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What is Entropy? The laws of thermodynamics. Entropy definition. Examples of entropy

Updated on January 28, 2011

The three laws of Thermodynamics

1. You cannot gain something from nothing because energy cannot be create nor destroyed.

2. Any closed system cannot be returned to its original state. Entropy only increases.

3. You cannot reach absolute zero in temperature. As you tend towards absolute zero, entropy decreases and vice versa. As you tend to wards absolute zero, entropy tends towards a constant. (it can only actually be a constant when it is 0, a state of perfect organisation).

They control the interactions of everything in the known universe, regardless of scale.

The first Law

The first law is essentially a prelude to the second, and there is little to build upon other than the fact that you cannot create something from nothing, and that matter cannot be destroyed, only changed.

The second Law

The second law is all about entropy.

Energy exists in one of two states. Either usable, or not usable. Entropy is the measurement of how much usable energy there is. The level of entropy within a closed system increases as the level of unusable energy increases (and also obviously, as the level of usable energy decreases).

Even though the energy cannot be destroyed, it gradually becomes obsolete and useless over time as entropy increases. Entropy cannot be reversed. As a result of the first and second laws combined, one can logically conclude that the universe is finite.

As entropy increase, randomness and disorganisation also increase.

  • Cold is low entropy, heat is high entropy.
  • The entropy of a solid is < that of a liquid
  • the entropy of a gas is > that of a solid
  • and of course, the entropy of a solid is therefore < that of a gas


If you think in the same way I do, your probably thinking right now:

But if entropy is irreversible; how can a gas have a higher entropy than a liquid or a solid when it is a cyclical system? I mean, you can turn gas back into water cant you?

Yes of course you can. The answer is thus:

Entropy can be reversed under one condition. If a system takes usable energy from outside of its own system, then the entropy % is reduced.

For example: The system of ice, H2O and Vapor will be called ABC\

Entropy % = E

Usable Energy = Z

Ice = A

H2O = B

Vapor = C

The universe will be called U

When A becomes B, its E increases. The same for B becoming C

When C becomes B, it takes Z from U and its E decreases.

When system ABC takes Z from U; the E of U increases and the Z of U decreases.

U has no other system to draw energy from. It is finite.

The Third Law

As you tend towards absolute 0, entropy will decrease. As heat increases, so does entropy.

You may or may not have noticed, but as you heat up solids, they have a tendency to eventually melt.

Liquids are hotter than solids

Liquids = more entropy than solids

heat = more entropy.

As you already know, the ice cube has a lower entropy than the gas.

But remember, you cannot actually reach absolute 0.

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