Fisica

28
Direct Current Circuits
CHAPTER OUTLINE
28.1 28.2 28.3 28.4 28.5 28.6 Electromotive Force Resistors in Series and Parallel Kirchhoff’s Rules RC Circuits Electrical Meters 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 terminal. Whenever a battery isdelivering 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 theanswer 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

*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 potentialbetween 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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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 first bulb to shortit 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) For 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. Theranking 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). (iv) Answer (b). (v) Answer (a). (vi) Answer (a). Q28.13 Two runs in series: of one lift and two runs: . Three runs in parallel: . . Junction

Gustav Robert Kirchhoff, Professor of Physics at Heidelbergand Berlin, was master of the obvious. A junction rule: The number of skiers coming into any junction must be equal to the number of skiers leaving. A loop rule: the total change in altitude must be zero for any skier completing a closed path. Q28.14 The bulb will light up for a while immediately after the switch is closed. As the capacitor charges, the bulb gets progressively dimmer. When thecapacitor is fully charged the current in the circuit is zero and the bulb does not glow at all. If the value of RC is small, this whole process might occupy a very short time interval. Q28.15 The hospital maintenance worker is right. A hospital room is full of electrical grounds, including the bed frame. If your grandmother touched the faulty knob and the bed frame at the same time, she could...