A capacitor is a device that stores electrical potential energy by building up a difference in charge on two pieces of metal.

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A capacitor is a device that stores electrical
potential energy by building up a difference in
charge on two pieces of metal.
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The ability of a capacitor to store charge is
called the capacitance of the capacitor.
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Capacitance is the ratio of net charge on each
plate to the potential difference between the
plates. C Q/?V (or Q VC)
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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 1 pF 10-12 F
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Capacitance for a parallel plate capacitor is
calculated fromC e0 A/de0 is the
permittivity of a vacuumA is the area of the
platesd is the distance between the plates
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The permittivity depends on the material between
the plates. For a vacuum (or air), the value is
8.85 x 10-12 C2/Nm2. Permittivity is greater
for other materials.
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The material between the plates is an insulator
and is called a dielectric. The presence of a
dielectric increases the capacitance.
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The molecules of a dielectric align themselves
with the electric field, negative end toward the
positive plate, and vice-versa.
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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)
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C e0 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.
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List four ways to increase the charge on a
capacitor.
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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.
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Devices that use capacitors include camera flash
devices, computer keyboards, defibrillators,
tasers and automobile blinkers.
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A charged capacitor stores electrical potential
energy. The amount is given by the following
equation PEelectric ½Q?V
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PEelectric ½Q?VSince Q ?VC,PEelectric
½C(?V)2andPEelectric Q2/2C
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Here are the two big capacitor equationsQ
VCPEelectric ½CV2
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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?
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