chapter18 - Chapter 18 Direct Current Circuits Sources of...

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Chapter 18 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
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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 ΔV = V b -V a ΔV = ε – Ir For the entire circuit, ε = IR + Ir
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Internal Resistance and emf, cont ε is equal to the terminal voltage when 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 I = I 2 R + I 2 r When R >> r, most of the power delivered by the battery is transferred to the load resistor
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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 ΔV = 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
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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
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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 Parallel, Example Equivalent resistance replaces the two original resistances Household circuits are wired so the electrical devices are connected in parallel Circuit breakers may be used in series with other circuit elements for safety purposes
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chapter18 - Chapter 18 Direct Current Circuits Sources of...

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