Thermodynamics
Heat Vs Temperature 4 Temperature is NOT heat! 4 Heat is energy (kinetic energy of atoms and molecules) 4 Temperature is the level of energy (related to the speed of the atoms or molecules)
Temperature Scales 4 Fahrenheit: 0 o for o water freezes, body temp 96 o –Chose the numbers for easy divisibility of measuring tools 4 Celsius or centigrade: o 4 Kelvin or Absolute: 273K-373K, Zero is coldest possible temp.
Kinds of thermometers 4 Expansion: mercury, metal, gas, alcohol 4 Chromatic: color emitted by hot object 4 Thermocouple: small electrical flow between two different hot metals 4 Crystal: alignment of different crystals at different temperatures
Expansion 4 All materials expand when heated. Expansion amount depends on original length, change in temperature, and the material’s coefficient of thermal expansion 4 Water is unusual in that it contracts a bit just before it freezes, leaving the coldest water at the top.
Quantity of Heat 4 Measured in calories. 1cal = 4.2J 4 Every substance has a specific heat capacity, c, which is the amount of energy required to raise its temperature one degree. 4 Water has a rather high c which is crucial in determining weather and climate
Change of Phase 4 Changing from solid to liquid, gas to liquid, etc. 4 Solid to gas is called sublimation –Like “dry” ice 4 Heat of fusion: energy to melt or freeze. For water, 80 cal/g 4 Heat of Vaporization: energy to condense or evaporate. For water at sea level, 540 cal/g.
Humidity 4 Warm air, by virtue of its lower density than cold air, can hold more water vapor. 4 The amount of water a volume of air holds is the absolute humidity. 4 The amount of water a volume of air holds compared with the amount of water it can hold is the relative humidity.
Methods of Heat Transfer 4 Conduction: bumping of molecules (contact required). Poor heat conductors are called insulators. 4 Convection: heat transfer through buoyant forces e.g clouds, “heat rising”. 4 Radiation: energy transported via electromagnetic radiation e.g. Infrared.
Human cooling process 4 Radiation: 60% (through skin) 4 Evaporation: 25% (sweat) 4 Conduction: 12% 4 Convection: 3% 4 Larger bodies have lower skin to heat- producing volume, harder to cool off.
Laws of Thermodynamics 4 0: Heat flows from hot to cold 4 1: Conservation of Energy 4 2: Efficiency must be <100% 4 3: Entropy: order to disorder
Heat engine 4 Heat flows from a hot source to a cold sink; some energy is pulled off to do work 4 Internal combustion engine is the archetypal heat engine 4 Refrigeration is the reverse –Invented by John Gorrie in 1840s –Pulls heat from cool sink to hot source –Energy input required
Calorimetry 4 Heat lost by one object is gained by another. –Q lost = Q gained 4 The two objects come to some equilibrium temperature.