SM_chapter28

SM_chapter28 - 28 Direct Current Circuits CHAPTER OUTLINE...

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28 Direct Current Circuits CHAPTER OUTLINE 28.1 Electromotive Force 28.2 Resistors in Series and Parallel 28.3 Kirchhoff’s Rules 28.4 RC Circuits 28.5 Electrical Meters 28.6 Household Wiring and Electrical Safety ANSWERS TO QUESTIONS Q28.1 No. If there is one battery in a circuit, the current inside it will be from its negative terminal to its positive termi- nal. 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. *Q28.2 The terminal potential difference is e Ir where I is the current in the battery in the direction from its negative to its positive pole. So the answer to (i) is (d) and the answer to (ii) is (b). The current might be zero or an outside agent might push current backward through the battery from positive to negative terminal. *Q28.3 121 *Q28.4 Answers (b) and (d), as described by Kirchhoff’s junction rule. *Q28.5 Answer (a). Q28.6 The whole wire is very nearly at one uniform potential. There is essentially zero difference in potential between the bird’s feet. Then negligible current goes through the bird. The resistance through the bird’s body between its feet is much larger than the resistance through the wire between the same two points. *Q28.7 Answer (b). Each headlight’s terminals are connected to the positive and negative terminals of the battery. Each headlight can operate if the other is burned out.
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122 Chapter 28 Q28.8 Answer their question with a challenge. If the student is just looking at a diagram, provide the materials to build the circuit. If you are looking at a circuit where the second bulb really is fainter, get the student to unscrew them both and interchange them. But check that the student’s understanding of potential has not been impaired: if you patch past the F rst bulb to short it out, the second gets brighter. *Q28.9 Answer (a). When the breaker trips to off, current does not go through the device. *Q28.10 (i) ±or both batteries to be delivering electric energy, currents are in the direction g to a to b, h to d to c, and so e to f. Points f, g, and h are all at zero potential. Points b, c, and e are at the same higher voltage, d still higher, and a highest of all. The ranking is a > d > b = c = e > f = g = h. (ii) The current in ef must be the sum of the other two currents. The ranking is e = f > g = a = b > h = d = c. *Q28.11 Closing the switch removes lamp C from the circuit, decreasing the resistance seen by the battery, and so increasing the current in the battery. (i) Answer (a). (ii) Answer (d). (iii) Answer (a). (iv) Answer (a). (v) Answer (d). (vi) Answer (a). *Q28.12 Closing the switch lights lamp C. The action increases the battery current so it decreases the terminal voltage of the battery. (i) Answer (b). (ii) Answer (a). (iii) Answer (a).
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SM_chapter28 - 28 Direct Current Circuits CHAPTER OUTLINE...

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