Physics for Scientists and Engineers 8ed - ch26 - PowerPoint Slides

Physics for Scientists and Engineers 8ed - ch26 - PowerPoint Slides

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Chapter 26 Capacitance and Dielectrics
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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
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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) Q C V
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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
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More About Capacitance 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 farad is a large unit, typically you will see microfarads ( μ F) and picofarads (pF)
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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
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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
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Parallel Plate Capacitor, final 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
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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 and the potential difference 4 / o e e Q Q R C ¶Qa V k Q a k = = = =
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Capacitance – Parallel Plates The charge density on the plates is σ = Q / A A is the area of each plate, which are equal Q is the charge on each plate, equal with opposite signs The electric field is uniform between the plates and zero elsewhere
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Capacitance – Parallel Plates, cont.
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