Entropy

[Prof Sporlan]

What is entropy and why does it exist?

Entropy can be simple defined as a measure of energy that is no longer available to perform useful work.

Consider the following: A piston is used to compress a gas, and in the process, the pressure and the temperature of the gas increase. Work, of course, must be applied to the gas for this to happen.

If we assume this process is <b>adiabatic</b>, i.e., heat from the gas doesn’t go anywhere, we know from the First Law that the work applied to the gas increases the internal energy of the gas by the amount of work

delta U = Q + W

where:
U = internal energy
Q = heat (Q = 0 for an adiabatic process)
W = work

The above also assumes no inefficiencies in the compression, e.g., friction does not exist. In the real world, however, not all of the work will be converted to internal energy. We will lose work thru friction, and some of the heat will transfer out into the environment. This is “lost” energy that is no longer available to perform useful work.

Note that this does not violate the First Law, which states all energy must be conserved. The First Law states that all energy must be accounted for.. there is no requirement that the energy be useful.

If we expand the gas, we can recover the work used to compress the gas. If the process is adiabatic and we have no inefficiencies, we will recover all of the work. This is what is referred to as a <b>reversible</b> process, where no energy is lost resulting in no change in entropy. In the real world, however, we cannot hope to recover all of the work.

Also consider what would happen if we allowed our high pressure gas to expand into the environment without doing any work, a <b>throttling</b> process. This would be much like letting the air out of a balloon. Here, no work would be recovered, and we would have a maximum change entropy.

[Prof Sporlan]

The Second Law simply states that as we transform energy from one state to another, a penalty is extracted, and that penalty is a reduction in the available energy to do useful work.

Entropy is simply a measure of energy that is no longer available to do useful work.

An increase in entropy means there is less energy available to do useful work in the future. It is often considered a change from an ordered state to a disordered state. One can see this easily enough in real life. Consider what your home might look like after 20 years of neglect…

Thermodynamic laws are considered the most simplistic and elegant laws of modern science. The Second Law is considered the fundamental law of natural science.

If our universe were static, we would how it would end. Entropy would continually increase until we reach maximum entropy, i.e., "heat death". The problem with the "heat death" scenario is the universe is expanding. If the universe were eventually to contract upon itself due to gravity into the "big crunch", would we zero out entropy in the process?

Creationists will often argue the Second Law disallows evolution, i.e., lower orders of life evolving to higher orders. The problem with this argument is earth is not isolated. It is for all practical purposes, a closed system, with an external source of energy, the sun.

Entropy in the universe is certainly increasing with time. Fortunately, the earth is not an isolated system like the universe, so it is possible for us to maintain entropy on the planet as long as the sun shines.