What is a quasi static process? Someone just asked me this, one of my students and I had to write a big explanation for that because most textbooks just dismiss it as a slow process. But then you ask them what is reversible and then again you get the answer it's a slow process. So what is a quasi static process and how does it differ from a reversible process? That's what we are going to look at in this video. Take a look at this water bottle. It contains a little amount of water. Now this water bottle is currently on the left side of the screen. Say I want to bring it to the right side. I have two options. So take a look at the water inside the bottle. Option 1, I suddenly shifted from here to here like this. So when I do this shift, you can see that the water inside the bottle begins to slosh about. Why does that happen? What happens because when I start from here and I push this bottle suddenly, the entire water does not move at the same time because inertia. Newton's first law says that an object continues to be in a state of rest or uniform motion until x to the acted upon by an external force. So if you look at this second half of water, it is not acted upon by an external force until the first half begins to climb up this side of the water. So if I push this bottle suddenly, what happens is this? This half begins to climb up this side. Then it exerts the force of this half and this half begins to climb up as well. And the aim of the whole exertion of force by one part of the water on another is to shift the water sideways. What does this have to do with quasi static processes? You see, what if I want to move the bottle without sloshing the water? Simple trick. Instead of moving it suddenly, I just start like this. And move it slowly. You see the water didn't slosh. Now why did the water slosh this time? It is that the left wall is pushing on the water. But different parts of the water are able to communicate this push between each other very quickly without having to undergo a large vertical displacement. And this is why the water does not slosh. What makes this movement easy to communicate and what makes the other movement difficult to communicate? The obvious difference is the speed. The speed of one movement is much slower than the speed of another. The second movement which I did was much slower than what? It was much slower than the speed of waves along the surface of the water. If I move slowly enough that waves move much faster than my movement, then these waves will have to carry a much smaller amplitude compared to the range of motion I want the water to fall. And this means that the waves will effectively be invisible and the whole water moves along as one unit. But if I move it suddenly, then the waves have to communicate this movement from one part of the water to another. And therefore they have to move with a large amplitude. And this large amplitude waves is what causes sloshing of the water. So what does all this have to do with quasi static processes? It's very simple. A thermodynamic process is basically when you compress or expand a gas that is accompanied by a change in its temperature. So when you compress or expand a gas just like the water sloshing example, you can compress it quickly generating a compression wave. This happens because the gas nearest to the piston is compressed fast when the piston is pushed in. And the rest of the gas is not yet compressed. And what happens is this compressed gas pushes on the amount of gas next to it and it compresses and then that compresses the part of gas next to it and so on until the wave reaches the end of the tube or the container. And it reflects from the end and comes back towards the piston, reflects from here, goes there and keeps going forward and backward until it is damped down by this thing called viscosity. And because when it is damped by viscosity, it loses its amplitude. The particles are not oscillating as much after certain. And the oscillation becomes invisible and the energy of that wave is converted into the heat of the system. This heat may be carried out through the walls or it may be placed inside the gas to raise its temperature. Now just like the example with the water bottle, if the piston is pushed in slowly, that is slower than the speed of compression waves through the gas, then the amplitude of the compression waves is going to remain small and they are going to damp back, they are going to go back and forth quickly and get damped up by viscosity, raising the temperature of the whole gas at a time. Not only the temperature, but if you look at compression, compression is actually a change in density of the gas. And a compression wave that is one part of the gas being compressed and the rest not being compressed means that different points on the gas have different densities. This is something which is non-quasi static. So now we come to the meaning of quasi static, which says that all parts of the gas must have number one, the same temperature and number two, the same density. This will be possible only if I push in the piston with the speed which is much slower than the speed of sound through that gas at that temperature. The question of piston at this speed, all parts of the gas are equally compressed simultaneously and there is no sudden compression wave generated which makes different parts of the gas to have different temperatures and different densities. In fact, different parts of the gas having different temperature will remove any meaning there is to the word temperature of the gas. Now if I am talking about the temperature of the gas, obviously I am assuming that all parts of the gas have the same temperature. Similarly, if I am talking about the density of the gas, I am assuming that all parts of the gas have the same density. But if the temperature and density are different for different parts of the gas, then that little T has no meaning anymore. I can't talk about the temperature of gas because different parts have different temperatures. So the necessity of quasi static condition is that all parts of the gas have the same temperature and the same density. So we can speak about the temperature of the gas as a whole and the density of the gas as a whole. And the method to achieve a quasi static condition, a quasi static change, a pseudo static change. In other words, is for you to compress and expand the piston at a speed which is much lower than the speed of sound through the gas.