Question

Could someone explain to me the Entropy and Second Law of Thermodynamics?

Answer 1

Entropy is the quantitative measure of disorder in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of "absolute entropy," physicists generally talk about the change in entropy that takes place in a specific thermodynamic process.

Calculating Entropy
In an isothermal process, the change in entropy (▲S) is the change in heat (Q) divided by the absolute temperature (T):

▲S = Q/T

In any reversible thermodynamic process, it can be represented in calculus as the integral from a processes initial state to final state of dQ/T.
The SI units of entropy are J/K (joules/degrees Kelvin).

Answer 2

Entropy & The Second Law of Thermodynamics

One way of stating the second law of thermodynamics is:
In any closed system, the entropy of the system will either remain constant or increase.
One way to view this is that adding heat to a system causes the molecules and atoms to speed up. It may be possible (though tricky) to reverse the process in a closed system (i.e. without drawing any energy from or releasing energy somewhere else) to reach the initial state, but you can never get the entire system "less energetic" than it started ... the energy just doesn't have anyplace to go.

Answer 3

The second law of thermodynamics states that in general the total entropy of any system will not decrease other than by increasing the entropy of some other system. Hence, in a system isolated from its environment, the entropy of that system will tend not to decrease. It follows that heat will not flow from a colder body to a hotter body without the application of work (the imposition of order) to the colder body. Secondly, it is impossible for any device operating on a cycle to produce net work from a single temperature reservoir; the production of net work requires flow of heat from a hotter reservoir to a colder reservoir. As a result, there is no possibility of a perpetual motion system. Finally, it follows that a reduction in the increase of entropy in a specified process, such as a chemical reaction, means that it is energetically more efficient.
It follows from the second law of thermodynamics that the entropy of a system that is not isolated may decrease. An air conditioner, for example, may cool the air in a room, thus reducing the entropy of the air of that system. The heat expelled from the room (the system), involved in the operation of the air conditioner, will always make a bigger contribution to the entropy of the environment than will the decrease of the entropy of the air of that system. Thus, the total of entropy of the room plus the entropy of the environment increases, in agreement with the second law of thermodynamics.
In mechanics, the second law in conjunction with the fundamental thermodynamic relation places limits on a system's ability to do useful work. The entropy change of a system at temperature T absorbing an infinitesimal amount of heat δq in a reversible way, is given by . More explicitly, an energy TRS is not available to do useful work, where TR is the temperature of the coldest accessible reservoir or heat sink external to the system. For further discussion, see Exergy.
Statistical mechanics demonstrates that entropy is governed by probability, thus allowing for a decrease in disorder even in a closed system. Although this is possible, such an event has a small probability of occurring, making it unlikely. Even if such event were to occur, it would result in a transient decrease that would affect only a limited number of particles in the system.