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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, as the value of is typically specified in units of capacitance rather than inverse capacitance. However, elastance is used in theoretical work in network analysis and has some niche applications, particularly at microwave frequencies.
The term elastance was coined by Oliver Heaviside through the analogy of a capacitor to a spring. The term is also used for analogous quantities in other energy domains. In the mechanical domain, it corresponds to stiffness, and it is the inverse of compliance in the fluid flow domain, especially in physiology. It is also the name of the generalized quantity in bond-graph analysis and other schemes that analyze systems across multiple domains.
Elastance ( S) is the reciprocal of capacitance, thus,Camara, p. 16-11
Expressing the values of as elastance is not commonly done by practical electrical engineers, but can be convenient for capacitors in series since their total elastance is simply the sum of their individual elastances. However, elastance is sometimes used by network theorists in their analyses. One advantage of using elastance is that an increase in elastance results in an increase in impedance, aligning with the behavior of the other two basic passive elements, resistance and inductance. An example of the use of elastance can be found in the 1926 doctoral thesis of Wilhelm Cauer. On his path to founding network synthesis, he developed the mesh analysis A:
where L, R, S, and Z are the network loop matrices of inductance, resistance, elastance, and impedance, respectively, and s is complex frequency. This expression would be significantly more complicated if Cauer had used a matrix of capacitances instead of elastances. The use of elastance here is primarily for mathematical convenience, similar to how mathematicians use rather than more common units for angles.Cauer, Mathis & Pauli, p.4. The symbols in Cauer's expression have been modified for consistency within this article and with modern practice.
Elastance is also applied in microwave engineering. In this field, are used as voltage-variable capacitors in devices such as frequency multipliers, parametric amplifiers, and variable filters. These diodes store charge in their p-n junction when , which generates the capacitor effect. The slope of the voltage-stored charge curve in this context is referred to as differential elastance.Miles, Harrison & Lippens, pp. 29–30
The term daraf was coined by Arthur E. Kennelly, who used it as early as 1920.
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 stated,
Here, permittance is Heaviside's term for capacitance. He rejected any terminology that implied a capacitor acted as a container for holding charge. He opposed the terms capacity (capacitance) and capacious (capacitive) along with their inverses, incapacity and incapacious.Howe, p.60 At the time, the capacitor was often referred to as a condenser (suggesting that the "electric fluid" could be condensed), or as a leyden,Heaviside, p.268 after the Leyden jar, an early capacitor, both implying storage. Heaviside preferred a mechanical analogy, viewing the capacitor as a compressed spring, which led to his preference for terms suggesting properties of a spring.Yavetz, pp.150–151
Heaviside's views followed James Clerk Maxwell's perspective on electric current, or at least Heaviside's interpretation of it. According to this view, electric current is analogous to velocity, driven by the electromotive force, similar to a mechanical force. At a capacitor, current creates a "displacement" whose rate of change is equivalent to the current. This displacement was seen as an electric strain, like mechanical strain in a compressed spring. Heaviside denied the idea of physical charge flow and accumulation on capacitor plates, replacing it with the concept of the divergence of the displacement field at the plates, which was numerically equal to the charge collected in the flow view.Yavetz, pp.150–151
In the late 19th and early 20th centuries, some authors adopted Heaviside's terms elastance and elastivity.See, for example, Peek, p.215, writing in 1915 Today, however, the reciprocal terms capacitance and permittivity are almost universally preferred by electrical engineers. Despite this, elastance still sees occasional use in theoretical work. One of Heaviside's motivations for choosing these terms was to distinguish them from mechanical terms. Thus, he selected elastivity rather than elasticity to avoid the need to clarify between electrical elasticity and mechanical elasticity.Howe, p.60
Heaviside carefully crafted his terminology to be unique to electromagnetism, specifically avoiding overlaps with mechanics. Ironically, many of his terms were later borrowed back into mechanics and other domains to describe analogous properties. For example, it is now necessary to differentiate electrical impedance from mechanical impedance in some contexts.van der Tweel & Verburg, pp.16–20 Elastance has also been used by some authors in mechanics to describe the analogous quantity, though stiffness is often preferred. However, elastance is widely used for the analogous property in the domain of fluid dynamics, particularly in fields such as biomedicine and physiology.See for example Enderle & Bronzino, pp.197–201, especially equation 4.72
The first reason is to explain electrical phenomena in terms of more familiar mechanical systems. For example, the differential equations governing an electrical RLC circuit (inductor-capacitor-resistor circuit) are of the same form as those governing a mechanical mass-spring-damper system. In such cases, the electrical domain is translated into the mechanical domain for easier understanding.
The second, more significant reason is to analyze systems containing both mechanical and electrical components as a unified whole. This approach is especially beneficial in fields like mechatronics and robotics, where integration of mechanical and electrical elements is common. In these cases, the mechanical domain is often converted into the electrical domain because network analysis in the electrical domain is more advanced and highly developed.Busch-Vishniac, pp.17–18
Since elastance is defined as the ratio of voltage to charge, its analogue in other energy domains is the ratio of a generalized force to a generalized displacement. Therefore, elastance can be defined in any energy domain. The term elastance is used in the formal analysis of systems involving multiple energy domains, such as in .Vieil, p.47
| +Definition of elastance in different energy domains |
| Elastance |
| Stiffness/elastanceHorowitz, p.29 |
| Rotational stiffness/elastance Moment of stiffness/elastance Torsional stiffness/elastance |
| Elastance |
| Warming factorFuchs, p.149 |
| PermeanceKarapetoff, p.9 |
| Inverse chemical capacitanceHillert, pp.120–121 |
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