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Thermodynamics. 1 st Law of Thermodynamics. Conservation of energy When heat flows to or from a system, the system gains or loses an amount of energy equal to the amount of heat transferred. Heat added to system = increase in internal energy + external work done by the system.
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1st Law of Thermodynamics • Conservation of energy • When heat flows to or from a system, the system gains or loses an amount of energy equal to the amount of heat transferred Heat added to system = increase in internal energy + external work done by the system
Adiabatic Processes • Compression or expansion of a gas where no heat enters or leaves the system • If we do work on a system by compression, we heat it up. • When we expand a system we cool it off • Remember blowing on your hands? • Warm air rises, expands, cools and forms clouds
2nd Law of Thermodynamics • Thermal energy never flows spontaneously from cold object to a hot object. • A machine cannot be 100% efficient • Entropy of closed systems always increases. • One of the greatest shortcoming of would-be inventors is lack of understanding of the 1st and 2nd law of Thermodynamics.
Maximum Efficiency of Heat Engine High Temperature Input Heat Work done Heat Exhaust Low Temperature
Efficiency T hot - Tcold • Ideal Efficiency = ------------------------ T hot If a steam turbine runs with an input of 400 K and an output of 300 K it has an efficiency of: Eturbine = (400-300)/400 = ¼ or 25%
Entropy • 2nd Law of Thermodynamics • The entropy of a thermally isolated system never decreases, it can only increase or stay the same. • Entropy is a measure of the disorder of a system. • Drop an egg and the process can never reverse and become an egg again. In a natural process, high quality energy tends to transform into lower quality energy – order tends to disorder
Class Problem • The air temperature at an altitude of 10 kilometers is a chilling –35°C. Cabin temperatures in airplanes flying at this altitude are comfortable because of air conditioners rather than heaters. Why?
Class Problem • The air temperature at an altitude of 10 kilometers is a chilling –35°C. Cabin temperatures in airplanes flying at this altitude are comfortable because of air conditioners rather than heaters. Why? • Airliners have pressurized cabins. The process of stopping and compressing outside air to near-sea-level pressures would normally heat the air to a roasting 55°C (130°F).So air conditioners must be used to extract heat from pressurized air.
Class Problem • A piece of iron has a temperature 10°C. A second identical piece of iron is twice as hot. What is the temperature of the second piece of iron? • 200 C • 2730 C • 2930 C • 3130 C • None of the above
Class Problem • The twice-as-hot iron is 293°C:Consider a stick that is 273 + 10 units long. This is like a thermometer that extends from absolute zero (–273°C) to 10°C. Can you see that a stick twice as long is 2 x 283 + 566 units long? (Or temperature-wise, 566 K?)
Class Problems • 1. To wholly convert a given amount of heat energy into mechanical energy is • A) possible using a simple machine. • B) possible using an atomic reactor. • C) possible using a steam engine. • D) impossible regardless of the technique used. • 2. The first law of thermodynamics is a restatement of the • A) law of heat addition. • B) Carnot cycle. • C) principle of entropy. • D) conservation of energy. • E) none of these • 3. Systems that are left alone, tend to move toward a state of • A) more entropy.B) less entropy.C) no entropy.
Class Problems • 4. Entropy measures • A) temperature at constant pressure. • B) temperature as volume increases. • C) temperature at constant volume. • D) temperature as pressure increases. • E) messiness. • 5. During an adiabatic compression of an ideal gas, • A) the temperature of the gas does not change. • B) the internal energy of the gas remains constant. • C) no heat is supplied to or removed from the gas. • D) no work is done on the gas. • E) None of the above choices are true. • 6. Entropy can be • A) neither created nor destroyed. • B) created but not destroyed. • C) sometimes destroyed but never created.
Class Problems • 7. One hundred joules of heat is added to a system that performs 60 joules of work. The internal energy change of the system is • A) 60 J. • B) 40 J. • C) 100 J. • D) 0 J. • E) None of the above choices are correct. • 8. A heat engine would have 100 percent efficiency if its input reservoir were • A) 1000 times hotter than the exhaust sink. • B) 100 times hotter than the exhaust sink. • C) 100 times cooler than the exhaust sink. • D) any finite temperature if the exhaust sink were at absolute zero. • E) at any finite temperature regardless of the heat sink temperature.
Class Problems • 9. An adiabatic process is characterized by the absence of • 1) temperature change. • 2) pressure change. • 3) entropy. • 4) heat exchange. • 5) None of the above choices are true. • 10. Two identical blocks of iron, one at 10 degrees C and the other at 20 degrees C, are put in contact. Suppose the cooler block cools to 5 degrees C and the warmer block warms to 25 degrees C. This would violate the • 1) 1st law of thermodynamics. • 2) 2nd law of thermodynamics. • 3) both of these • 4) neither of these • 11. Suppose you put a closed, sealed can of air on a hot stove burner. The contained air will undergo an increase in • 1) pressure. • 2) temperature. • 3) temperature and pressure. • 4) internal energy, temperature and pressure. • 5) internal energy.
Class Problems • 12. Your refrigerator is in your kitchen. If you try to cool your kitchen on a hot day by operating your refrigerator with its door open, the kitchen temperature will • increase. • decrease. • remain unchanged. • 13. The ideal efficiency for a heat engine operating between temperatures of 2700 K and 300 K is • 1) 89%. • 2) 24%. • 3) 10%. • 4) 80%. • 5) none of these