 Hello, welcome to my channel. My name is Adrian. And today in understanding thermodynamics, we are going to look at the property pressure. By the end of this video, you will be able to explain the concept of pressure. You will know what is absolute and total pressure, what is ambient pressure, what is gauge pressure, as well as what is hydrostatic pressure. And you will also know why pressure is an intensive variable. So what is the definition of pressure? Now pressure is defined as P and is given by the equation P equals F divided by A where F is the force in newtons, A is the area in meter squared and the units of pressure is given in Pascal. Now let's have a quick example. Say a person with a mass of 100 kilograms stand on two bricks. Each brick is 10 centimeter wide and 20 centimeter long and the total area of both bricks is thus 0.04 meter squared. And then the pressure that this bricks are exerting on the ground is 24.5 kilo Pascal. Now imagine the same person gets on a bicycle. The tire contact area is a lot smaller compared to the bricks. What then will be the required pressure in some of the contact area is so much reduced. Now the air in car tires are approximately 200 kilo Pascal. And what do you imagine is the pressure of steam in a power station? Let me know in the comments below. Now let's consider this piston cylinder arrangement. And you can see gas molecules collide with the walls of the cylinder as well as with the piston at the top. Now the setup is in a vacuum and therefore there's no gas molecules colliding on the outside surface of the setup. And the upward force exerted by the colliding gas molecules counterbalance the downward force exerted by the combined mans of the piston and the two small weights that you can see in green. But looking at that, we can say the force that is exerted by the colliding molecules is directly proportional to the mass of this molecules, the velocity of the molecules squared and the rate at which collisions is taking place, which is n. Now adding molecules to a fixed volume, which happens when you inflate the tube of a bicycle or the tires on your car will therefore increase the number of molecules because you're adding air and therefore also the rate of collisions because there's now more molecules to collide with each other. And this will cause an increase in pressure. Right, so now we can look at total pressure and total pressure is the total force that's exerted by the colliding molecules divided by the total area which they are colliding against. Total pressure is often called absolute pressure. And if there is no molecules or atoms colliding with a surface in the case of a vacuum, the force exerted will be zero and therefore the pressure will also be zero. Next, let's look at ambient pressure. Now ambient pressure is the total pressure exerted by the atmospheric molecules, the air molecules around us. And at sea level, this pressure is 101.325 kilopascal. On top of Mount Everest, where there's not a lot of atmospheric molecules, this pressure is greatly reduced to approximately 34 kilopascal. Now imagine if you inflate a balloon at sea level and you go and hike up to Mount Everest with this balloon, what do you think will happen with the balloon? Let me know in the comments below. Next, let's consider gauge pressure. Now gauge pressure is the difference between the total pressure inside the container, which in this case is the tire of a car and the total pressure outside. And usually the pressure outside is equal to the ambient pressure. Let us investigate the effect of ambient pressure on gauge pressure. So like I said, gauge pressure is the difference between the total pressure inside of this container, this case tire and the total pressure outside, which is usually ambient pressure. Now let us calculate the gauge pressure. If this car tire has an absolute pressure of 300 kilopascal. Now at sea level, our gauge pressure would be approximately 200 kilopascal because it's 300 kPa minus 101 kPa, which is atmospheric pressure at sea level. But if we take the same tire to Mount Everest, then our gauge pressure will be 266 kilopascal. That is because the atmospheric pressure has greatly reduced and then the difference between the absolute pressure inside the tire and the atmospheric pressure is bigger. Right, so here is a problem that we can consider. So we have two cylinders with a piston head at the top. And there's a piston head again with a little valve here at the bottom. Now for the first instance, this piston cylinder is containing air. And the cross-sectional area of the piston is 0.02 meter squared. And the mass of the piston head is 100 kilogram. And the ambient pressure outside the cylinder is 87 kilopascal. Now calculate the total and gauge pressure inside the cylinder before and after some air was let out of the setup through the valve. You can pause this video and have a go and then we will discuss it. Now let's consider the pressure inside the cylinder. Now in order for the air to hold the piston head stationary, it needs to offset the weight of the piston head as well as the ambient pressure pushing down on the piston head as well. So the total pressure inside is a sum of the ambient pressure plus the weight of piston head. And we can calculate the pressure exerted on the piston head by multiplying its mass with G, which is 9.81 and dividing it by the cross-sectional area, which is 0.02 and we get a value of 49 kilopascal. And now we can add them together. So the total pressure inside the cylinder is 136 kilopascal. Now to get the gauge pressure, we are going to subtract the outside pressure from the inside pressure. And in this case it's 136 minus 87, 87 being the atmospheric pressure and we get a gauge pressure of 49 kilopascal. Now note this will be the same irrespective of how many air is inside the cylinder. So if you let out some air and you measure the gauge pressure again, you will get the same value of 49 kilopascal. And this nicely illustrates why pressure is an intensive variable. Now consider the setup here where we have a container connected and it has a pressure P1, temperature T1 and a volume V1. And now we change it by taking away the connection in the middle and we end up with two separate boxes. And we can see that the volume is half because we've divided it equally into two. So the volume of each box is half of the first volume and the pressure stays exactly the same. Nothing has happened with the pressure. What do you imagine happened with the temperature? Did it half or did it stay the same? Let me know in the comments. So by looking at that, the fact that pressure stays the same after we've half the volume shows that pressure is in fact an intensive variable. Now lastly, let's consider hydrostatic pressure. Now imagine we fill the cylinder with water to a depth of H meter. The pressure exerted by the water on the piston is equal to the force exerted by the water divided by the cross sectional area. And we can expand this equation and you'll see that areas cancel out and then we end up with the hydrostatic pressure as being rho, which we assume is uniform, times G, the gravimetric acceleration constant, times the height of the water column. So in summary, the total pressure is equal to the total force divided by the total cross sectional area and that will give us a value in Pascal. Now the lowest value that you can get for pressure is zero. Total pressure will always be bigger than zero. Ambient pressure is pressure exerted by the atmospheric air molecules and this varies with height above sea level. Gauge pressure can be calculated by subtracting the total pressure outside from the total pressure inside the container and gauge pressure can in fact be negative, zero or positive. And we know that hydrostatic pressure can be calculated with rho GH. Thank you very much for watching course notes which these videos are based on is available on my website, rdronsblog.com. You're welcome to connect with me on Twitter. My Twitter handle is at ASVN90 where you can ask me any questions and I'll be happy to answer them. Thank you very much for watching and I will see you in the next video. Bye.