Thermodynamics

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

WeatherRead Text Lots of stuff on this

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

Low Temperature

Work done

Input Heat

Heat Exhaust

Heat engines

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?

1. 200 C

2. 2730 C

3. 2930 C

4. 3130 C

5. 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 1) increase. 2) decrease. 3) 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