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 proved heat is form of energy

First Law of Thermodynamics

• Heat energy supplied to closed system equals work done by system plus change in internal energy of system.
• Q = ΔE + 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

Expansion and Work

• Expanding gases 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

Specific Heats of Gases

• 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

Heat Engines

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

Efficiency of Heat Engines

• Work done = heat energy used
• Efficiency = work done/heat input
• Carnot showed max. efficiency for any heat engine = temp. difference between hot source and cold sink divided by temp. of source
• eideal = (T_hot - T_cold )/ T_hot
• For max. efficiency, industry uses high temp. source, large body of water for sink.

Second Law of Thermodynamics

• It is impossible to convert all heat energy into useful work
• Some heat will remain and must be expelled into low temp. sink
• Heat will never of itself flow from a cold object to a hot object
• Absolute zero is unattainable

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
• 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
• Δ S = ΔQ/T (units of J/K)
• Entropy increased in melting, evaporation, organic decay
• Total energy of universe is constant, but usable energy decreases due to increased entropy

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
• Diesel Engines: heat from compression ignites fuel; efficient, powerful, but heavy
• Gas Turbines: air compressed by turbine forced through combustion chamber; used on airplanes
• Jet Engines: expanding combustion gases forced out rear of engine; action-reaction & conserv. 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; motor condenses vapor in compressor
• When allowed to vaporize, it extracts its heat of vaporization
• Basis for refrigerators, air conditioners