Prof Sporlan
08-12-2003, 02:57 PM
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.
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.