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Unformatted text preview: hapter 18 PHY213 1 Chapter 18 Direct Current Circuits 18.1-18.4 only, 18.5 is covered in lab. No 18.6-18.8 Homework Set #3, Due Tuesday, June 24 th Ch 18: CQ-2,4,6,9,10,14,21; P-3,5,8,14,16,20,27 Ch 19: CQ-2,3,6,10,14,18; P-3,4,8,15,18,29,30,34,46,48 Sources of emf s The source that maintains the current in a closed circuit is called a source of emf s Any devices that increase the potential energy of charges circulating in circuits are sources of emf s Examples include batteries and generators s SI units are Volts s The emf is the work done per unit charge emf and Internal Resistance s A real battery has some internal resistance s Therefore, the terminal voltage that you would typically measure is not equal to the batteries emf (the open circuit terminal voltage) More About Internal Resistance s The schematic shows a battery with emf , , and the internal resistance, r s The terminal voltage is V = V b-V a s V = Ir s the terminal voltage drops with increasing current s For the entire circuit, = IR + Ir hapter 18 PHY213 2 Internal Resistance and emf, cont s is equal to the terminal voltage when the current is zero s Also called the open-circuit voltage s R is called the load resistance s The current depends on both the resistance external to the battery and the internal resistance Internal Resistance and emf, final s When R >> r, r can be ignored s Setting r=0 is an idealized battery. s Generally assumed in problems s Power relationship s I = I 2 R + I 2 r s When R >> r, most of the power delivered by the battery is transferred to the load resistor Resistors in Series s When two or more resistors are connected end-to-end, they are said to be in series s The current is the same in all resistors because any charge that flows through one resistor flows through the other s The sum of the potential differences across the resistors is equal to the total potential difference across the...
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