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• There's a concept that's crucial to chemistry and physics.

• It helps explain why physical processes go one way and not the other:

• why ice melts,

• why cream spreads in coffee,

• why air leaks out of a punctured tire.

• It's entropy, and it's notoriously difficult to wrap our heads around.

• Entropy is often described as a measurement of disorder.

• That's a convenient image, but it's unfortunately misleading.

• For example, which is more disordered -

• a cup of crushed ice or a glass of room temperature water?

• Most people would say the ice,

• but that actually has lower entropy.

• So here's another way of thinking about it through probability.

• This may be trickier to understand, but take the time to internalize it

• and you'll have a much better understanding of entropy.

• Consider two small solids

• which are comprised of six atomic bonds each.

• In this model, the energy in each solid is stored in the bonds.

• Those can be thought of as simple containers,

• which can hold indivisible units of energy known as quanta.

• The more energy a solid has, the hotter it is.

• It turns out that there are numerous ways that the energy can be distributed

• in the two solids

• and still have the same total energy in each.

• Each of these options is called a microstate.

• For six quanta of energy in Solid A and two in Solid B,

• there are 9,702 microstates.

• Of course, there are other ways our eight quanta of energy can be arranged.

• For example, all of the energy could be in Solid A and none in B,

• or half in A and half in B.

• If we assume that each microstate is equally likely,

• we can see that some of the energy configurations

• have a higher probability of occurring than others.

• That's due to their greater number of microstates.

• Entropy is a direct measure of each energy configuration's probability.

• What we see is that the energy configuration

• in which the energy is most spread out between the solids

• has the highest entropy.

• So in a general sense,

• entropy can be thought of as a measurement of this energy spread.

• Low entropy means the energy is concentrated.

• High entropy means it's spread out.

• To see why entropy is useful for explaining spontaneous processes,

• like hot objects cooling down,

• we need to look at a dynamic system where the energy moves.

• In reality, energy doesn't stay put.

• It continuously moves between neighboring bonds.

• As the energy moves,

• the energy configuration can change.

• Because of the distribution of microstates,

• there's a 21% chance that the system will later be in the configuration

• in which the energy is maximally spread out,

• there's a 13% chance that it will return to its starting point,

• and an 8% chance that A will actually gain energy.

• Again, we see that because there are more ways to have dispersed energy

• and high entropy than concentrated energy,

• the energy tends to spread out.

• That's why if you put a hot object next to a cold one,

• the cold one will warm up and the hot one will cool down.

• But even in that example,

• there is an 8% chance that the hot object would get hotter.

• Why doesn't this ever happen in real life?

• It's all about the size of the system.

• Our hypothetical solids only had six bonds each.

• Let's scale the solids up to 6,000 bonds and 8,000 units of energy,

• and again start the system with three-quarters of the energy in A

• and one-quarter in B.

• Now we find that chance of A spontaneously acquiring more energy

• is this tiny number.

• Familiar, everyday objects have many, many times more particles than this.

• The chance of a hot object in the real world getting hotter

• is so absurdly small,

• it just never happens.

• Ice melts,

• cream mixes in,

• and tires deflate

• because these states have more dispersed energy than the originals.

• There's no mysterious force nudging the system towards higher entropy.

• It's just that higher entropy is always statistically more likely.

• That's why entropy has been called time's arrow.

• If energy has the opportunity to spread out, it will.

There's a concept that's crucial to chemistry and physics.

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B1 US TED-Ed entropy energy configuration spread solid

# 【TED-Ed】What is entropy? - Jeff Phillips

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IS LIU posted on 2018/03/01
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