spring_07_exam2_phy2054

spring_07_exam2_phy2054 - PHY2054 Spring 2007 Prof Eugene...

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PHY2054 Spring 2007 1 Prof. Eugene Dunnam P r o f . P a u l A v e r y M a r . 6 , 2 0 0 7 Exam 2 Solutions 1. At room temperature conditions of 25 ° C, a circuit containing a metal resistor connected to a 50 V battery has a current of 2.00 A. When the wire is heated to 85 ° C the current drops to 1.94 A. What is the temperature coefficient of resistivity of this metal? (1) 0.00052 / ° C (2) 0.013 / ° C (3) 0.031 / ° C (4) 0.0025 / ° C (5) 0.77 / ° C The initial resistance is 50 / 2.00 = 25 Ω and the finally resistance is 50 / 1.94 = 25.77 . The temperature coefficient of resistivity is therefore 0.77 / 25.00 / 60 ° = 0.00052 / ° C 2. A solar panel measures 80 cm × 50 cm. In direct sunlight, the panel delivers 3.2 A at 15 V. If the intensity of sunlight striking the panel is 1000 W/m 2 , what is the efficiency of the solar panel in converting solar energy into electrical energy? (1) 12.0% (2) 18.0% (3) 24.0% (4) 6.0% (5) none of these The input power is 1000 × 0.80 0.50 = 400 W. The output power is 15 3.2 = 48 W. The power conversion efficiency is therefore 48 / 400 = 0.12 = 12.0%. 3. A high-voltage transmission line delivers 750 MW of power at 250 kV to a consumer area over a 100 mile total distance. If the resistance in the wire is 0.1 Ω /mile, what is the power loss due to ohmic heating of the line? (1) 90 MW (2) 1875 kW (3) 100 kW (4) 30 MW (5) none of these
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PHY2054 Spring 2007 2 The total current is I = 750 × 10 6 / 250 10 3 = 3000 A. The total resistance is 0.1 100 = 10 Ω . Thus the total ohmic heating power is P = 3000 2 10 = 90 10 6 W = 90 MW. 4. A resistor of resistance R is connected to a battery (voltage V) and dissipates power P . Replac- ing the resistor with one of resistance 2 R will result in what power dissipated? (1) none of these (2) 2 P (3) P / 4 (4) 4 P (5) P The dissipated power in general is P = V 2 / R. If the resistance is doubled then the power be- comes P / 2. 5. An electric lantern contains a heavy-duty battery (usually 6-volt) in series with an incandes- cent bulb and a switch. Here are electrical data from a typical lantern: Battery terminal voltage (switch OFF) 5.6 V Battery terminal voltage (switch ON) 4.0 V Circuit current (switch ON) 0.4 A This battery's internal resistance in ohms is: (1) none of these (2) 14 (3) 1.4 (4) 2.4 (5) 5.0 When the switch is off, the measured potential difference is the full amount or 5.6 V. When the switch is closed, current I = 0.4 A flows through the circuit (implying a total resistance of 14 ).
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This test prep was uploaded on 04/17/2008 for the course PHY 2054 taught by Professor Avery during the Spring '08 term at University of Florida.

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spring_07_exam2_phy2054 - PHY2054 Spring 2007 Prof Eugene...

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