Chapter
15 & 16
Electrostatics:
Forces & Energy
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?
Solution:
Using ,
solve for Q:
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 .
Factors Affecting Capacitance
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.
Solution:
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?
Solution:
(b)
The charge on the plates decreases.
**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 ?
Solution:
Find
the capacitance:
Solve
for
:
Substitute
the numerical values:
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:
On to example of Circuits with Capacitors |