Two conductors that are not connected and are separated by an insulator constitute a capacitor. When a source of EMF such as a cell is connected to such an arrangement, current flows momentarily, transferring change (in the form of electrons) from one conducting plate (the + plate) to the other. When a quantity of charge Q (measured in units of coulombs) has been transferred, the voltage across the plates equals the voltage V across the voltage source. For a fixed arrangement of conductors and insulator, the ratio Q/V is a constant called the capacitance, C. The relationship can be written in the three forms:
Q = CV C = Q/V V = Q/C
with ‘V’ in volts, ‘Q’ in coulombs and ‘C’ in farads
Basic principles of the capacitor. The relationship Q/V shown in the graph, is defined as the capacitance, C.