Capacitance Notes
March 20
Today:
• Questions re: text Q's/P's
• Questions re: Electrostatics Problems 5
• Capacitance, C (F)
• HW for Friday: Online
Honors Physics
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Capacitance Notes
Honors Physics
March 20
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Capacitance Notes
March 20
Capacitor: A device that stores energy by holding a net charge at a certain
voltage. Most common type is referred to as a "parallel-plate capacitor".
Capacitors have a lot of applications in modern electronics; just about any
physical button (e.g., on a remote control) has a capacitor that governs its
function.
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Capacitance Notes
March 20
Capacitance, C (F): Any capacitor has a Capacitance, C, that describes the
amount of charge it can store for a certain voltage. By definition,
C=Q
V
where Q is the amount of charge it will store (note: a capacitor stores an
equal amount of + and - charge, so the overall net charge is zero; Q is the
amount of + and - charge it holds), and V is the voltage difference across the
two plates.
The unit of Capacitance is the Farad (F), named for Michael Faraday. A
capacitance of 1 F means the capacitor will hold 1 C (a huge amount) of
charge at a voltage of 1 V (less than a AAA battery). Most capacitors are
micro-Farad (E-6 F) or nano-Farad (E-9 F).
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Capacitance Notes
March 20
Example 1: When a certain capacitor is connected to a 12 V potential, it
can store 2.4 E-10 C of charge. What is its capacitance?
Example 2: A 3 E-8 F capacitor is connected to a 6 V potential. How much
charge will it store?
Example 3: What voltage is required to store 6 E-10 C of charge on a 3 E-9
F capacitor?
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Capacitance Notes
March 20
Parallel-plate capacitors: The most common form of capacitor is a parallelplate capacitor. It features two parallel plates (pieces) of metal that are
separated by a distance d (m). The two plates generally have the same
surface area A (m2).
For this, we can determine the capacitance of a parallel-plate capacitor
based solely on geometry:
with representing the "Dielectric Constant", and
representing the
"Permittivity of Free Space", a constant equal to 8.85 E-12 C2/Nm2.
The term "Dielectric" refers to an insulator; essentially, if you slip an
insulator between the two parallel plates, you will improve its ability to
hold charges; in effect, you increase its capacitance.
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Capacitance Notes
March 20
Example: A parallel plate capacitor is formed from plates with a surface
area of 0.0025 m2 that are separated a distance of 0.005 m. If there is only
air between the plates (dielectric constant = 1), what is the capacitance of
these plates?
If connected to a 20 V power source, how much charge would be stored on
the plates?
What is the strength of the Electric Field between the plates?
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