Heat Engines
Heat making work
Outline
•	Thermometers
•	Thermal Expansion
•	Heat Engines and the First Law of Thermodynamics
•	Heat Engine efficiency and the Second Law of Thermodynamics
Thermometers
•	How do we measure temperature?
–	Thermal expansion of mercury or alcohol
–	Thermal expansion of bimetallic strips
•	Fixed points of temperature
–	Ice and water - 0°C, water and steam - 100°C
–	Absolute zero - -273°C = 0 K
–	T(K) = T(°C) + 273 K

Thermal Expansion
•	Most things expand when heated.
–	Exception: water just above freezing gets denser when heated!
•	Why? See the molecular motion model
•	Influx of heat raises the temperature by increasing the Kinetic energy of the molecules.
•	Expanding gases can push – i.e. do work!
Examples of heat engines
•	External Combustion engines
–	Stirling Engine
–	Steam Engine
•	Internal Combustion engines
–	Gasoline Engine
–	Diesel Engine
•	Drinking bird, Christmas Whirligig, etc.
Stirling Engine
Steam Engine
Heat Engine principles
•	Heat Engine Cycle
•	Addition of Heat
•	Expansion leading to Work done.
•	Expulsion of Waste Heat.
•	Return to original state.
•	Qh = W + Qc

1st Law of Thermodynamics
•	ÄE = Q - W
•	“The change in Energy” = “The Heat added” - “The Work done”.
•	Another statement of the Conservation of Energy, including Heat.
•	In a heat engine, after each cycle, ÄE = 0.
•	So, W = Q   ----  BUT Q = Qh – Qc!
Engine Efficiency
•	 All known heat engines produce “waste heat”.
•	An “efficient” engine should produce less waste.
•	Efficiency – e
–	 e = W / Qh = 1 – Qc / Qh
•	Example: Every cycle, an engine receives 2,000 Joules of heat and does 500 Joules of work.
–	 e = 500 J / 2,000 J = .25  OR 25 %.
–	Since W = Qh - Qc , then Qc = 1,500 J
–	4 units of heat are used to produce just 1 unit of useful work!
Ideal Engines
•	Can’t we do any better?
•	An ideal engine:
–	 e ideal = 1 – Tc / Th 
•	Example: An engine draws heat from a boiler at Th = 800 K, and expels heat to the outside at Tc = 300 K.
–	 e ideal = 1 – 300 K / 800 K = ?
–	 e ideal = .625 or 62.5% efficient.
2nd Law of Thermodynamics
•	 eideal always greater than the actual e.
•	So, even the best engine must “waste” heat.
•	2nd Law: Every engine takes heat from high T, and wastes it at low T. 
•	Consequences:
–	Heat can never be made to do only Work
–	Waste heat is less useful, because it is at a lower T.
2nd Law says