law of thermodynamics. First law relates to energy and how it can neither be created nor

destroyed. But the second law relates to entropy. What is entropy? It is a measure of the amount

of disorder in a process. And so if you watch the word entropy I am increasing entropy of

entropy. In other words I am increasing the disorder. And it is much more likely that

it is to fall apart then all those pieces were to spontaneously come back together again.

So the second law of thermodynamics relates to processes that can either be reversible

or irreversible. Reversible means they can go either way. And the amount of entropy will

not change. But in an irreversible process the amount of entropy will increase over time.

Now entropy is a state function. That means it is just measured at one point or one state

in time. It measures disorder. Other alternative definitions, it is the lack of energy to do

work. And so a way to think of it is increasing chaos of the system or its organization of

the system. The nice thing about AP Physics is you do not have to quantify entropy. You

just have to know its quality, in other words what it is. In a closed system the amount

of entropy will never decrease. It always increases over time. And if we think of the

whole universe, the universe, entropy of the universe is going to increase over time as

well. Now that is kind of a vague concept. What does it mean to increase the entropy

of the universe. Let me give you some solid examples. And so I have two videos here. Video

A and B. Let me start playing them. They are both the same video but one is played in the

forward direction and one is played in the reverse direction. And so can you figure out

which one is forward and which on is reverse? It is hard to tell. If I remember right A

is actually played in the forward direction. So we can think of this as a reversible process.

It is just as likely to happen in the A direction as it is in the B direction. But let me show

you another video. Can you figure out which one of these is played in the forward and

which one is played in the reverse? Well you probably have never seen B as a video. You

do not see milk spontaneously move outside of a cup. So we know A is in the direction.

Now we are dealing with an irreversible process. It is totally probable to happen in the A

direction but it is statistically improbable if not impossible to happen in the B direction.

Let’s watch this. This is another one. So which one of these is in the forward direction?

Well B now is in the forward direction. So now we are getting at what it is to be an

irreversible process. In other words in an irreversible process, and I am just tugging

on the block at the bottom, it is moving in that direction. Now could the reverse occur?

Perhaps. But it is going to be statistically improbable for it to occur. So now we have

an irreversible process. What is going to happen to the entropy? It is going to increase

over time. And sometimes you will hear it referred to, entropy, as time’s arrow. It

is going to move in the direction of time in an irreversible process. Let me give you

an example of that. On the left we have gas molecules in a container. On the right we

have those same gas molecules in a container. So which do you think took place first? Which

of these is early and which is later in time? Well the right answer is going to be like

that. It is going to become more disordered in time. So what you can do is just draw the

arrow of time. And then entropy is just going to be in that same exact direction. And so

the second law of thermodynamics says in a closed system, a system isolated from its

surroundings, entropy will never decrease. In other words entropy is going to increase

over time. In other words in a closed system as time advances entropy is going to advance

as well. But this might see counter intuitive. How could I make something like this video

and this computer, they seem to have a huge amount of order. They do not see to be chaotic

as well. And so it seems like time and entropy in this case are reversed. Over time we have

something that is more complex. Well you have to step back again. And remember what I said

very carefully. Entropy never decreases but that is in a closed system. This computer

and this video are not a closed system. I made them more ordered by making the surroundings

less ordered. And that leads into this whole idea that over time entropy is going to increase

in the environment. And so did you learn to connect qualitatively the second law of thermodynamics

and the state function of entropy? I hope so. And I hope that was helpful.