Impedance of components
Let's recap what we now know about voltage and curent in linear components. The
impedance is the general term for the ratio of voltage to current. Resistance is the special case of impedance when φ = 0, reactance the special case when φ =
± 90°. The table below summarises the impedance of the different components. It is easy to remember that the voltage on the capacitor is
behind the current, because the charge doesn't build up until after the current has been flowing for a while.
The same information is given graphically below. It is easy to remember the frequency dependence by thinking of the DC (zero frequency) behaviour: at DC, an inductance is a short circuit (a piece of wire) so its impedance is zero. At DC, a capacitor is an open circuit, as its circuit diagram shows, so its impedance goes to infinity.
