Capacitor
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A capacitor stores energy in an electric field between conductors separated by an insulating region. Capacitors provide coupling, bypassing, filtering and tuning in electronic and radio circuits.
Capacitance and energy
Q = C V E = ½ C V²
C is capacitance in farads (F), Q is the magnitude of charge on either plate in coulombs, V is voltage between the plates, and E is stored energy in joules. Common units are μF (10^-6 F), nF (10^-9 F) and pF (10^-12 F).
A 1000 μF capacitor charged to 12 V stores 0.072 J. Its two plates carry opposite charges; the device need not have a net charge overall.
Frequency response
The magnitude of the reactance of an ideal capacitor is:
X_C = 1 / (2 π f C)
f is frequency in hertz and X_C is in ohms. At 1 MHz, an ideal 100 nF capacitor has about 1.59 Ω of reactance. Its impedance is -jX_C: the current leads the voltage by 90 degrees in the ideal sinusoidal model.
In a simple RC charging circuit, the time constant is τ = RC. After one time constant, an initially uncharged capacitor reaches about 63.2% of its final voltage. For 10 kΩ and 10 μF, τ is 0.1 s.
Real components
Real capacitors have leakage, equivalent series resistance and lead or package inductance. Above their self-resonant frequency, their behaviour can become inductive. The simple reactance formula is not enough to select an RF bypass component.
Voltage rating, dielectric type, temperature and polarity matter. Many electrolytic capacitors are polarised. A stored voltage can remain after the circuit is switched off.