mention. A farad is quite a large amount of capacitance. Most of the capacitors used in electronics are measured in smaller units using the standard SI prefixes. These units are pF (picofarads) (1 pF = 10
-12 farads, a tiny amount of capacitance); nF (nanofarads) (1 nF = 10
-9 farads), and µF (microfarads) (1 µF = 10
-6 farads), a moderate amount of capacitance). You can actually buy capacitors with values from under 1 pF to hundreds of thousands of µF, and supercapacitors with capacitance of many farads. That's a range of over 14 orders of magnitude, which dwarfs the range for other types of components such as resistors. Many different materials and construction techniques are used to cover this wide range though.
Now I hope you understand what a capacitor
does but there is a lot to know about how they are used in a typical circuit. The main applications are decoupling, coupling, timing, slowing down and speeding up signals, filtering, and charge transfer. Some of those categories overlap. I'll post some more when I have time.
Edit: Correction. I may post more... someday
