pk-ch.18-1x - 2/4/2010 Chapter 18 So far Current,...

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2/4/2010 1 Chapter 18 s Direct Current Circuits So far s Current, resistance, resistivity, Ohm’s Law s Power s Superconductivity R V R I P A l R IR V / 2 2 = = = = ρ Today s Circuits s Electromotive force s Internal resistance s Resistors in series or in parallel s Other (more complex) circuits 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 is not equal to the emf
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2/4/2010 2 More About Internal Resistance s The schematic shows the internal resistance, r s The terminal voltage is ∆V = V b -V a s ∆V = ε – Ir s For the entire circuit, ε = IR + Ir Internal Resistance and emf, cont s ε 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 Generally assumed in problems s Power relationship s I ε = I 2 R + I 2 r s
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pk-ch.18-1x - 2/4/2010 Chapter 18 So far Current,...

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