783_Physics ProblemsTechnical Physics

783_Physics ProblemsTechnical Physics - Chapter 27 P27.65...

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Chapter 27 123 P27.65 (a) P = IV so I V == × = 800 10 667 3 . W 12.0 V A. (b) t U × × 200 10 250 10 7 3 . . J 8.00 10 W s 3 and ∆∆ xv t × = 20 0 2 50 10 50 0 3 .. . ms s km bg ej . P27.66 (a) We begin with R A TT AT T +− + + ρ ρα α A A 00 0 0 11 12 , which reduces to R RT T T T = + +′− 0 0 αα . (b) For copper: 0 8 170 10 m , ° −− 390 10 31 . C , and ′= × ° 17 0 10 61 C R A 0 0 8 3 2 200 0100 10 108 × × = π A . . a f . The simple formula for R gives: R =+ × ° ° ° = 1 08 1 3 90 10 100 20 0 1 420 . . CC C ΩΩ a f a f while the more complicated formula gives: R = + × °° + × ° ° = 1 800 1 1 7 01 0 8 0 0 1418 . . . . a f a f a f a f . P27.67 Let be the temperature coefficient at 20.0°C, and be the temperature coefficient at 0 °C. Then ρρ ° 0 0 0 T .C a f , and =′+′ −° 10 T C a f must both give the correct resistivity at any temperature T. That is, we must have: 0 0 0 1 0 +−° = + ° C a f a f . (1) Setting T =
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