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Thermodynamics Mechanical Equivalent of Heat Heat produced by other forms of energy Internal Energy: total available potential & kinetic energy of particles Adding heat increases internal energy Joule’s experiment with the paddles stirring the water proved heat is form of energy Expansion and Work Many thermodynamic processes involve expansion or compression of gases Expanding gases exert pressure (force/area) and can do work (car engine) Work done equals pressure times volume change for constant pressure For expansion with pressure change, work equals area under curve of pressure vs. volume graph First Law of Thermodynamics Heat energy supplied to closed system equals work done by system plus change in internal energy of system. Q = DU + W 1st Law is actually conservation of energy restated to include heat energy If no work is done by system, heat added equals change in internal energy Adiabatic Processes A process where no heat is added or allowed to escape Process must happen very quickly or be well insulated Adiabatic compression of gas causes temp. increase Adiabatic expansion of gas causes cooling Isothermal Processes No temperature change occurs Isothermal expansion requires heat input from surroundings Isothermal compression requires heat emission to surroundings Specific Heats of Gases Gases have two specific heats, one for constant volume (cv ) , and one for constant pressure (cp ) (cv ) < (cp ) because work must be done against pressure to change volume Second Law of Thermodynamics It is impossible to convert all heat energy into useful work Device that uses heat to do work can never be 100% efficient Some heat will remain and must be expelled into low temp. sink Heat will never flow from a cold object to a hot object by itself without work input Absolute zero is unattainable Heat Engines Any device that turns heat into mechanical energy Anything that burns fuel or uses steam to move or do work Ideal heat engine cycle: takes heat from high temp. source, does work using part of the energy, expels remaining heat energy into low temp. heat sink. Ideal Heat Engine Diagram Efficiency of Heat Engines Efficiency = work done/heat input Work done = heat energy used Sadi Carnot (1796 – 1832) showed max. efficiency for any heat engine = temp. difference between hot source and cold sink divided by temp. of source eideal = (Thot - Tcold )/ Thot For max. efficiency, industry uses high temp. source, large body of water for sink. Types of Heat Engines Steam Engines: first heat engines; Watt, Fulton, Newcomen; external combustion Steam Turbine: uses high pressure steam to turn wheel with many cupped fan-like blades Gasoline Engines: internal combustion engine; heat from expanding combustion gases drive piston Types of Heat Engines Diesel Engines: heat from compression ignites fuel; efficient, powerful, but heavy Gas Turbines: air compressed by turbine forced through combustion chamber; used on airplanes Types of Heat Engines Jet Engines: expanding combustion gases forced out rear of engine; action-reaction & conservation of momentum create forward thrust Rockets: like jets, but carries own oxidizer to work outside atmosphere. Thrust depends on velocity of exhaust gases Heat Pumps Reverse cycle of heat engine; work from electric motor causes heat to flow from cool area to warm area Uses easily condensed vapor (freon); motor condenses vapor in compressor; When allowed to vaporize, it extracts its heat of vaporization Basis for refrigerators, air conditioners Heat Pump Diagram Entropy A measure of the disorder of a system Is related to the amount of energy that cannot be converted into mechanical work Natural systems tend toward greater disorder (greater entropy) Controls the direction of time Entropy Work must be done to decrease entropy of system Heat is disordered energy, increased entropy Change in entropy of system equals heat added divided by absolute temperature DS = DQ/T (J/K) Entropy Entropy increased in melting, evaporation, organic decay Total energy of universe is constant, but usable energy decreases due to increased entropy