ThermodynamicsThermodynamics

Mechanical Equivalent of Heat Heat produced by other forms of energy Heat produced by other forms of energy Internal Energy: total available potential & kinetic energy of particles Internal Energy: total available potential & kinetic energy of particles Adding heat increases internal energy Adding heat increases internal energy Joule’s experiment with the paddles stirring the water proved heat is form of 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 Many thermodynamic processes involve expansion or compression of gases Expanding gases exert pressure (force/area) and can do work (car engine) Expanding gases exert pressure (force/area) and can do work (car engine) Work done equals pressure times volume change for constant pressure 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 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. Heat energy supplied to closed system equals work done by system plus change in internal energy of system. Q =  U + W Q =  U + W 1st Law is actually conservation of energy restated to include heat energy 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 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 A process where no heat is added or allowed to escape Process must happen very quickly or be well insulated Process must happen very quickly or be well insulated Adiabatic compression of gas causes temp. increase Adiabatic compression of gas causes temp. increase Adiabatic expansion of gas causes cooling Adiabatic expansion of gas causes cooling

Isothermal Processes No temperature change occurs No temperature change occurs Isothermal expansion requires heat input from surroundings Isothermal expansion requires heat input from surroundings Isothermal compression requires heat emission to surroundings Isothermal compression requires heat emission to surroundings

Specific Heats of Gases Gases have two specific heats, one for constant volume (c v ), and one for constant pressure (c p ) Gases have two specific heats, one for constant volume (c v ), and one for constant pressure (c p ) (c v ) < (c p ) because work must be done against pressure to change volume (c v ) < (c p ) 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 It is impossible to convert all heat energy into useful work Device that uses heat to do work can never be 100% efficient Device that uses heat to do work can never be 100% efficient Some heat will remain and must be expelled into low temp. sink 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 Heat will never flow from a cold object to a hot object by itself without work input Absolute zero is unattainable Absolute zero is unattainable

Heat Engines Any device that turns heat into mechanical energy Any device that turns heat into mechanical energy Anything that burns fuel or uses steam to move or do work Anything that burns fuel or uses steam to move or do work Ideal heat engine cycle: Ideal heat engine cycle: takes heat from high temp. source, takes heat from high temp. source, does work using part of the energy, does work using part of the energy, expels remaining heat energy into low temp. heat sink. expels remaining heat energy into low temp. heat sink.

Ideal Heat Engine Diagram

Efficiency of Heat Engines Efficiency = work done/heat input Efficiency = work done/heat input Work done = heat energy used 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 Sadi Carnot (1796 – 1832) showed max. efficiency for any heat engine = temp. difference between hot source and cold sink divided by temp. of source e ideal = (T hot - T cold )/ T hot e ideal = (T hot - T cold )/ T hot For max. efficiency, industry uses high temp. source, large body of water for sink. 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 Engines: first heat engines; Watt, Fulton, Newcomen; external combustion Steam Turbine: uses high pressure steam to turn wheel with many cupped fan-like blades 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 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 Diesel Engines: heat from compression ignites fuel; efficient, powerful, but heavy Gas Turbines: air compressed by turbine forced through combustion chamber; used on airplanes 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 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 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 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 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 Basis for refrigerators, air conditioners

Heat Pump Diagram

EntropyEntropy A measure of the disorder of a system A measure of the disorder of a system Is related to the amount of energy that cannot be converted into mechanical work Is related to the amount of energy that cannot be converted into mechanical work Natural systems tend toward greater disorder (greater entropy) Natural systems tend toward greater disorder (greater entropy) Controls the direction of time Controls the direction of time

EntropyEntropy Work must be done to decrease entropy of system Work must be done to decrease entropy of system Heat is disordered energy, increased entropy Heat is disordered energy, increased entropy Change in entropy of system equals heat added divided by absolute temperature Change in entropy of system equals heat added divided by absolute temperature  S =  Q/T (J/K)  S =  Q/T (J/K)

EntropyEntropy Entropy increased in melting, evaporation, organic decay Entropy increased in melting, evaporation, organic decay Total energy of universe is constant, but usable energy decreases due to increased entropy Total energy of universe is constant, but usable energy decreases due to increased entropy