Электр серпімділігі және оның физиологиялық мағынасы

Mechanical stiffness, or inverse of electrical capacitance or fluid flow compliance

the physiological term
Electrical elastance is the reciprocal of capacitance. The SI unit of elastance is the inverse farad (F−1). The concept is not widely used by electrical and electronic engineers. The value of capacitors is invariably specified in units of capacitance rather than inverse capacitance. However, it is used in theoretical work in network analysis and has some niche applications at microwave frequencies. The term elastance was coined by Oliver Heaviside through the analogy of a capacitor as a spring. The term is also used for analogous quantities in some other energy domains. It maps to stiffness in the mechanical domain, and is the inverse of compliance in the fluid flow domain, especially in physiology. It is also the name of the generalised quantity in bond graph analysis and other schemes analysing systems across multiple domains.

The SI unit of elastance is the reciprocal farad (F−1). The term daraf is sometimes used for this unit, but it is not approved by SI and its use is discouraged. The term is formed by writing farad backwards, in much the same way as the unit mho (unit of conductance, also not approved by SI) is formed by writing ohm backwards. The term daraf was coined by Arthur E. Kennelly. He used it from at least 1920.

The terms elastance and elastivity were coined by Oliver Heaviside in 1886. Heaviside coined a great many of the terms used in circuit analysis today, such as impedance, inductance, admittance, and conductance. Heaviside's terminology followed the model of resistance and resistivity with the ance ending used for extensive properties and the ivity ending used for intensive properties. The extensive properties are used in circuit analysis (they are the "values" of components) and the intensive properties are used in field analysis. Heaviside's nomenclature was designed to highlight the connection between corresponding quantities in field and circuit. Elastivity is the intensive property of a material corresponding to the bulk property of a component, elastance. It is the reciprocal of permittivity. As Heaviside put it,
Permittivity gives rise to permittance, and elastivity to elastance.|Oliver Heaviside
Here, permittance is Heaviside's term for capacitance. He did not like any term that suggested that a capacitor was a container for holding charge. He rejected the terms capacity (capacitance) and capacious (capacitive) and their inverses incapacity and incapacious. The terms current in his time for a capacitor were condenser (suggesting that the "electric fluid" could be condensed out) and leyden after the Leyden jar, an early form of capacitor, also suggesting some sort of storage. Heaviside preferred the analogy of a mechanical spring under compression, hence his preference for terms that suggested a property of a spring. This preference was a result of Heaviside following James Clerk Maxwell's view of electric current, or at least, Heaviside's interpretation of it. In this view, electric current is a flow caused by the electromotive force and is the analogue of velocity caused by a mechanical force. At the capacitor, this current causes a "displacement" whose rate of change is equal to the current. The displacement is viewed as an electric strain, like a mechanical strain in a compressed spring. The existence of a flow of physical charge is denied, as is the buildup of charge on the capacitor plates. This is replaced with the concept of divergence of the displacement field at the plates, which is numerically equal to the charge collected on the plates in the charge flow view. For a period in the nineteenth and early twentieth centuries, some authors followed Heaviside in the use of elastance and elastivity. Today, the reciprocal quantities capacitance and permittivity are almost universally preferred by electrical engineers. However, elastance does still see some usage by theoretical writers. A further consideration in Heaviside's choice of these terms was a wish to distinguish them from mechanical terms. Thus, he chose elastivity rather than elasticity. This avoids having to write electrical elasticity to disambiguate it from mechanical elasticity. Heaviside carefully chose his terms to be unique to electromagnetism, most especially avoiding commonality with mechanics. Ironically, many of his terms have subsequently been borrowed back into mechanics and other domains in order to name analogous properties. For instance, it is now necessary to distinguish electrical impedance from mechanical impedance in some contexts. Elastance has also been borrowed back into mechanics for the analogous quantity by some authors, but often stiffness is the preferred term instead. However, elastance is widely used for the analogous property in the domain of fluid dynamics, especially in the fields of biomedicine and physiology.

Mechanical–electrical analogies are formed by comparing the mathematical description of the two systems. Quantities that appear in the same place in equations of the same form are called analogues. There are two main reasons for forming such analogies. The first is to allow electrical phenomena to be explained in terms of the more familiar mechanical systems. For instance, an electrical inductor capacitor resistor circuit has differential equations of the same form as a mechanical mass spring damper system. In such cases the electrical domain is converted to the mechanical domain. The second, and more important, reason is to allow a system containing both mechanical and electrical parts to be analysed as a unified whole. This is of great benefit in the fields of mechatronics and robotics. In such cases the mechanical domain is most often converted to the electrical domain because network analysis in the electrical domain is highly developed.

Maxwell's analogy is not the only way that analogies can be constructed between mechanical and electrical systems. There are any number of ways to do this. One very common system is the mobility analogy. In this analogy force maps to current instead of voltage. Electrical impedance no longer maps to mechanical impedance, and likewise, electrical elastance no longer maps to mechanical elastance.