It is often listed on data sheets for electrolytic capacitors used in switching power supplies. At high frequencies, the ESR is the limiting factor in the performance of the capacitor. The quality factor, Q, is the ratio of the imaginary impedance to the real impedance. For inductors, a high Q indicates a more reactively pure component. A low Q indicates a substantial series resistor.
Q varies with frequency.
With resistors, often all that is stated is that the resistor has low inductance. A low D indicates a nearly pure capacitor. Inc. All rights reserved.