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Capacitors are very useful in electronics circuits they block direct currents and only allow alternating currents through them.
The capacitor has resistance to alternating currents and is called a capacitive reactance Xc.
Complex numbers are useful here to represent the capacitve reactance as a complex impedance in the cartesian form a + jb, where a represents pure d.c. resistance and jb is pure capacitve reactance. Because a capacitor is made up of two metal plates seperated by an insulator there is no d.c. resistance and the term for capacitve reactance is simply 1/jwc or -j/wc. (See Argand Diagrams)