Hero’s Engine Introducing Engines and Heat Pumps

Heat Engines Any device that uses heat to perform work

3 Essential Features 1.Heat comes from hot reservoir. 2.Part of input heat performs work 3.Remainder of heat, Q C <Q H, goes to a cold reservoir

Efficiency (e) = Energy output/Energy Input To be efficient, an engine must produce a large amount of work for as little input heat as possible. Car engines are only about 25% efficient! They output work (rotating) by taking in heat (Q) from some htot “resevoir” (source) of heat. If a machine takes in 100 J of energy to do 50J of work it is _____ % efficient (or,we say, e = 0.5)

In Terms of Temperature Calculate the maximum efficiency for an ideal engine running with a hot source at 100 C and a cold source at O C. (Switch to kelvin first!)

Carnot’s Principle The most efficient engine operates around a reversible process in which both system and environment can be returned the exact states they were in before the process. This can’t happen, but the closer you get to it the more efficient you engine is.

Example The water near the surface of a tropical sea is at K while 700m beneath the surface it’s K. It has been proposed that we can use this difference in temperature to make a heat engine. Find the efficiency of it. 6%

The Carnot Cycle 1.Isothermal expansion 1-2, W=Q, heat in is work out 2.Adiabadic expansion 2-3, expanding so temp must drop. Work still positive 3.Isothermal compression 3-4, work is negative so Q is out 4.Adiabatic compression 4-1, work negative so into system, raising temp and pressure

Overall Effect Heat has been moved