Physical description #
All capacitors are thin plates of conductive material separated by a thin layer of insulating material. For the cylindrical electrolytic capacitors in your kit, they are made of two narrow strips of metal foil separated by a strip of thin brown paper, rolled up like a cinnamon roll.
Capacitors are measured in farads. 1 F is 1 coulomb/volt, a measure of how much charge it takes to raise the voltage of the capacitor by 1 volt. Oddly, most of the capacitors we use in electronics are small relative to 1 F, usually in the range of microfarads (uF, 10^-6) down to picofarads (pF, 10^-12).
Voltage-current relationship #
Capacitors behave like frequency-dependent resistors. The governing equation is: \(I = C dV/dt\) , where \(I\) is current, \(C\) is capacitance, and \(dV/dt\) is the rate at which the voltage is changing. The capacitance is a physical property of a capacitor, and is more or less constant. When the frequency of a signal is high, \(dV/dt\) is high, so more current flows. This makes the apparent resistance of capacitor low at high frequency. The reverse is also true: a capacitor appears high resistance to low frequency signals.
But what is the point of them? #
Capacitors can be used in two main ways: as a small battery and as a filter.