Second law of thermodynamics in simple terms [Mechanical engineering]

# Second law of thermodynamics in simple terms

In simple words, what is the second law of thermodynamics and why do creationists talk about it so much?

The second law effectively states that the integral over a closed system of q/T, q the heat, T the temperature, increases with time, and it does not decrease. One of the consequences of that is while you can completely convert mechanical energy to heat, there are very strict limits on how much heat you can convert to work. Basically, the second law says heat flows from hot to cold, and NEVER from cold to hot. Your dinner on your plate never suddenly drops a degree of temperature and smears itself across the ceiling.

Heat is random kinetic energy, thus in a gas the molecules are actually travelling very fast, but because of the randomness of the motion, they are not going anywhere in particular. So, what the second law also says is that a random mix does not spontaneously sort itself. If you put pink solution into water, eventually it mixes and you get a lighter pink, but that never sorts itself and returns to a mix of pink and clear. However, you can sort a mixture by doing work on it. So, basically, as q/T increases, the system also becomes more random, not less random, provided no extra energy is input.

The issue I think you find yourself concerned with is the concept that life requires order. If everything is gradually degrading, how can life emerge? Think of iron; left to its own devices, it rusts, but the rust can be converted back to iron by reducing with carbon and putting in energy. Things can be reversed; all it requires in an input of energy, and what you will find with every analysis is that within the system, q/T (entropy) increases, because some of that input energy ends up as low grade heat, which cannot be reconverted to work.

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### FAQ

##### what is the second law of thermodynamics in laymans terms? | Yahoo Answers

Any closed system will, over time, increase in entropy. Also, a system can at times decrease in entropy, so long as the net effect is an increase in entropy.
Essentially, it is saying that, over time, a closed system will become more disorderly (things tend to 'fall apart', as it was so eloquently put previously). Therefore, an 'increase in entropy' refers to a 'decrease in order' and vice-versa.

##### What are the limitations of first law of thermodynamic in terms of second law

1. A major limitation of the first law of thermodynamics is that its merely indicates that in any process there is an exact equivalence between the various forms of energies involved, but it provides no information concerning the spontaneity or feasibility of the process. For example, the first law does not indicate whether heat can flow from a cold end to a hot end or not.
2. If in the expansion of a gas the opposing pressure is infinitesimally smaller than the pressure of the gas, the expansion takes place infinitesimally slowly i.e. reversible. If however, the opposing pressure is muc…