What does a capacitor actually do?
It stores energy in an electric field between two conducting plates separated by an insulator. Charge builds on the plates until the voltage across them matches whatever is driving them. Unlike a battery, which stores energy chemically and releases it slowly at a fairly steady voltage, a capacitor stores far less energy but can deliver and absorb it almost instantly. That speed is what makes it useful for smoothing supplies, filtering signals and buffering sudden current demands.
How do you calculate charge in a capacitor?
Q equals C times V — charge in coulombs is capacitance in farads multiplied by voltage. The farad is an enormous unit, so real components are marked in microfarads, nanofarads and picofarads. A 100 µF capacitor at 12 V holds 100 × 10⁻⁶ × 12 = 1.2 × 10⁻³ coulombs, or 1,200 microcoulombs. Keeping the powers of ten straight is where most errors in this calculation happen.
What is the RC time constant?
When a capacitor charges through a resistor the voltage rises exponentially rather than linearly, and the time constant τ = R × C sets the pace. After one time constant the capacitor has reached about 63% of the supply voltage, after three about 95%, and after five roughly 99% — treated as fully charged for practical purposes. A 100 µF capacitor through a 10 kΩ resistor gives τ = 1 second, so it is effectively charged after five.
Why is the farad such a large unit?
Because one farad means storing one coulomb of charge per volt, and a coulomb is an enormous quantity of charge — about 6.24 × 10¹⁸ electrons. Ordinary capacitors are in the microfarad range or below, and even a supercapacitor rated at several farads is physically substantial. The unit was defined from the underlying physics rather than from anything convenient to build, which is why practically every real component is marked with a prefix.
What is the difference between a capacitor and a battery?
A battery stores energy chemically, holds a roughly constant voltage as it discharges, and packs far more energy per unit volume. A capacitor stores energy physically in a field, its voltage falls steadily as it discharges, and it holds perhaps a thousandth as much energy for the same size. What a capacitor offers instead is power: it can dump its energy in microseconds and be recharged endlessly without wearing out, where a battery would be damaged by either.