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**Unformatted text preview: **Chapter 20: ELECTRIC CIRCUITS 1. Electromotive Force and Current 2. Ohms Law 3. Electromotive Force and Circuits 4. Energy and Power in Electric Circuits 5. Resistors in Series and in Parallel 6. Kirchhoffs Rules 7. Electric Measuring Instruments 8. Resistance-Capacitance Circuits 9. Physiological Effects of Currents 10.Power Distribution Systems COLLEGE PHYSICS, Part II ELECTROMOTIVE FORCE AND CURRENT Electric current can be defined simply as any movement of electric charge. A more accurate definition of electric current is the following: Although current is directly related with motion, or displacement, it is a scalar quantity. Still current has direction. By convention, the direction of current is the direction of the motion of positive charges , or the opposite direction of the motion of negative charges. In metallic conductors the real carriers of electric current are the free electrons. In the absence of electric field (potential difference) they are involved in high-speed random motion , and in the presence of electric field they undergo directed motion , or drift . In order to keep them moving, we need to keep a constant electromotive force , or a constant potential difference (voltage). When this force equilibrates with the frictional force , the charge carriers move (drift) with a constant speed. In a closed circuit the current is the same at any cross section; charges normally do not build up anywhere. Example: The current in a circuit is 5.0 mA. How many electrons flow across the cross section of the circuit per second? t Q I = C s s C t I Q 3 3 10 . 5 1 10 . 5-- = = = The number of electrons would be: 16 19 3 10 12 . 3 10 602 . 1 10 . 5 = = =-- C e Q n RESISTANCE AND OHMS LAW The higher the potential difference ( V ) between the two ends of a conductor, the higher the field ( E ), and therefore the force ( F ) acting on the mobile charges in the conductor. Eventually, the current, I , is directly proportional to the applied voltage, V . This low of proportionality between V and I is known as Ohms law. The ratio V / I is called the resistance of a conductor. Resistivity The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area. The resistivity is the property of a given material and does not depend on its shape or dimensions. You can compare electric current with water pumped through a hose. Temperature Dependence of Resistance and Superconductivity For metals, the resistivity increases with temperature. At high temperatures, this dependence is linear: ) ( 1 T T T- + = where and are resistivities at temperature T and at a reference temperature, such as 0 o C, and is a constant called temperature coefficient of resistivity . At lower temperatures, the temperature dependence of resistivity has a more complex, nonlinear character....

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