Capacitive reactance

The opposition of a capacitor to AC, due to its capacitance:

X_C = 1 / (2π f C) (ohm)

• X_C is inversely proportional to f and to C. For DC (f = 0) X_C is infinite (blocks DC); for high frequencies X_C is small. Current: I = V/X_C.
• Contrast with the coil: raising the frequency makes a coil oppose the current more, but a capacitor less.

Example 2b. For C = 20 μF: at 50 Hz X_C = 1/(2π × 50 × 20×10⁻⁶) = 159 Ω; at 500 Hz X_C = 15.9 Ω.