Week5_2

# Week5_2 - emf A source of emf(electromotive force is an...

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emf A source of emf (electromotive force) is an entity that maintains the constant voltage of a circuit The emf source supplies energy, it does not apply a force, to the circuit The battery will normally be the source of energy in the circuit Could also use generators

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Sample Circuit We consider the wires to have no resistance The positive terminal of the battery is at a higher potential than the negative terminal There is also an internal resistance in the battery
Internal Battery Resistance If the internal resistance is zero, the terminal voltage equals the emf In a real battery, there is internal resistance, r The terminal voltage, Δ V = ε -Ir

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emf, cont The emf is equivalent to the open-circuit voltage This is the terminal voltage when no current is in the circuit This is the voltage labeled on the battery The actual potential difference between the terminals of the battery depends on the current in the circuit
Load Resistance The terminal voltage also equals the voltage across the external resistance This external resistor is called the load resistance In the previous circuit, the load resistance is the external resistor In general, the load resistance could be any electrical device

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Power The total power output of the battery is P = I Δ V =I ε This power is delivered to the external resistor (I 2 R) and to the internal resistor (I 2 r) P = I ε = I 2 R + I 2 r The current depends on the internal and external resistances I Rr ε = +
Resistors in Series When two or more resistors are connected end- to-end, they are said to be in series For a series combination of resistors, the currents are the same in all the resistors because the amount of charge that passes through one resistor must also pass through the other resistors in the same time interval The potential difference will divide among the resistors such that 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
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Week5_2 - emf A source of emf(electromotive force is an...

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