MadSci Network: Physics

Re: The Laws of Thermodynamics

Area: Physics
Posted By: Michael Onken, WashU
Date: Tue Jun 18 11:05:08 1996

The first thing to understand about the three Laws of Thermodynamics is that they apply only to energy. This is evident from the term "thermodynamics": as fluid dynamics examines the movements of fluids, thermodynamics examines the movements of heat (or in a broader sense, energy). That said, here are the Thermodynamic Laws:

1st Law - Conservation of Energy

If the state of a system is changed by applying work or heat or both, then the change in the energy of the system must equal the energy applied. In equation form this is written as: DE = Q + W, where DE is the change in energy, Q is the added heat, and W is the work done on the system. This is also important if the system is doing work (W), then the system cannot do this work without losing energy. Essentially the First Law states that (without a nuclear reaction) energy, like mass, cannot be created or destroyed.

2nd Law - Tendency toward Equilibrium

This is very difficult to understand, even for me, so I will begin with the empirical (observed) phenomena. If you take two bottles and fill one with a gas and make the other a vacuum, and then connect the bottles, the gas will quickly reach a state of equilibrium where it fills both bottles with equal pressure and temperature, both values lower than those in the first bottle. Since the First Law states that the energy of the system is the same after as before the expansion, then there must be a new value, S (entropy), which increased with the loss of heat (Q). Entropy is defined as the "capacity for change" of a system. If the state of a system is changed but the entropy is not changed (DS = 0), then the process was reversible (able to be changed back to the original state without added energy). If the state of a system is changed and the entropy increases (DS > 0), then the process was irreversible, or spontaneous. NOTICE, the DS (change in entropy) for a state change cannot be less than zero! Since most processes are irreversible, it is said that universal entropy is always increasing - since entropy is the driving force behind equilibrium (not chaos), this means that the universe is constantly moving toward a less dynamic state.

The formal statement of the Second Law is: it is impossible to move heat, by a cyclical process, from something at lower temperature to something at higher temperature unless work is added to the system. Since any two things at different temperatures brought together will come to equilibrium at the same temperature, with increased entropy for the spontaneous change, to force the heat to move in the opposite direction requires some external source of energy (work) to make up for the change in entropy. A good example is the refrigerator: if you leave the door to your refrigerator open for an hour (don't try this with perishables on the shelves), the temperature in the kitchen will go up, since there is more heat coming from the coils in the back than is being absorbed by the coils in the box.

3rd Law - Absolute Zero

Without going into Gibbs free energy and enthalpy, suffice it to say that at lower temperatures the change in entropy for spontaneous reaction decreases. This observation lead to the postulate, that as the temperature of a pure substance approached some lower limit (called absolute zero) the entropy for would approach zero. After plotting the temperatures and energy changes for several spontaneous reactions, it is possible to work backwards to find the value of absolute zero, which is -273C (about -460F). The Kelvin scale places 0K at absolute zero and then uses Celsius for increment size, so that water freezes at 273K and boils at 373K. With further math, the nature of absolute zero was defined as the Third Law: If the entropy of each element at absolute zero can be taken as zero, then all elements above absolute zero must have a finite, positive entropy; however, because entropy cannot be reduced to zero by finite means (as per the Second Law), no system can reach absolute zero.

These laws may seem remote to most people, but they can be applied to every aspect of science, from astrophysics to zoology. In summary: the First Law says energy is conserved; the Second Law says everything moves toward equilibrium because of something called entropy; and the Third Law says that there is a lowest temperature, called absolute zero, where this entropy stuff is zero.

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