hw8 - Physics 21 Fall, 2008 Solution to HW-8 YF 25-2 A...

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Physics 21 Fall, 2008 Solution to HW-8 YF 25-2 A silver wire 2.6 mm in diameter transfers a charge of 420 C in 80 min. Silver contains 5 . 8 × 10 28 free electrons per cubic meter. (a) What is the current in the wire? (b) What is the magnitude of the drift velocity of the electrons in the wire? (a) I = total charge total time = 420 C 4800 sec =0 . 088 A (b) v d = I neA =1 . 77 × 10 - 6 m/s , where the area A = πr 2 for r =2 . 6mm / 2=1 . 3 × 10 - 3 m. 25-31 A copper transmission cable 130 km long and 10.0 cm in diameter carries a current of 150 A. (a) What is the potential drop across the cable? (b) How much electrical energy is dissipated as thermal energy every hour? (a) First ±nd the resistance of the copper cable using the resistivity of copper ( ρ . 72 × 10 - 8 Ωm from the table in the text) and the length L and cross sectional area A of the cable: R = ρL A . 285 Ω . Then the potential drop is V = IR =42 . 7V . (b) The power loss is P = IV = I 2 R watts (or Joules per second). The loss in one hour is 3600 sec × (150 A) × (42 .

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This note was uploaded on 02/09/2009 for the course PHYS 021 taught by Professor Hickman during the Fall '08 term at Lehigh University .

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hw8 - Physics 21 Fall, 2008 Solution to HW-8 YF 25-2 A...

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