Transcript Thermodynamics

Thermodynamics
Heat Vs Temperature
 Temperature is NOT heat!
 Heat is energy (kinetic energy of atoms and
molecules)
 Temperature is the level of energy (related
to the speed of the atoms or molecules)
Temperature Scales
 Fahrenheit: 0o for brine, @32o water
freezes, body temp 96o
– Chose the numbers for easy divisibility of
measuring tools
 Celsius or centigrade: 0-100o
 Kelvin or Absolute: 273K-373K, Zero is
coldest possible temp.
Kinds of thermometers
 Expansion: mercury, metal, gas, alcohol
 Chromatic: color emitted by hot object
 Thermocouple: small electrical flow
between two different hot metals
 Crystal: alignment of different crystals at
different temperatures
Expansion
 All materials expand when heated.
 Expansion amount depends on original
length, change in temperature, and the
material’s coefficient of thermal expansion
a
 Water is unusual in that it contracts a bit
just before it freezes, leaving the coldest
water at the top.
Quantity of Heat
 Measured in calories. 1cal = 4.2J
 Every substance has a specific heat
capacity, c, which is the amount of energy
required to raise its temperature one degree.
 Water has a rather high c which is crucial in
determining weather and climate
Change of Phase
 Changing from solid to liquid, gas to liquid,
etc.
 Solid to gas is called sublimation
– Like “dry” ice
 Heat of fusion: energy to melt or freeze.
For water, 80 cal/g
 Heat of Vaporization: energy to condense or
evaporate. For water at sea level, 540 cal/g.
Humidity
 Warm air, by virtue of its lower density than
cold air, can hold more water vapor.
 The amount of water a volume of air holds
is the absolute humidity.
 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
 Conduction: bumping of molecules (contact
required). Poor heat conductors are called
insulators.
 Convection: heat transfer through buoyant
forces e.g clouds, “heat rising”.
 Radiation: energy transported via
electromagnetic radiation e.g. Infrared.
Human cooling process
 Radiation: 60% (through skin)
 Evaporation: 25% (sweat)
 Conduction: 12%
 Convection: 3%
 Larger bodies have lower skin to heat-
producing volume, harder to cool off.
Laws of Thermodynamics
 0: Heat flows from hot to cold
 1: Conservation of Energy
 2: Efficiency must be <100%
 3: Entropy: order to disorder
Heat engine
 Heat flows from a hot source to a cold sink;
some energy is pulled off to do work
 Internal combustion engine is the archetypal
heat engine
 Refrigeration is the reverse
– Invented by John Gorrie in 1840s
– Pulls heat from cool sink to hot source
– Energy input required
Calorimetry
 Heat lost by one object is gained by another.
– Qlost = Qgained
 The two objects come to some equilibrium
temperature.