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582_Physics ProblemsTechnical Physics

582_Physics ProblemsTechnical Physics - 584 P20.19 Heat and...

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584 Heat and the First Law of Thermodynamics P20.19 Q m c T m L = = Cu Cu N vap N 2 2 e j 1 00 293 77 3 48 0 0 414 . . . . kg 0.092 0 cal g C C cal g kg ⋅° ° = = b ga f b g m m *P20.20 The original gravitational energy of the hailstone-Earth system changes entirely into additional internal energy in the hailstone, to produce its phase change. No temperature change occurs, either in the hailstone, in the air, or in sidewalk. Then mgy mL y L g = = = × F H G I K J = × 3 33 10 9 8 1 1 3 40 10 5 4 . . . J kg m s kg m s J m 2 2 2 P20.21 (a) Since the heat required to melt 250 g of ice at 0°C exceeds the heat required to cool 600 g of water from 18°C to 0°C, the final temperature of the system (water + ice) must be 0 ° C . (b) Let m represent the mass of ice that melts before the system reaches equilibrium at 0°C. Q Q mL m c T m m f w w i cold hot C J kg kg J kg C C C g, so the ice remaining g g g = − = − ° × = − ⋅° ° ° = = = 0 3 33 10 0 600 4 186 0 18 0 136 250 136 114 5 b g e j b
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