PHYS
Ch 24 (Vol 2 Ch 8) - Capacitance.pptx

# Ch 24 (Vol 2 Ch 8) - Capacitance.pptx - Volume 2 Chapter 8...

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Volume 2 Chapter 8 - Capacitance 2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 1 Physics 2426 Ashok Kumar

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2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 2 Capacitors Our first step into the field of electricity and electrical components is the capacitor. Any pair of conductors that are separated by some insulation can act as a capacitor. When connected to a voltage source, such as a battery, the capacitor will store charge and energy.
2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 3 Capacitors A standard capacitor can be treated as a simple pair parallel plates, for which we already know the field. When attached to a battery with Voltage V, the battery acts as a pump, moving electrons from one plate (+Q) to the other (-Q). The amount of charge moved, Q, is proportional to V

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Capacitors Since is proportional to , we define a constant of proportionality called Capacitance , . is the magnitude of the charge on either plate, not both. has units of Farads where 1 Farad = 1 Coulomb/Volt 2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 4
Capacitors Lets compute for a parallel plate capacitor, , and So , and 2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 5

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Capacitors For a capacitor with A = 20.0 cm x 3.0 cm & d = 1 mm Find C, Q & E, when V = 12 V 2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 6
Capacitors – Cylindrical Capacitor (coax) For a cylindrical capacitor of length, From Gauss’s Law , so 2/18/18 Physics 2426 Ch 24 (Vol 2 Ch8) - Ashok Kumar 7

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Capacitors are now made with capacitances of 1 farad or more, but they are not parallel-plate capacitors. Instead, they are activated carbon, which acts as a capacitor on a very small scale. The capacitance of 0.1 g of activated carbon is about 1 farad. Some computer keyboards use capacitors; depressing the key changes the capacitance, which is detected in a circuit.

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Example 24-2: Cylindrical capacitor. A cylindrical capacitor consists of a cylinder (or wire) of radius R b surrounded by a coaxial cylindrical shell of inner radius R a . Both cylinders have length l which we assume is much greater than the separation of the cylinders, so we can neglect end effects. The capacitor is charged (by connecting it to a battery) so that one cylinder has a charge + Q (say, the inner one) and the other one a charge Q . Determine a formula for the capacitance. l

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Example 24-3: Spherical capacitor. A spherical capacitor consists of two thin concentric spherical conducting shells of radius r a and r b as shown. The inner shell carries a uniformly distributed charge Q on its surface, and the outer shell an equal but opposite charge Q . Determine the capacitance of the two shells.

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