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Physics 2

Physics 2. Chapter 12 Sections 5 & 6. Second Law. Heat energy flows spontaneously from a substance at a high temperature to a substance at a low temperature and does not flow spontaneously in the reverse direction Ex – cold drink in a warm room warms up

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Physics 2

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  1. Physics 2 Chapter 12 Sections 5 & 6

  2. Second Law • Heat energy flows spontaneously from a substance at a high temperature to a substance at a low temperature and does not flow spontaneously in the reverse direction Ex – cold drink in a warm room warms up cold drink in a warm room won’t get cooler

  3. Heat Engine • Any device that uses heat energy to perform work High T surroundings QH Heat engine W QC Low T surroundings

  4. Heat Engine Cont’d. For all heat engines: • Heat supplied at relatively high T, QH • Part of heat used to do W • What’s left is heat given off at lower T, QC QH = W + QC

  5. Efficiency • Ratio of work done by an engine to the heat supplied e = work done = W heat supplied QH ex- if an engine does 200 J of work for every 1000 J of heat supplied e = 200/1000 = .2 or 20% efficient

  6. Sample Efficiencies Engine Type calculated measured max eff max eff __________________________________ Steam engine .29 .40 Gas engine .60 .25 Diesel engine .56 .35

  7. Sample Problem An automobile engine has an efficiency of 22% and produces 2510 J of work. How much heat is rejected by the engine?

  8. Refrigerator/Air Conditioner • Opposite of a heat engine 1) work done to run compressor 2) compressor carries heat out of compartment Low T compartment QC fridge W QH High T surroundings

  9. Problem • A typical heat engine is an irreversible process • Heat only flows as long as ΔT • Once ΔT = 0 no heat flows and no work can be done Hot Object Cold Object

  10. Carnot’s Principle • Efficiency of a heat engine is a maximum when the engine operates reversibly • Reversible process – one in which the system and its surroundings can be returned to exactly the same states they were in before the process occurred - any process that involves an energy dissipating mechanism, like friction, can’t be reversible

  11. Carnot Engine e = 1 – TC / TH T in Kelvin efficiency of all Carnot engines operating between the same 2 temp is the same regardless of the specifics of the system Hot reservoir, TH QH engine W QC Cold reservoir, TC

  12. Sample Problem Water near the surface of a tropical ocean has a temperature of 25 C, while water 700 m beneath the surface has a temperature of 7 C. It has been proposed that the warm water be used as the hot reservoir and the cool water as the cold reservoir of a heat engine. A) Find the maximum possible efficiency for such an engine. B) Determine the minimum input heat that would be needed if a number of these engines were to produce an amount of work equal to the 8.5 x 1019 J of energy that the US consumed in 1980.

  13. Carnot Engine cont’d. • Efficiency of a Carnot engine increases as difference between TC and TH increases • Carnot engine is reversible and has max efficiency possible of any engine operating under these conditions • All engines that use irreversible processes are always less efficient under the same conditions

  14. Sample Problem Suppose that 1200 J of heat are used as input for an engine under 2 different conditions. The first time the heat is supplied by a hot reservoir whose temperature is 650 K. The second time, the heat first flows irreversibly through a copper rod into a second reservoir whose temperature is 350 K, and then the heat enters the engine. In either case, a 150 K reservoir is at hand to use as the cold reservoir. For each case, determine the maximum amount of work that can be obtained from the 1200 J of heat.

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