prallech18 - Chapter 18 Direct-Current Circuits 18.1...

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1 Chapter 18 Direct-Current Circuits 18.1 Sources of EMF 18.2 Resistors in Series 18.3 Resistors in Parallel 18.4 Kirchhoff’s Rules 18.5 RC Circuits Sources of emf The source that maintains the current in a closed circuit is called a source of emf Any devices that increase the potential energy of charges circulating in circuits are sources of emf Examples include batteries (chemical) and generators (mechanical) DC converted from AC (adapters) Solar cells SI units are Volts, e – The emf is the work done per unit charge historically: emf = electro-motive force - clearly wrong because it is a source of voltage, not force (math is the same as resistors in series)
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2 emf and Internal Resistance A real battery has some internal resistance Therefore, the terminal voltage is not equal to the emf The schematic shows the internal resistance, r The terminal voltage is ΔV = V b -V a ΔV = ε – Ir For the entire circuit, ε = IR + Ir Power: I e = I 2 R + I 2 r When R >> r, most of the power delivered by the battery is transferred to the load resistor • One wire: – Effectively adding lengths: • R eq = r (L 1 +L 2 )/A – Since R is proportional to L add resistance: R R = 2R R eq = R 1 + R 2
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3 Current and Voltage • Resistors connected end-to-end: – If charge goes through one resistor, it must go through other. I
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This note was uploaded on 03/15/2010 for the course PHY 102 taught by Professor Luo during the Spring '08 term at SUNY Buffalo.

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prallech18 - Chapter 18 Direct-Current Circuits 18.1...

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