Capacitance
Capacitance is a measure of the ability of a device to store charge.
Definition of Capacitance, C
Units: coulomb/volt = farad, F
Example 16-9
A capacitor of 0.75 µF is charged to a voltage of 16 V. What is the magnitude of the charge on each plate of the capacitor?
Parallel-Plate Capacitor
Capacitance of a Parallel-Plate Capacitor
is
the permittivity of the substance between the parallel-plates. The chart
15-3 on page 550 in the book has the values needed. For air 
.
Example 16-10
A parallel-plate capacitor is constructed
with plates of area 
and
separation 0.550 mm. Find the magnitude of the charge on each plate of this
capacitor when the potential difference between the plates is 20.1 V.
Example 16-11
A parallel-plate capacitor is connected to a battery that maintains a constant potential difference V between the plates. If the plates are pulled away from each other, increasing thier separation, does the magnitude on the plates (a) increase, (b) decrease, or (c) remain the same?
Capacitance of a Parallel-Plate Capacitor Filled with a Dielectric:
where 
is
the dielectric constant found also in table 15-3.
Example 16-12
A parallel-plate capacitor is constructed
with plates of area and
separation 0.550 mm. The space between the plates is filled with a dielectric
with dielectric constant .
When the capacitor is connected to a 12 V battery, each of the plates has
a charge of magnitude 
.
What is the value of the dielectric constant ?
Capacitors in Combination
Capacitors in Parallel
The voltage across the combination of capacitors is equal to the voltage across the battery.
The net charge stored is the sum of the individual amounts stored on each capacitor.
The capacitance is the sum of the individual capacitors.
Capacitors in Series
The amount of charge on each capacitor is equal.
The voltage is the sum of each voltage across the individual capacitors.
The total capacitance:
Example 16-13
Determine the capacitance of the single capacitor that is equivalent to the parallel combination of capacitors shown in the figure, and find the charge on the 12.0 µF capacitor.
Example 16-14
Four capacitors are connected in series
with a battery, as shown in the figure.
(a) Find the capacitance of the equivalent capacitance.
(b) Find the charge on the 12.0 µF capacitor.
Example 16-15
Find the equivalent capacitance between a and b for the combination of capacitors shown in the figure.
Electrical Energy Stored in a Capacitor
The total energy storedin a capacitor with a charge Q and potential difference V can be written:
Example 16-16
A camera flash unit stores energy in a 150 µF capacitor at 200 V. How much electric energy can be stored?
Example 16-17
A parallel-plate capacitor has plates
of dimension 2.0 cm X 3.0 cm separated by a 1.0 mm thickness of paper. (a)
Find the capacitance of this device. (b) What is the maximum charge that
can be placed on the capacitor? (Maximum voltage is 
.)
