Placeholder Image

Subtitles section Play video

  • 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.

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 US TED-Ed entropy energy configuration spread solid

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

  • 1412 194
    IS LIU posted on 2018/03/01
Video vocabulary