sect3 - 2nd Law of Thermodynamics A process must satisfy...

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1 ABE 201 Biological Thermodynamics 1 Section 3 2 nd Law of Thermodynamics -A process must satisfy 1 st law to occur, but a process obeying 1 st law DOES NOT mean it will actually occur. Satisfies the First and the Second law of Thermodynamics Heat Satisfies the First Law , but it cannot take energy of the Cold Ambient and add back to heat the coffee Heat is transferred between the cup and the cold ambient Example 1 T amb < T coffee First Law Satisfied in both cases as long as energy balances (direction not important) k p U E E Q W Δ + Δ + Δ = - Heat Engine Work can be converted to heat directly. Converting heat to work requires the use of some special device called “heat engines”. Hot Reservoir T H Q in Cold Reservoir T C Q out W net =W out -W in Heat Engine Think the heat engine as a “black box” congaing a working fluid which accepts heat from a high temperature source, turns some into work, and rejects the remainder to a low temperature sink, operating on a cycle. Example 2 Work Heat It satisfies the First and the Second Law . Friction in the fluid generates heat that is transferred to the ambient to lower temperature Work Heat It satisfies the First Law but not the Second Law Thermal Energy Reservoir A(hypothetical) body with a large thermal energy capacity that can absorb or supply finite amounts of heat without undergoing a change in temperature. Hot reservoir (heat source): A reservoir that supplies heat energy. Sun, a furnace, a person in a small room Cold reservoir (heat sink): A reservoir that absorbs heat energy. Atmosphere, river, lake, ocean, the room surrounding a refrigerator
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2 Steam Power Plant-Heat Engine Thermodynamic Cycles W out W in Boiler Turbine Condenser Pump Hot Reservoir T H Q H Cold Reservoir T C Q C 1 2 3 4 POWER CYCLE Turbine Heat lost Working fluid: Water The water substance that flows through the four components proceeds through a thermodynamics cycle because the water continually retraces the same change in state, i.e. it comes to the original initial state once a cycle is completed. 1. Mass balance : No mass in, No mass out 2. 1 st law energy balance: Steam Power Plant-Heat Engine the 1st law energy balance of water in this cycle, ( ) ( ) , , 0, 0 ( ) ( ) ( ) ( ) k p in out out in k p in out out in in out out in U E E Q Q W W U E E Q Q W W Q Q W W Δ + Δ + Δ = - - - Δ Δ Δ = = - - - - = - Hot Reservoir T H Q in Cold Reservoir T C Q out W net =W out -W in Heat Engine However, the 1 st law of thermodynamics does not tell us the how much heat is converted to work, how much heat is thrown away, direction of energy flow, what is the efficiency of the engine. barb2right 2 nd law of thermodynamics is introduced! Kelvin Planck Statement barb2right concerns cycles that use heat transfer to produce work/power (heat engines) barb2right No process on a cyclic operation is possible to convert all heat to work Hot Reservoir T H Heat Engine Q in Cold Reservoir T C Q out W net =W out -W in Heat Reservoir T H Heat Engine Q in W net =W out -W in Clausius Statement barb2right
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