joseph (dj23889) – Homework #5 – yu – (44020) This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Suppose that a voltage surge (increase) produces 140 V for a moment. By what percentage will the output of a 90 V, 119 W lightbulb increase, assuming its resistance does not change? Answer in units of %. 002 10.0 points When a certain air-filled parallel-plate capacitor is connected cross a battery, it acquires a charge (on each plate) of 130 µC. While the battery connection is maintained, a dielectric slab is inserted into and fills the region between the plates. This results in the accumulation of an additional charge of 220 µC on each plate. What is the dielectric constant of the dielectric slab? 003 10.0 points Which of the following will cause the electric resistance of certain materials known as superconductors to suddenly decrease to essentially zero? 1. Cooling the material below a certain threshold temperature 2. Stretching the material into a wire of sufficiently small diameter 3. Placing the material in a sufficiently large magnetic field 4. Increasing the voltage applied to the material beyond a certain threshold voltage 5. Increasing the pressure applied to the material beyond a certain threshold pressure 004 10.0 points An incandescent bulb has a resistance of 23 Ω when it is at room temperature (25 ◦ C) and 1 100 Ω when it is hot and delivering light to the room. The temperature coefficient of resistivity of the filament is 0.005 (◦ C)−1 , where the base resistance R0 is determined at 0◦ C. What is the temperature of the bulb when in use? Assume that the temperature dependence of resistivity is linear. Answer in units of ◦ C. 005 10.0 points A 15 Ω resistor and a 3.0 Ω resistor are connected in series with an emf source. The potential difference across the 3.0 Ω resistor is measured with a voltmeter to be 24 V. Find the potential difference across the emf source. Answer in units of V. 006 (part 1 of 2) 10.0 points A person notices a mild shock if the current along a path through the thumb and index finger exceeds 82 µA. Find the maximum allowable potential difference without shock across the thumb and index finger for a dry-skin resistance of 1.9 × 105 Ω. Answer in units of V. 007 (part 2 of 2) 10.0 points Find the maximum allowable potential difference without shock across the thumb and index finger for a wet-skin resistance of 1500 Ω. Answer in units of V. 008 (part 1 of 2) 10.0 points Consider the system of equivalent capacitors. 2 µF 2 µF 2 µF 2 µF a b 2 µF 2 µF EB Find the equivalent capacitance Cab of the network of capacitors. joseph (dj23889) – Homework #5 – yu – (44020) 2 µF 3 a 3. C = 2 µF 4. C = 1 µF 3 96.6 V c µF 47.7 µF 2. C = 8 11. 53.3 µF 1. C = 3 µF 2 d b 3 85. µF 5. C = 6 µF 6. C = 4 µF 7. C = 4 µF 3 8. C = 12 µF 009 (part 2 of 2) 10.0 points What potential difference must be applied between points a and b so that the charge on each plate of each capacitor will have a magnitude of 6 µC? 1. Vab = 9 V 2. Vab = 3 V 3. Vab = 4 V 4. Vab = 18 V 5. Vab = 2 V 3 6. Vab = 6 V 7. Vab = 3 V 2 8. Vab = 2 V 010 (part 1 of 4) 10.0 points Four capacitors are connected as shown in the figure. Find the capacitance between points a and b of the entire capacitor network. Answer in units of µF. 011 (part 2 of 4) 10.0 points What is the charge on the 47.7 µF centeredupper capacitor? Answer in units of µC. 012 (part 3 of 4) 10.0 points A dielectric with dielectric constant 2.89 is inserted into the 53.3 µF capacitor (lowercentered capacitor) while the battery is connected. What is the charge on the 85.3 µF lowerright capacitor? Answer in units of µC. 013 (part 4 of 4) 10.0 points If the battery is removed before the dielectric in the above question is inserted, what will be the charge on the 85.3 µF lower-right capacitor? Answer in units of µC. 014 (part 1 of 2) 10.0 points A 1.2 mile long copper wire that has a resistance of 20 ohm is cut in half. What is the resistance of one of the halves? Answer in units of ohm. 015 (part 2 of 2) 10.0 points What will be the new resistance if the two halves are doubled over and used as one wire? Answer in units of ohm. 016 (part 1 of 2) 10.0 points Two capacitors of 22 µF and 2.2 µF are connected parallel and charged with a 58 V power joseph (dj23889) – Homework #5 – yu – (44020) supply. Calculate the total energy stored in the two capacitors. Answer in units of J. 017 (part 2 of 2) 10.0 points What potential difference would be required across the same two capacitors connected in series in order for the combination to store the same energy as in the first part? Answer in units of V. 018 10.0 points A lightbulb marked “75 W at 129 V” is screwed into a socket at one end of a long extension cord in which each of the two conductors has a resistance of 0.889 Ω. The other end of the extension cord is plugged into a 129 V outlet. Find the actual power of the bulb in this circuit. Answer in units of W. 019 10.0 points A certain coffeepot draws 4.0 A of current when it is operated on 120 V household lines. If electrical energy costs 10 cents per kilowatt-hour, how much does it cost to operater the coffeepot for 2 hours? 1. 4.8 cents 2. 16 cents 3. 9.6 cents 4. 2.4 cents 5. 8.0 cents 020 10.0 points The magnitude of the excess charge on each conductor of a parallel-plate capacitor is 78 µC. What is the potential difference between the conductors if the capacitance of the system is 0.0377 µF? Answer in units of V. 3 021 (part 1 of 2) 10.0 points A battery with an emf of 10.5 V and internal resistance of 1.43 Ω is connected across a load resistor R. If the current in the circuit is 1.19 A, what is the value of R? Answer in units of Ω. 022 (part 2 of 2) 10.0 points What power is dissipated in the internal resistance of the battery? Answer in units of W. 023 (part 1 of 2) 10.0 points The wire in a house circuit is rated at 19.5 A and has a resistance of 0.12 Ω. What is its power rating? Answer in units of W. 024 (part 2 of 2) 10.0 points How much heat does the wire give off in 26 min? Answer in units of J. 025 (part 1 of 2) 10.0 points Consider Earth and a cloud layer 920 m above Earth to be the plates of a parallel-plate capacitor. If the cloud layer has an area of 0.7 km2 = 7 × 105 m2 , what is the capacitance? The value of the permittivity of free space is 8.8542 × 10−12 C2 /N · m2 . Answer in units of F. 026 (part 2 of 2) 10.0 points If an electric field strength greater than 3 × 106 N/C causes the air to break down and conduct charge (lightning), what is the maximum charge the cloud can hold? Answer in units of C. 027 (part 1 of 4) 10.0 points An energy plant produces an output potential of 9600 kV and serves a city 184 km away. A high-voltage transmission line carries 1240 A to the city. The effective resistance of a transmission line [wire(s)] is 3.73 Ω/km times the distance from the plant to the city. What is the potential provided to the city, joseph (dj23889) – Homework #5 – yu – (44020) i.e., at the end of the transmission line? Answer in units of kV. 028 (part 2 of 4) 10.0 points How much power is dissipated due to resistive losses in the transmission line? Answer in units of W. 029 (part 3 of 4) 10.0 points Assume the plant charges $ 0.084 /kW · hr for electric energy. At this rate, how much does it cost to transmit energy to the city (by the transmission line heating the atmosphere) each hour? Answer in units of dollars/hr. 030 (part 4 of 4) 10.0 points Consider the money lost by the transmission line heating the atmosphere each hour. Assume the energy plant produces the same amount of power; however, the output electric potential of the energy plant is 20% greater. How much money per hour is saved by increasing the electric potential of the power plant? Answer in units of dollars/hr. 4 ...
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