chapter20i_PC

# chapter20i_PC - Chapter 20 Direct Current Circuits Sources...

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Chapter 20 Direct Current Circuits

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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 and generators SI units are Volts The emf is the work done per unit charge
emf and Internal Resistance A real battery has  some internal  resistance Therefore, the  terminal voltage is  not equal to the emf

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More About Internal  Resistance The schematic shows  the internal resistance, r The terminal voltage is  Δ V = V b -V a Δ V =   – Ir ε For the entire circuit,    ε = IR + Ir
Internal Resistance and emf,  cont  is equal to the terminal voltage when  ε the current is zero Also called the  open-circuit voltage R is called the  load resistance The current depends on both the  resistance external to the battery and  the internal resistance

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Internal Resistance and emf,  final When R >> r, r can be ignored Generally assumed in problems Power relationship ε  = I 2  R + I 2  r When R >> r, most of the power  delivered by the battery is  transferred to the load resistor
Resistors in Series When two or more resistors are connected  end-to-end, they are said to be in  series The current is the same in all resistors  because any charge that flows through one  resistor flows through the other The sum of the potential differences across  the resistors is equal to the total potential  difference across the combination

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Resistors in Series, cont Potentials add Δ V = IR 1  + IR 2  = I (R 1 +R 2 ) Consequence of  Conservation of Energy The equivalent  resistance has the  effect on the circuit as  the original combination  of resistors
Equivalent Resistance –  Series R eq  = R 1  + R 2  + R 3  + … The equivalent resistance of a  series combination of resistors is  the algebraic sum of the individual  resistances and is always greater  than any of the individual resistors

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Equivalent Resistance –  Series: An Example Four resistors are replaced with their  equivalent resistance
Resistors in Parallel The potential difference across each resistor  is the same because each is connected  directly across the battery terminals The current, I, that enters a point must be  equal to the total current leaving that point I = I 1  + I 2 The currents are generally not the same Consequence of Conservation of Charge

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Equivalent Resistance –
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