2 nd Law of Thermodynamics Heat Engines. 2 nd Law Heat flows naturally from high temperature to low temperature, never in reverse.

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Presentation transcript:

2 nd Law of Thermodynamics Heat Engines

2 nd Law Heat flows naturally from high temperature to low temperature, never in reverse

Heat Engine A device that changes thermal energy into mechanical work When heat is allowed to flow from high temp to low temp, some of the heat transferred can be transformed into work

Q H = W + Q L

Steam Engine Steam in through intake pushes piston out Motion of flywheel pushes piston back and forces gas out valve Repeats

Temp difference is required in order to get work out If the temperature was the same, the work done on the piston in the intake would be the same as the work done by the piston on the exhaust stroke – no net work In a real engine there is a condenser making the exhaust reservoir cooler so the heat will naturally flow out of the piston chamber More work is done on the piston pushing out than is done by the piston pushing in so there is a net gain in work on the piston

Efficiency (e) Ratio of work you get out to the heat you put in e = W/Q H Q H = W + Q L W = Q H - Q L e = Q H – Q L / Q H e = 1 – (Q L / Q H )

Example A car engine has an efficiency of 20% and produces an average of 23000J of work. How much heat is discharged?

Carnot Engine Theoretical engine with the maximum efficiency In this engine, the intake and exhaust steps are considered to be reversible with no difference in the work or heat In real engines turbulence and friction cause a loss of heat and it is not reversible

Max Efficiency In carnot engine Q L / Q H is proportional to T L / T H e max = 1 – (T L / T H ) Real engines will not be at carnot efficiency, most between 60 and 80 percent of the max efficiency

Notice that the max possible efficiency depends on the temp difference between the hot and cold reservoir 100% efficiency is impossible, would require a T L of 0 K and this temp is not possible No device can transform all heat added into work – violates 2 nd law Note that the greater the difference in T H and T L the more efficient the engine is

Example A steam engine operates between 500°C and 270°C. What is the max possible efficiency? An engine manufacturer makes the claim that it inputs 9.0kJ at 375K and outputs 4.0kJ at 225K. Is this claim possible?