SAT II Subject Test Physics: Thermal Physics/Thermodynamics - Heat and Temperature

• Molecules
All matter is composed of molecules that are relatively far apart and exert force on each other only when they collide.
• Pressure of Gas
The pressure of gas is caused by collisions of gas molecules with the walls of the container.
• Compare to gas, molecules of liquids are comparatively close to each other, in solid the molecules are much closer together, there are considerable bonding force between the molecules.
• temperature is a measure of the average kinetic energy per molecule of a substance
The above definition may sound strange to you and me, but hey this is science and it can be nerdy.
• The average kinetic energy of the oxygen molecules is the same as that of the hydrogen molecules. However oxygen molecule will move more slowly because oxygen molecule has greater mass based on kinetic energy formula:
  
• Standard pressure is the pressure that would be produced by a column of 76cm high mercury
• Kelvin scale is used as an absolute temperature scale and this scale has no negative temperatures. The absolute zero is the coldest temperature in theory.
  
• Expansion and contraction
When solid is heated, it expands in length, area and volume
• If a hollow solid (e.g. a coin with a hole in the middle) is heated, the empty space increase in volume just as though it were made of the same material as the solid.
• As water is cooled from 100 °C, it contracts until is reaches 4 °C. Then it expands if it is cooled further.
  
• If the pressure on a gas is kept constant, heating the gas will result in an increase in its volume
  
  1. Charles' law
  If the pressure on a gas is kept constant, its volume is directly proportion to the absolute temperature
  
  2. Boyle's law
  If the temperature of a gas is kept constant, it volume of the gas varies inversely with the pressure
  
  3. General Gas Law (combines Charles' law and Bayle's law)
  
  4. SI unit for pressure is Pascal
  One Pascal is 1 Newton per square meter
• "Internal energy" or "work" or "heat being transferred" are all measured in "Joules"
Food contains energy and this is also officially measured in "Joules". Most people are familiar with calorie, which is a pre-SI metric unit of energy. One "Calorie" approximates the energy needed to increase the temperature of 1 gram of water by 1 °C, which is about 4.2 Joules. Check the food label of your grocery and you are most likely to find energy measured in both Calorie and Jules.

• Specific Heat Capacity (sp. ht.)
Specific heat capacity of a substance is the amount of heat required to raise the temperature of 1gram of the substance by 1°C.
• Specific heat capacity of solid is different from the liquid form of the same substance, eg.
Heat capacity of ice is: 2.05 joules/g.°C Heat capacity of water is: 4.19 joules/g.°C
• Change of phase
During the change of phase, heat represents an increase in potential energy rather than kinetic energy. Therefore the temperature remains the same.
• During heat transfer, heat lost by the hot object = heat gained by the cold object
• Heat of fusion of a substance is the amount of heat required to melt a unit mass of the substance without changing of temperature
• Melting point is practically independent of atmosphere pressure
• Boiling point is normally directly related to the atmosphere pressure, in the high mountains, where the atmosphere pressure is lower, the boiling point of water is much lower than on the ground
• Regelation
Regelation is melting under increased pressure (melting point is lower than usual) and refreezing when the pressure is reduced. This only occurs when substance, such as ice that expands upon freezing. You can observe this with ice skating. The melting point change is normally very small compared to the pressure change. E.g. for ice to melt at -4°C rather than 0°C, 500 atmosphere pressure is required.
• A lower melting point can also be resulted from dissolving solids in the liquid. E.g. 10% salt solution melt at -6°C and 20% salt solution melt at -16°C. That is why you sometimes see worker add salt on the road to help melting the ice.
• Evaporation
Evaporation is a cooling process with fast moving molecules at the surface escape from the liquid
• Boiling
Heat of vaporization of a substance is the amount of heat needed to vaporize a unit mass (1g) without changing its temperature.
• Effect of pressure increase
Boiling point decrease Melting point changes very little
• Sublimation
Substance changes directly from solid to vapor without going through liquid phase
• Condensation
Change from vapor to liquid, occasionally used to refer to the reverse of sublimation that is the direct conversion of a vapor to a solid (e.g. frost)