 
Phys 4, Section
2
Jan 30, 2002
Capacitors
A capacitor is a device composed of two conductive plates separated by an
insulator. A capacitor is charged by moving charge from one plate to the other.
Thus the charge on the two plates is equal and opposite, with the total charge
on the device being zero. Its capacitance is defined at the ratio of the voltage
across the two conductors to the charge on the positive plate. C = Q/V (units of
farads) Actually, a farad is a very large unit, so in practice, microfarads and
nanofarads, even picofarads, are more common units. The capacitance of a
capacitor depends only on its geometry. The classic capacitor is assumed to be
two large flat plates of area A separated by a distance d which is
very small compared to the area. The space between the plates is filled with air
or vacuum.
In fact, many capacitors have insulating material, a dielectric, between the
plates. This increases the capacitance. In the course, we will not use
dielectric materials.
Note that the capacitance is measured by assuming some test charge
configuration, e.g., +Q on one plate, Q on the other.
However, the capacitance is a geometric quantity and has the same value even
when the capacitor is uncharged.
Capacitor Combinations
Series: 1/C_{eff} = 1/C_{1} + 1/C_{2} + … Parallel:
C_{eff} = C_{1} + C_{2} + …
Last modified on February 11, 2002
