Thermodynamics and P-V Diagrams - YouTube

Hi. It’s Mr. Andersen and this AP Physics essentials video 85. It is on thermodynamics

and p-v diagrams or pressure volume diagrams. And this is a picture of a fire piston. You

can put some tinder, like cotton in the bottom of a fire piston, you can slam your hand down

on it and since heat does not have time to escape from that piston, we heat it up and

the thing starts on fire. And natives have known this for a long time. This is a native

fire starter. You put some tinder on the inside, you slam it down and then it starts a fire.

And so a part of the conservation of energy is the first law of thermodynamics. Energy

can neither be created nor destroyed. But we are going to study how this applies in

a typical idealized gas in a piston like this. And so our equation is delta U, so that is

the change internal energy is equal to Q + W. Q is going to be heat and W is going to

be work. And so we can add energy to this system, this internal energy by heating it

up or we can apply a force over a given distance to it. We can do work on it. And so what we

are looking at is how energy can be transferred from the surroundings into the system and

back again. Now we are not going to loose any energy. Remember the energy is going to

be conserved. And a good way to measure this, using a piston, is to graph the pressure and

the volume over time. And so we are going to start with the piston kind of relaxed and

we are going to have a really high volume, so we have a large volume here, but it is

going to be a relatively low pressure. And then I am going to compress that piston. So

the surroundings are going to do work on the system. And as they do that you can see we

have an increase in the pressure inside. Now we did it slow enough that we are allowing

that temperature to stay constant throughout that whole process. And so what is neat about

a pressure volume diagram is the area under that curve is going to be the amount of work

that we do. Now a common mistake in physics is to figure out who is doing the work. Is

it the system or is it the surroundings? And so in any piston like this, the area under

the curve is the work being done by the gas. And so you can see that we are going from

right to left and so this is negative work. And so what work is being done by the gas?

It is negative work. Well it is negative work of a gas that means it is positive work of

the surroundings. We are adding work on the system and so we are increasing the amount

of internal energy on the inside. This would be an isothermal process, since we are keeping

this temperature the same. Now if physics 1 and 2 you do not have to do calculations,

you just have to know qualitatively what is going on. But we could have a different process

that will model here. It could be isovolumetric. We could lock the volume of that piston and

we could just add energy through heat. We have have isobaric when we keep the pressure

the same. Or in the fire piston we can do adiabatic cooling or heating. So that is when

we are doing it so quickly that we are not allowing heat to kind of move in and out.

And so let’s start with a little simulation here. This is a PHET simulation. We have the

gas molecules on the inside. We can add heat right here. And then we can push back and

forth on the piston on this side so we can do work. We can apply a force for a given

distance. And so let’s watch what happens when we run this simulation. The energy is

in the molecules on the inside. What happens as I add temperature to it? I am increasing

their speed and so I am increasing their energy, the energy of the system increases. If I cool

it down, energy of the system decreases. How else could I add energy? Remember it is W.

It is work. So we could push in on it. What is happening to the pressure, and therefore

the energy on the inside? It is increasing. And now that total internal energy is decreasing

over time. And so it is nice to be able to use a simulation, but the two things that

we can measure are the volume and the pressure. And so if we just keep track of the volume

and the pressure over time we can figure out what kind of a process is going on. And so

this is a P-V diagram. And so let’s start with an isobaric process. In an isobaric process

we are going to keep the pressure the same. But we are going to be able to change the

volume over time. And so a way to do that is to allow this piston to move in and out.

And so we can change the temperature. What is going to stay the same? It is going to

be this same pressure on the inside. So that piston is going to move back and forth but

we can add energy into the system or we can pull energy out. Since it is free to move

the pressure is going to stay the same. And so if you see on a P-V diagram this horizontal

line, then you know it is isobaric. What else could we do? We could do isovolumetric. What

does that mean? We are going to keep the volume the same. You can see the volume is always

the same so it is just going to move back and forth on here. We are going to increase

or decrease the pressure. And so what would that look like, using our piston? We are going

to lock it in place. Now we can add temperature, we could pull that temperature out. We could

also have isothermal. Isothermal is always going to look like a curve like this. This

would be an isothermal curve. And so how do we do that with a piston? Well we will use

something like a heat sync where we can keep the same temperature throughout that whole

process. And then finally we could do adiabatic. That is where you either pull it out or move

it in really really quickly, so fast that we can not loose temperature to the surroundings

and you are going to get a curve that looks something like that. Okay. So let’s kind

of model those using this PHET simulation. And so let me kind of get this thing going.

There we go. And so now what we are going to do is we are going to keep the volume constant.

So we have locked it is place. How do we get energy into the system? We could add heat.

As we add heat we increase temperature, pressure is going to go up. Let’s say we add ice

on the bottom. We are going to pull energy out of it. Heat is going to flow from the

system to the surroundings and the pressure is going to decrease. What is staying the

same that whole time? The volume is going to stay the same. For the second one let’s

keep the temperature constant now. So we have a constant temperature. So you can see that

I can not even change my heat controls here. So how am I going to get energy into or out

of the system? It has to be through work. So if I apply a force from the outside what

is happening to the pressure on the inside? It is going up. If I do negative work from

the outside, what is going to happen to the pressure? The pressure is going to drop off.

And so we are going to get that nice curve on a P-V diagram. And then finally we can

look at an isobaric process. In an isobaric we are going to keep that pressure the same.

What does that mean? This piston can simply move back and forth. It is going to be the

same pressure exerted by the gas. If we cool it down those molecules move slower, the piston

is simply going to be move in. If we were to add temperature to it, as we increase temperature

we are adding heat into the system. What is going to happen? Those molecules are going

to move quickly. But since the pressure remains the same the piston is going to move out.

And so what would the P-V diagram look like in an isobaric process? Since that piston

can move back and forth the pressure is always going to remain the same. And so the volume

is going to change over time. And so remember the area under a pressure volume diagram is

going to be the amount of work that we do. And so since we are taking a large volume

and moving to a small volume, the work is going to be in that direction, and so we would

be doing negative work. Now remember that is negative work of the gas itself, what is

inside the piston. So what is the positive work? That is from outside. That is going

to be the work of the surroundings. And so did you learn to figure out how internal energy

in a system can change? It is really only two things. We could add heat or Q or we could

do work. Do you know, looking at a P-V diagram which of the three or four processes this

is? So this would be isobaric. This would be isovolumetric. This would be isothermal.

And this is going to be adiabatic. And then finally could you find the area under the

curve? And if you know the are under the curve could you figure out how much work is being

done by the gas, because that is what is going to be under the curve?

I hope so. And I hope that was helpful.