The ability of a device to stored charge is called its capacitance.
Certain materials called dielectrics can be used to increase
the capacitance of a capacitor.
Since separated charges has potential energy, a charged capacitor
stores energy.
Capacitance
Capacitor: two conductor separated by a distance,
example parallel-plate capacitor.
Charging a capacitor: Charge can be moved from one
conductor to the other thus "charging" the capacitor.
In practice, how do we move the charges from one conductor to
the other?
Capacitance: The voltage difference between the two
conductor is linear proportional to the net charge on each conductor,
hence V/Q is indepedent of the amount charge but only depends
on the geometry of the conductors. Derive result for parallel-plate
capacitor. This ratio is called the capacitance of the capacitor.
Note: The higher the capacitance, the large amount
of charge the capcitor can hold at a given voltage difference
or likewise it has a smaller voltage for a given amount of charge
stored.
Dielectric Materials
Increasing the capacitance: we can increase the capacitance
if we can lower the voltage difference for agiven amount of charge
stored. To lower the voltage, we need to lower the E-field, which
can be done by putting a polarizable materials (dielectrics)
in between the conductors. The induced dipole moments create
a E-field in the opposite direction of that of the capacitor.
Examples of dielectric materials: Table 21-1, P. 697.
Electrostatic Energy Stored in charged capacitor
The energy stored in acharged capacitor can be expressed
in terms of its capacitance (C), the charge (Q), and/or the voltage
difference (V), Eq. 21-16.
We can also interpret that the energy is stored in the E-field.