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Chapter 26

PHYS 142 Chapter 26 Smart Notes.docx

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PHYS 142
Yoichi Miyahara

PHYS 142 Chapter 26 Smart Notes  26.1: Definition of Capacitance o Capacitors  A combination of two conductors is called a capacitor. The conductors are called plates. If the conductors carry charges of equal magnitude and opposite sign, a potential difference exists between them. o Capacitance  Definition  The capacitance C of a capacitor is defined as the ratio of the magnitude of the charge on either conductor to the magnitude of the potential difference between the conductors  To understand capacitance, think of similar notions that use a similar word. The capacitance of a capacitor is the amount of charge the capacitor can store per unit of potential difference.  Capacitance is always a positive quantity.  Equation   SI Units  1F (farad) = 1 C/V 1   26.2: Calculating Capacitance o Capacitance of an Isolated Charged Sphere  o Parallel Plate Capacitors  As a capacitor is being charged by a battery, electrons flow into the negative plate and out of the positive plate.  Consider the situation before any charges have had a chance to move in response to this change. Because no charges have moved, the electric field between the plates has the same value but extends over a shorter distance. 2 Therefore the magnitude of the potential difference between the plates is smaller.  When the potential difference between the plates again matches that of the battery, the flow of charge stops.  26.3: Combinations of Capacitors o In studying electric circuits, we use a simplified pictorial representation called a circuit diagram. Such a diagram uses circuit symbols to represent various circuit elements.  o Parallel Combination of Capacitors  The individual potential differences across capacitors connected in parallel are the same and are equal to the potential difference applied across the combination.   Sum of the charges on the individual capacitors 3   Equivalent Capacitor   Equivalent Capacitance for Capacitors in Parallel  o Series Combination  Voltage between the Two Capacitors   Equivalent Capacitance for Capacitors in Series   26.4: Energy Stored in a Charged Capacitor o Because positive and negative charges are separated in the system of two conductors in a capacitor, electric potential energy is stored in the system. The discharge can often be observed as a visible spark. If you accidentally touch the opposite plates of a charged capacitor, your fingers act as a pathway for discharge and the result is an electric shock. The degree of shock you receive depends on the capacitance and the voltage applied to the capacitor. 4  Note: Work must be done to move additional charge through the potential difference between plates.  When the switch is closed  The battery establishes an electric field in the wires and charges flow between the wires and capacitor.  Some of the chemical energy in the battery is converted to electric potential energy associated with the separation of positive and negative charges on the plate.
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