Physics for Scientists and Engineers 8ed - ch28 - PowerPoint Slides

# Physics for Scientists and Engineers 8ed - ch28 - PowerPoint Slides

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Chapter 28 Direct Current Circuits

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Direct Current When the current in a circuit has a constant direction, the current is called direct current Most of the circuits analyzed will be assumed to be in steady state , with constant magnitude and direction Because the potential difference between the terminals of a battery is constant, the battery produces direct current The battery is known as a source of emf
Electromotive Force The electromotive force (emf), ε , of a battery is the maximum possible voltage that the battery can provide between its terminals The emf supplies energy, it does not apply a force The battery will normally be the source of energy in the circuit The positive terminal of the battery is at a higher potential than the negative terminal We consider the wires to have no resistance

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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, V = ε I r Use the active figure to vary the emf and resistances and see the effect on the graph
Active Figure 28.1 PLAY ACTIVE FIGURE

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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 just the external resistor In general, the load resistance could be any electrical device These resistances represent loads on the battery since it supplies the energy to operate the device containing the resistance

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Power The total power output of the battery is This power is delivered to the external resistor ( I 2 R ) and to the internal resistor ( I 2 r ) I V I ε ℘ = ∆ = 2 2 I R I r ℘ = +
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 V = I R 1 + I R 2 = I ( R 1 + R 2 ) Consequence of Conservation of Energy The equivalent resistance has the same effect on the circuit as the original combination of resistors
Resistors in Series – Example Use the active figure to vary the battery voltage and the resistor values Observe the effect on the currents and voltages of the individual resistors PLAY ACTIVE FIGURE

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## This note was uploaded on 05/22/2010 for the course PHYS 2326 taught by Professor Staff during the Summer '08 term at HCCS.

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Physics for Scientists and Engineers 8ed - ch28 - PowerPoint Slides

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