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

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.