week5_chapter26_web - Circuits and Circuit Elements...

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Week 5, Chapter 26 Capacitance and Dielelectrics Capacitance and Dielectrics Circuits and Circuit Elements Electric circuits are the basis for the vast majority of the devices used in society. Circuit elements can be connected with wires to form electric circuits. Capacitors are one circuit element. Others will be introduced in other chapters Introduction Capacitors Capacitors are devices that store electric charge. Examples of where capacitors are used include: radio receivers filters in power supplies to eliminate sparking in automobile ignition systems energy-storing devices in electronic flashes Introduction Makeup of a Capacitor A capacitor consists of two conductors. These conductors are called plates. When the conductor is charged, the plates carry charges of equal magnitude and opposite directions. A potential difference exists between the plates due to the charge. Section 26.1 Definition of Capacitance The capacitance , C , of a capacitor is defined as the ratio of the magnitude of the charge on either conductor to the potential difference between the conductors. The SI unit of capacitance is the farad (F). The farad is a large unit, typically you will see microfarads (mF) and picofarads (pF). Capacitance will always be a positive quantity The capacitance of a given capacitor is constant. The capacitance is a measure of the capacitor’s ability to store charge . The capacitance of a capacitor is the amount of charge the capacitor can store per unit of potential difference. Q C V Section 26.1 Parallel Plate Capacitor Each plate is connected to a terminal of the battery. The battery is a source of potential difference. If the capacitor is initially uncharged, the battery establishes an electric field in the connecting wires. Section 26.1
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Parallel Plate Capacitor, cont This field applies a force on electrons in the wire just outside of the plates. The force causes the electrons to move onto the negative plate. This continues until equilibrium is achieved. The plate, the wire and the terminal are all at the same potential. At this point, there is no field present in the wire and the movement of the electrons ceases. The plate is now negatively charged. A similar process occurs at the other plate, electrons moving away from the plate and leaving it positively charged. In its final configuration, the potential difference across the capacitor plates is the same as that between the terminals of the battery. Section 26.1 Capacitance – Isolated Sphere Assume a spherical charged conductor with radius a. The sphere will have the same capacitance as it would if there were a conducting sphere of infinite radius, concentric with the original sphere. Assume V = 0 for the infinitely large shell Note, this is independent of the charge on the sphere and its potential.
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This note was uploaded on 02/04/2011 for the course PHYSICS 121 taught by Professor Fayngold during the Spring '10 term at NJIT.

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week5_chapter26_web - Circuits and Circuit Elements...

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