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Starting electronics
Alternatively, we may define the farad (as we promised we
would, earlier in the chapter) as being the capacitance which
will store a charge of one coulomb when a voltage of one volt
is applied.
We can now understand why it is that changing the capacitor
value changes the time constant, and hence changes the
associated time delay in the changing voltage across the capacitor.
Increasing the capacitance, say, increases the charge
stored. As the current flowing is determined by the resistance
in the circuit, and is thus fixed at any particular voltage, this
increased charge takes longer to build up or longer to decay
away. Reducing the capacitance reduces the charge, which is
therefore more quickly stored or more quickly discharged.
Similarly, as the resistor in the circuit defines the current
flowing to charge or discharge the capacitor, increasing or
decreasing its value must decrease or increase the current,
therefore increasing or decreasing the time taken to charge or
discharge the capacitor. This is why the circuit’s time constant
is a product of both capacitance and resistance values.
Capacitance values
Finally, we can look at how the size and construction of
capacitors affects their capacitance. The capacitance of a
basic capacitor for example, consisting of two parallel, flat
plates separated by a dielectric may be calculated from the
formula:
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