What the heck is "entropy"?

Question

The most no-nonsense definition I've ever found is dS = δQ / T, which to me could as well translate to сяскыйкийуз!

Question: when the sun goes off and all oceans in earth freeze, then the ice will have a lower entropy than the liquid water. So, it's lowering it's entropy by giving away energy?? I thought it only worked the other way? Or is that one of the "pockets" the Eri mentioned?

Answer 1

Entropy, in layman's terms, can be thought of as a measure of the "disorder" of a system.

For example, consider a contained divided in half. On one side of the barrier is oxygen gas. On the other side is a vacuum (empty). This is a very orderly state, or a low-entropy state. It is highly unlikely a situation like this would arise naturally. Now we open a hole in the barrier. Oxygen flows through the hole until it's spread evenly throughout the entire container. This is as a very disorderly state, or a high-entropy state.

The second law of thermodynamics states that the entropy of a *closed* system always goes up. This means that if nothing gets in or out of a system, the entropy will never spontaneously decrease. It will always go up. The reason for this is simple probability. How likely is it that all the molecules of oxygen gas would suddenly move to one side of the chamber, leaving the other side as a vacuum? It's so unlikely that it is never observed. The reverse is extremely likely, however. Gas bunched up on one side of the container is very likely to spread through the entire container. This is so likely that it is always observed.

For the system to go from a high-entropy (disorderly) state to a low-entropy (orderly) state, there must be outside intervention (such as a human installing a barrier and pumping all the oxygen out of one side. However, outside intervention means the system is not a closed one, so the second law of thermodynamics doesn't apply.

Entropy is also a way of measuring the direction of time. If you drop a glass, it will shatter. However, you could play a video of this event backwards, and it will show shards of glass spontaneously forming into a single piece and leaping into the air. This is clearly a highly improbable event, so we call the first event (breaking glass) forward passage of time, and the second event (shards forming into a glass) backwards passage of time.

The glass is the low-entropy (orderly) state, and the broken shards are the high-entropy (disorderly) state. The tendency of things, excluding outside intervention, for things to become disorderly, is what entropy is all about.

Answer 2

True - one can't go out and buy an entropy meter like he might do with a pressure gage or thermometer. Entropy is a derived parameter instead. To start, what are those terms in the equation you mention? The dS, and delta Q terms are small changes in S, entropy, and Q, heat transfer. So, the equation that "makes no sense" is about a process of change. The properties that are changing, S and Q, while T is constant, are associated with a volume, or system of mass (particles).
Pick a system of mass particles, maybe the glass of beer in front of you. Assume that its temperature is colder than that of the ambient temperature. At some instant of time, the glass & beer absorb thermal energy at nearly constant temperature. So, dS = dQ / T > 0, because heat transfer is from warmer to cooler bodies, and your next gulp is of a higher entropy (& temperature) beer. Not that you can see it or measure it, but you merely understand that it's a calculated parameter using measurable and other calculated quantities.
The simple equation you asked about is key to Hawking Radiation emitted by black holes.

Answer 3

One way to look at entropy is the amount of randomness or disorder in a system.

Think of a deck of cards all organized by suit (hearts, diamonds, spades, then clubs) and they by value lowest to highest.
Its very easy to distinguish one position in the deck from another - the 14th card will be the ace of diamonds, for example.
Now mix it up a bit so that the hearts could be anywhere even if the other suits are still in order, and you have more entropy because its less ordered and less easy to distinguish one position from another.
Now through the entire deck up into the air in front of a big fan, let them blow around, and then pick them up at random.
Now there is no way to know what card is where - that would be maximum entropy of the deck of cards.

Entropy can also be considered a measure of the amount of energy in a system that is not available to do work. A large boulder at the top of a cliff has a lot of potential kinetic and gravitational energy. So low entropy (can do a lot of work if we let it).
But a boulder at the bottom of the cliff has no energy available to do work so entropy is maximum (a lot of mass but it can't do any work by falling as it could at the top of the cliff).

Answer 4

The number of microstates available to a system. The entropy of the universe is always increasing, but small pockets can decrease in entropy when energy is put into the system - but there is a corresponding increase elsewhere.

EDIT: All those answers above are also true. And to whoever thumbed me down, I'd like to know where you got YOUR Ph.d. in physics.

Answer 5

Entropy is the measure of degree of randomness.

It actually refers to disorder.
If, we take the 3 states of matter into consideration, the order of entropy increases as follows;
solid
Spontaneous changes occur with an increase in entropy. Spontaneous changes tend to smooth out differences in temperature, pressure, density, and chemical potential that may exist in a system, and entropy is thus a measure of how far this smoothing-out process has progressed.

Answer 6

The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity.

Answer 7

Hi. The constant flow of energy from hotter to colder. This is an irreversible trend. (Don't you just hate it when a formula makes things seem more complicated than they are?)