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Chapter 24 Thermodynamics. The study of heat and its transformation into mechanical work. If we increase the thermal motion of atoms what happens? Temperature increases. There seems to be no upper limit of temperature. However, there is a lower limit. Absolute Zero. - PowerPoint PPT Presentation
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•No more energy can be extracted from a substance and the temperature can not be lowered any further.
The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings.
Signs (+/-) will tell you if energy is entering or
leaving a system.+ indicates energy enters a system
.- indicates energy leaves a system.
First Law of Thermodynamics
thermo: heat dynamics: force------------------------------------------------------Work done on water by paddle-wheelis converted to thermal energy.------------------------------------------------------First Law:
Heat added = Work done
or
Heat added = Thermal energy increase + = Work done by system
Work is the transfer of energy that takes place when an object is moved against an opposing force
i.e. a system does work when it expands against an external pressure
Car engine: petrol burns & produces gases which push out pistons in the engine and transfer energy to the
wheels of car
•Work stimulates uniform motion
• Heat and work can be considered as energy in transit
Ex: An engine cylinder compresses a volume of gas by 0.04 m3. How much work is done by the cylinder if it exerts a constant pressure of 7.5 x 105 Pa?
Ex: When 5000 J of heat is added to an engine, the
engine does 1250 J of work. What is the change in the internal energy of
engine?
Adiabatic•The process of compression or expansion of a gas so that no heat enters or leaves a system.
Starting position, intake stroke, and compression stroke.
Ignition of fuel, power stroke, and exhaust stroke.
1. intake/induction stroke
2. compression stroke
3. power stroke4. exhaust stroke
2nd Law of Thermodynamics
Heat flows spontaneously from a body of a higher
temperature to a one at a lower temperature.
•The basic idea of a heat engine is that mechanical work can be obtained only when heat flows from hot to cold temperatures.
• "Typical engines are only 15 to 20 percent efficient in consuming oil, as they lose a lot of energy when they emit hot exhaust gas into the air."
2. Convert some of this energy into mechanical work.
3. Expel the remaining energy to some lower temperature reservoir, usually called a “sink”.
Power plants and industrial factories are among the major contributors to the problem of thermal pollution. These facilities draw water from nearby lakes and streams, which they use to cool their machinery and steam-driven equipment. Although many such facilities now take care not to contaminate the water with chemical pollutants, few return the heated water to its original temperature before dumping it back into the lakes and streams from which it came. The heated water warms local bodies of water by as much as 10° C (18° F), making the water uninhabitable for fish and other organisms.
3rd Law of Thermodynamics
There is no process by which a body may be
cooled to a temperature of 0 K (Absolute Zero).