Exercise #13 Air Conditioner
Heat removed from room (and added to AC system) QL = cmT = (0.72)(800)(32-20) =
What is work? W = QL/K = 6912/2.5 = 2764 kJ P = W/t = 2764 kJ/15 min = 2764000 J/
900 s
Reversibility
e.g. a piston is heated and raises a weight
A reversible process must not change any other system anywhere
Mechanical Reversibility
In order to reverse them you would have to completely convert heat into work
Virtually every process converts
some work into heat, so mechanical irreversibility cannot be avoided
Isothermal Work
e.g. rub two blocks together under water in a lake Heat is produced but no temperature change
e.g. get it to run a perfect engine common examples:
Friction, stirring, or compression of systems in contact with air or water
Adiabatic Work Work done on insulated systems that changes
the internal energy
Work is converted completely into internal energy and raises the temperature of the system
To reverse, must restore temperature by removing heat and converting completely to work
Examples: Friction, stirring or compression of insulated systems
Dissipation
Dissipative effects produce external mechanical irreversibility
Any real machine involves dissipation and is thus irreversible
i.e. frictionless
Thermal Irreversibility
Heat flowing from hotter to cooler systems
To reverse need to have heat flow from cool to hot
Example:
can re-freeze, but that requires work
Perpetual Motion Three kinds of perpetual motion 1st kind:
violates 1st law
2nd kind: violates 2nd law
3rd kind: violates 2nd law
Ideal and Real Systems
Real systems are not reversible
We can approximate reversibility is several ways: Use a heat reservoir
Carnot Cycle A Carnot engine is a device that operates
between two reservoirs (at high and low T) with adiabatic and isothermal processes An isothermal addition of heat QH at TH
An isothermal subtraction of heat QL at TL
Engine Applet http://www.rawbw.com/~xmwang/javappl/
carnotC.html
Carnot Info Carnot cycles can operate with many
different systems:
Carnot cycle defined by: only two heat reservoirs and thus only two
temperatures
All other cycles involve heat transfers across temperature changes and thus are irreversible
Carnot Refrigerator
If you reverse a Carnot engine, you get a Carnot refrigerator Adiabatic rise from TL to TH
Adiabatic fall from TH to TL
If the two reservoirs are the same, the heats and work are the same for a Carnot refrigerator and engine
Carnot’s Theorem
Reversible processes are the most efficient
Carnot efficiency is an upper limit for any engine
Corollary
Efficiency only depends on the temperatures of the reservoirs
Thus: Maximum efficiency of any engine
depends only on the temperatures of the reservoirs