Admittance

In electrical engineering, admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the reciprocal of impedance, analogous to how conductance and resistance are defined. The SI unit of admittance is the siemens (symbol S); the older, synonymous unit is mho, and its symbol is ℧ (an upside-down uppercase omega Ω). Oliver Heaviside coined the term admittance in December 1887. Heaviside used Y to represent the magnitude of admittance, but it quickly became the conventional symbol for admittance itself through the publications of Charles Proteus Steinmetz. Heaviside probably chose Y simply because it is next to Z in the alphabet, the conventional symbol for impedance. Admittance is defined as where Y is the admittance, measured in siemens Z is the impedance, measured in ohms Resistance is a measure of the opposition of a circuit to the flow of a steady current, while impedance takes into account not only the resistance but also dynamic effects (known as reactance). Likewise, admittance is not only a measure of the ease with which a steady current can flow, but also the dynamic effects of the material's susceptance to polarization: where is the admittance, measured in siemens. is the conductance, measured in siemens. is the susceptance, measured in siemens. The dynamic effects of the material's susceptance relate to the universal dielectric response, the power law scaling of a system's admittance with frequency under alternating current conditions. The impedance, Z, is composed of real and imaginary parts, where R is the resistance, measured in ohms X is the reactance, measured in ohms Admittance, just like impedance, is a complex number, made up of a real part (the conductance, G), and an imaginary part (the susceptance, B), thus: where G (conductance) and B (susceptance) are given by: The magnitude and phase of the admittance are given by: where G is the conductance, measured in siemens B is the susceptance, also measured in siemens Note that (as shown above) the signs of reactances become reversed in the admittance domain; i.