Hero’s Engine Introducing Engines and Heat Pumps.
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Transcript of Hero’s Engine Introducing Engines and Heat Pumps.
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, QC<QH, 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. HQ
We
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
H
cH
T
TTe
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 298.2 K while 700m beneath the surface it’s 280.2 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 Cycle1. Isothermal expansion 1-2,
W=Q, heat in is work out2. 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