Transcript A capacitor is a device that stores electrical potential energy by

A capacitor is a device
that stores electrical
potential energy by
building up a difference
in charge on two pieces
of metal.
The ability of a
capacitor to store
charge is called the
capacitance of the
capacitor.
Capacitance is the ratio
of net charge on each
plate to the potential
difference between the
plates. C =
(or Q = VC)
Q/ΔV
The unit is the farad (F) ,
which is one coulomb per volt
(C/V). A typical capacitor
ranges from microfarads to
picofarads.
mF to pF
1 mF = 10-6 F
-12
1 pF = 10 F
Capacitance for a
parallel plate capacitor
is calculated from:
C = ε0 A/d
ε0 is the permittivity of a vacuum
A is the area of the plates
d is the distance between the
plates
The permittivity depends
on the material between
the plates. For a vacuum
(or air), the value is
-12
2
2
8.85 x 10 C /N•m .
Permittivity is greater for
other materials.
The material between the
plates is an insulator and
is called a dielectric. The
presence of a dielectric
increases the
capacitance.
The molecules of a
dielectric align
themselves with the
electric field, negative
end toward the positive
plate, and vice-versa.
The dielectric effectively
reduces the charge at each
plate, so the plate can
store more charge for a
given voltage. More charge
means more capacitance
for same voltage. (Q = VC)
C = ε0 A/d
As the equation indicates,
the larger the plates of a
capacitor, the higher the
capacitance; and the further
apart the plates are, the
lower the capacitance.
List four ways to
increase the
charge on a
capacitor.
Once a capacitor is charged,
the source can be removed and
the capacitor will retain the
charge. If the two plates are
connected by a conducting
material, the capacitor will
rapidly discharge. This
discharge continues until the
potential is zero.
Devices that use
capacitors include:
camera flash devices,
computer keyboards,
defibrillators, tasers
and automobile
blinkers.
A charged capacitor
stores electrical
potential energy.
The amount is given by
the following equation:
PEelectric = ½QΔV
PEelectric = ½QΔV
Since Q = ΔVC,
PEelectric = ½C(ΔV)2
and:
2
PEelectric = Q /2C
Here are the two big
capacitor equations:
Q = VC
PEelectric =
2
½CV
A 3.0 mF capacitor is
connected to a 12 V battery.
What is the magnitude of
the charge on each plate of
the capacitor, and how
much electrical potential
energy is stored in the
capacitor?