fall_05_exam2_sol_phy2049

# fall_05_exam2_sol_phy2049 - PHY2049 Fall 2005 Prof Andrew...

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PHY2049 Fall 2005 1 Prof. Andrew Rinzler Prof. Paul Avery Oct. 19, 2005 PHY2049, Fall 2005 Exam 2 Solutions 1. A wire made of a particular material has resistance R . Another wire made of the same material that has 1/4 the length and 1/4 the diameter has resistance? (1) 4 R (2) 16 R (3) R / 2 (4) R / 4 (5) R / 16 Resistance is proportional to length and inversely proportional to cross-sectional area. Since the length is 1/4 of the old length and the area is 1/16 the old area, the new resistance is 4 times as great or 4R. 2. A 2.00 kW heating element from a hair drier designed to operate at 120 V has a length of 80.0 cm. A 10.0 cm section of the element is removed. What power in watts is used by the now short- ened heating element at 120 V? (1) 2290 (2) 1750 (3) 2250 (4) 1810 (5) need the resistivity The new resistance R new is 7/8 the old resistance R. The total power, which is 2 new / V R , is thus 8/7 times the old power or 2290 W.

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PHY2049 Fall 2005 2 3. For the circuit shown the resistors all have resistance R . The battery has internal resistance r . What is the power dissipated by the battery? (1) () 2 2/ 2 VR r + (2) 2 4/ 3 4 r + (3) 2 4 r + (4) 43 4 VR r + (5) 22 + The total external resistance can be found from the rules of adding parallel and series resis- tances to be R / 2. The total resistance is then r + R / 2.
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fall_05_exam2_sol_phy2049 - PHY2049 Fall 2005 Prof Andrew...

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