Simple Nature - Light and Matter

absolute zero, the temperature at which all random molecular motion ceases, and equals 273 Kat the freezing point of water. We can get away with using the Celsius scale as long as we areonly interested in temperature differences; a difference of 1 degree C is the same as a differenceof 1 degree K.It is an observed fact that ideal gases obey the ideal gas law,P V = nkT ,and this equation can be explained by the kinetic theory of heat, which states that the gasexperts pressure on its container because its molecules are constantly in motion. In the kinetictheory of heat, the temperature of a gas is proportional to the average energy per molecule.Not all the heat energy in an object can be extracted to do mechanical work. We thereforedescribe heat as a lower grade of energy than other forms of energy. Entropy is a measure ofhow much of a system’s energy is inaccessible to being extracted, even by the most efficient heatengine; a high entropy corresponds to a low grade of energy. The change in a system’s entropywhen heat Q is deposited into it is∆S = Q TThe efficiency of any heat engine is defined as.efficiency =energy we get in useful formenergy we pay for,and the efficiency of a Carnot engine, the most efficient of all, isefficiency = 1 − T LT H.These results are all closely related. For instance, example 11 on page 312 uses ∆S = Q/T andefficiency = 1 − T L /T H to show that a Carnot engine doesn’t change the entropy of the universe.Fundamentally, entropy is defined as the being proportional to the natural logarithm ofthe number of states available to a system, and the above equation then serves as a definitionof temperature. The entropy of a closed system always increases; this is the second law ofthermodynamics.Chapter 6, Waves, page 339Wave motion differs in three important ways from the motion of material objects:Waves obey the principle of superposition.together.When two waves collide, they simply addThe medium is not transported along with the wave. The motion of any given point inthe medium is a vibration about its equilibrium location, not a steady forward motion.The velocity of a wave depends on the medium, not on the amount of energy in the wave.(For some types of waves, notably water waves, the velocity may also depend on the shapeof the wave.)953