Elastance

The unit of elastance is the volt per coulomb or, unofficially, the daraf (D). So if a capacitor has capacitance C = 1 nF, it has elastance S = 1 GD.

The elastance of a capacitor, its charge, and the voltage across it are therefore related as follows:
Q = CV so Q = V/S

and
V = Q/C so V = QS

Because capacitances add reciprocally in series and add directly in parallel, elastances add directly in series and add reciprocally in parallel:
CapacitanceElastance
Series 1/C = 1/C1 + 1/C2 S = S1 + S2
Parallel C = C1 + C2 1/S = 1/S1 + 1/S2
Elastance has the same series/parallel behavior as resistance and inductance.

All of this probably doesn't seem very useful. It isn't. No one ever uses elastance. We will only refer to elastance in one or two pages in this set of lessons, and then only because elastance has the useful property that the voltage across a capacitor is equal to its charge multiplied by its elastance.

I swear I am not making this up. See the American Institute of Physics Handbook, page 5-6.


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