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# Why Capacitance?

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A question asked frequently is why use capacitance? Resistive sensors have been used for a long time…so why change?

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### Why Capacitance?

1. 1. Why Capacitance? One question asked frequently; why use capacitance? Resistive sensors have been used for a long time…so why change? Here is an attempt to answer this question: 1. What is capacitance? a. It is the ability of a system to store charge. So if you take a set of parallel plates and apply a potential difference to it (Voltage), then for a brief time interval, electrons move from one plate to the other (a small current is generated) until it reaches a state of equilibrium (the current stops). The charge gets stored between the two parallel plates. b. The amount of charge that can stored (its Capacitance) depends on the area between the plates, the distance between the plates and the dielectric constant between the plates. c. The formula for the Capacitance of the system can be shown as: i. C = Er*E0*A/d where Er is the dielectric Constant, E0 is the Permittivity of free space, A is the area of overlap between the plates and d is the gap between the plates ii. For Air, Er=1 and E0 is a constant equal to 0.00854 picoFarad/mm iii. So if you have two small overlapping discs of radius=5.6 mm and the gap between them equal to 0.25 mm, then the Capacitance of the system- C - is given by C = (1.0)*(0.00854)*(3.14159)*(5.6)2/(0.25) or 3.4 pF iv. If the gap between the plates changes to 0.17 mm then the capacitance increases to 4.9 pF v. That is a change of 1.5 on 3.4 or 44.11%