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Unformatted text preview: 28 CHAPTER OUTLINE 28.1 Electromotive Force 28.2 Resistors in Series and Parallel 28.3 Kirchhoffs Rules 28.4 RC 28.5 Electrical Meters 28.6 Household Wiring and Electrical Safety Circuits Direct Current Circuits ANSWERS TO QUESTIONS Q28.1 The load resistance in a circuit is the effective resistance of all of the circuit elements excluding the emf source. In energy terms, it can be used to determine the energy delivered to the load by electrical transmission and there appearing as internal energy to raise the temperature of the resistor. The internal resistance of a battery represents the limitation on the efficiency of the chemical reaction that takes place in the battery to supply current to the load. The emf of the battery represents its conversion of chemical energy into energy which it puts out by electric transmission; the battery also creates internal energy within itself, in an amount that can be computed from its internal resistance. We model the internal resistance as constant for a given battery, but it may increase greatly as the battery ages. It may increase somewhat with increasing current demand by the load. For a load described by Ohms law, the load resistance is a precisely fixed value. Q28.2 The potential difference between the terminals of a battery will equal the emf of the battery when there is no current in the battery. At this time, the current though, and hence the potential drop across the internal resistance is zero. This only happens when there is no load placed on the batterythat includes measuring the potential difference with a voltmeter! The terminal voltage will exceed the emf of the battery when current is driven backward through the battery, in at its positive terminal and out at its negative terminal. Q28.3 No. If there is one battery in a circuit, the current inside it will be from its negative terminal to its positive terminal. Whenever a battery is delivering energy to a circuit, it will carry current in this direction. On the other hand, when another source of emf is charging the battery in question, it will have a current pushed through it from its positive terminal to its negative terminal. Q28.4 Connect the resistors in series. Resistors of 5.0 k , 7.5 k and 2.2 k connected in series present equivalent resistance 14.7 k . Q28.5 Connect the resistors in parallel. Resistors of 5.0 k , 7.5 k and 2.2 k connected in parallel present equivalent resistance 1.3 k . 129 130 Direct Current Circuits Q28.6 Q28.7 In series, the current is the same through each resistor. Without knowing individual resistances, nothing can be determined about potential differences or power. Q28.8 In parallel, the potential difference is the same across each resistor. Without knowing individual resistances, nothing can be determined about current or power....
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