What a capacitor is
Two pieces of metal facing each other with a gap between them. Read as a circuit that is a break, and yet current runs in both wires the moment you connect it — which is the whole part, and the reason a board has a dozen of them on it.
A resistor lets current through and charges you voltage for it. A capacitor does something no other part in the drawer does: it lets current run in the wires while nothing at all crosses the middle of it.
Close the switch and watch the gap.
Three things happened there, and the third is the one people miss.
Charge piled up on the plates — positive on one, negative on the other, and exactly as much of each. A field appeared between them, growing as the charge did. And then, with the switch still closed and the battery still connected, the current stopped.
It passes a change, not a voltage
That is the sentence to keep. Current flows into a capacitor only while the voltage across it is changing. Hold the voltage still and the current is zero, whatever the voltage is.
Which explains the two things a capacitor is ever fitted to do. It blocks DC, because a steady voltage is a voltage that has stopped changing. And it passes a signal, because a signal is nothing but change.
The sheet is the useful half of that picture. Nothing gets past it in either direction — and water still moves in the pipe every time the pump changes its mind, which is exactly the trick a capacitor pulls on a circuit.
Charge per volt is the farad
Push harder and more charge goes on. Double the voltage and you double the charge, and the ratio between them does not move:
C = Q ÷ V — charge in coulombs, voltage in volts, and the answer in farads.
A farad is an absurd unit. A coulomb is about six billion billion electrons, and a one-farad capacitor holds one of them for every volt you put across it — so almost every part you will meet is measured in millionths of a farad or smaller:
- pF, picofarads — a millionth of a millionth. Radio, crystals, the stray capacitance between two tracks that you did not fit and cannot remove.
- nF, nanofarads — thousandths of a millionth. The 100 nF beside every chip.
- µF, microfarads — millionths. The big ones on a supply rail.
A marking of 104 on a ceramic is the same code as a resistor's bands: 10 with
four zeros, in picofarads. 100,000 pF, which is 100 nF, which is 0.1 µF. All
three of those appear on schematics and all three are the same part.
Next: what is actually between those plates on a real part, and why a 100 µF is the size of a thumb while a 100 nF is smaller than a grain of rice.
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