Lesson 2.9 Evaporation

1. Yes. The water will continue to evaporate. The water will not boil because its vapor pressure at room temperature will not be anywhere near the atmospheric pressure.
2. Yes.
3. Yes. The gasoline will evaporate more quickly than the water. Gasoline evaporates more easily than water at the same temperature (it has a higher vapor pressure.)
4. Yes. No, they do not all move at the same speed.
5. As the molecules collide with each other, some molecules gain enough energy to break free from the forces of attraction that hold them in the liquid phase and they escape into the vapor phase.
6. Yes. When they collide with the surface of the liquid, they can re-enter the liquid phase. Also if they lose energy while in the gas phase, they can combine with other molecules and form droplets.
7. Yes. Liquid molecules have less energy than vapor molecules at the same temperature. The difference is the heat of vaporization.
8. Yes. The liquid will evaporate until it exerts a pressure equal to its vapor pressure at that temperature.
9. Liquid continues to evaporate and condense. At equilibrium, the rate of evaporation is the same as the rate of condensation.
10. The partial pressure of the liquid.
11. Yes. The vapor pressure – and the partial pressure – is defined by the temperature of the system.
12. When the temperature of a liquid is increased, its vapor pressure increases. At a particular temperature, this vapor pressure will equal the atmospheric pressure and the liquid starts to boil.
13. At a particular temperature, this vapor pressure will equal the pressure in the container and the liquid will start to boil.
14. The atmospheric pressure is lower at higher elevations. The temperature needed to create a vapor pressure equal to the lower pressure is lower.
15. Pressure cookers trap steam inside the container. This increases the pressure and the temperature increases as a result.

Lesson 2.10 Refrigeration

1. A system is a part of the universe that can be defined by some boundary or a part of the universe that is under consideration.
2. Open systems can exchange both heat and matter with their surroundings. Closed systems can exchange heat between the system and the surroundings, but matter cannot be exchanged.
3. When a gas is compressed adiabatically, the system does not lose any energy to the surroundings and the temperature increases.
4. The temperature of a material is a measure of the average kinetic energy of the particles that make up the material. The average kinetic energy increases as the temperature increases.
5. Yes. The temperature increases.
6. Not always. It depends on the gas.
7. The temperature will decrease.
8. The temperature will decrease.
9. a) The purpose of the cooling coil outside the refrigerator is to allow the hot, compressed gas to transfer heat to the air outside the refrigerator. In doing so, it liquefies.
   b) The Purpose of the compressor is to provide a difference in pressure between the coil outside the refrigerator and the coil inside the refrigerator. By compressing the gas, it increases the temperature of the gas and allows it to transfer energy to the air outside the refrigerator.
   c) The expansion valve partially blocks the flow of liquid to the cooling coil. This creates a difference in pressure between the liquid entering the expansion valve and the fluid inside the coil. The drop in pressure caused by the expansion valve allows the liquid to boil or vaporize rapidly and cool.
10. No. The room will actually get hotter because the refrigerator is not completely efficient. Apart from the fact that the refrigerator just moves heat from one zone to another, the energy used by the compressor requires electrical energy that ends up as additional thermal energy the room.

Lesson 2.11 Thermodynamics - 1

1. The Kelvin scale is the absolute temperature scale that has graduations that are the same size as those in the Celsius scale. 0 Kelvin is equivalent to -273 ºC.

2. Energy can neither be created nor destroyed. When heat is added to a system, it changes to an equal amount of some other form of energy.

3. The internal energy increases.

4. Heat engines convert thermal energy into mechanical energy.

5. a) Energy (heat) is added - at a relatively high temperature.

b) Some of the energy from that input heat is used to perform work.

c) Unused energy is removed at a lower temperature.

6. Label the following engines as internal combustion or external combustion:

a) Steam turbine - External Combustion

b) Diesel engine - Internal Combustion

c) Sterling engine - External Combustion

7. The Carnot efficiency is the maximum possible efficiency for a heat engine operating between a high source with absolute temperature T(hot) and a cold sink with absolute temperature T(cold) is:

[1-T(cold)/T(hot)] x 100%

8. Ideal efficiency = [T(hot) - T(cold)] / T(hot) = (500 - 150) / 500 = 0.7 or 70%

Lesson 2.12 Thermodynamics – 2

1. No. The Stirling engine is an external combustion engine.
2. The compression cycles are rapid and rates of heat transfer from gases to solid surfaces around the are so slow that the gas does not lose an appreciable amount of energy to the surroundings during compression. The compression can thus be regarded as adiabatic.
3. The compression stroke (or cycle) and the power stroke (or cycle).
4. Compression, ignition, expansion (power), exhaust and induction (drawing new air and fuel into the cylinder.)
5. The primary difference is that the fuel does not need to be ignited by a spark plug. The temperature increase caused by the compression of the air provides sufficient temperature for ignition of the fuel. Most diesel engines inject fuel directly into the cylinder when the piston reaches the top of its compression stroke whereas gasoline engines compress a mixture of fuel and air and rely on a spark to ignite the mixture.
6. Fuels that ignite too easily may auto-ignite (as in a diesel engine) before the piston reaches the top of its stroke. High-octane fuels do not ignite as easily as low-octane fuels do. If auto-ignition occurs, excess pressure is exerted on the piston as it tries to continue moving upwards and the piston "slaps" against the side of the cylinder. This can damage the engine. To prevent this, engines that have higher degrees of compression require higher octane fuels.
7. The scrambled egg.
8. The second law states that the entropy of the universe increases or entropy in a closed system can never decrease.
9. Living organisms appear to create order from disorder. The molecules in a living organism exist in great order than the surroundings. This only happens because the entropy of the rest of the universe is increased to a greater amount than the entropy of the organism is decreased.