Capacitance and Inductance

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Some typical capacitors

Capacitors collect and hold charge. The larger the capacitor, the more electrons it can hold for a given voltage:

where is capacitance, is charge, and is voltage. Since current, , is the flow of charge versus time, we may alternately express capacitance as relating current to a voltage change versus time:

A useful way of thinking of a capacitor is as two parallel plates with a wire connected to each side. Indeed, many capacitors are actually built this way. Imagine that we have a positive charge and put it on one side of the capacitor. Electrons, being naturally attracted to a negative charge, will collect at the other plate, if they are free to move, because that’s the closest they can get to positive charge that is attracted them. If these electrons are not given another path to the opposite plate, they will collect until their charges sum to , whereupon the repulsion of like charges offsets the attraction of the positive charge, and we will have achieved equilibrium. In the steady state, the charge difference between the two plates means there is a potential difference, or voltage , between them. For a given separation between them, larger plates make for bigger capacitors, as measured by capacitance . Capacitance is measured in Farads, with typical capacitors ranging from hundreds of picofarads (slightly more than the capacitance between nearby wires) to farads (will weld metal, and maybe explode, if you touch the leads together).

Capacitors in Series

Capacitors series.png

Capacitors in series

Capacitors in series add reciprocally. You can maybe see this by considering the total voltage drop across two capacitors in series. Clearly, each capacitor must individually see only a fraction of that voltage drop, and so will not collect as much charge. Since capacitance is the charge collected for a given voltage, the total capacitance must have decreased.:

Capacitors in Parallel

Capacitors parallel.png

Capacitors in parallel

Capacitors in parallel add, as should be apparent from examining the parallel plate capacitor discussed above. Two pairs of parallel plates behave just as one larger pair of plates whose area is the sum of the two original plates:



Some typical inductors

Inductors in Series

Inductors series.png

Inductors in series

Inductors in series add:

Inductors in Parallel

Inductors parallel.png

Inductors in parallel

Inductors in parallel add reciprocally: