Pressure is a physical quantity that is calculated as follows: divide the pressure force by the area on which this force acts. Pressure strength is determined by weight. Any physical object exerts pressure because it has at least some weight. The article will examine in detail the pressure in the gases. The examples will illustrate what it depends on and how it is changing.
The difference in pressure mechanisms of solid, liquid and gaseous substances
What is the difference between liquids, solids and gases from each other? The first two have volume. Solids retain their shape. Gas placed in a vessel occupies its entire space. This is due to the fact that gas molecules practically do not interact with each other. Therefore, the gas pressure mechanism is significantly different from the pressure mechanism of liquids and solids.
Put the weight on the table. Under the influence of gravity, the weight would continue to move down through the table, but this does not happen. Why? Because the molecules of the table approach the molecules of which the weight is made, the distance between them decreases so much that repulsive forces arise between the particles of the weight and the table. In gases, the situation is completely different.
Atmospheric pressure
Before considering the pressure of gaseous substances, we introduce the concept without which further explanations are impossible - atmospheric pressure. This is the effect that the surrounding air (atmosphere) has. The air only seems to us weightless, in fact it has weight, and to prove this, we will conduct an experiment.
We will weigh the air in a glass vessel. He enters there through a rubber tube in the neck. We remove air using a vacuum pump. Weigh the flask without air, then open the tap, and when the air enters, its weight will be added to the weight of the flask.
Vessel pressure
We will understand how gases act on the walls of blood vessels. Gas molecules practically do not interact with each other, but they do not scatter from each other. So, they still reach the walls of the vessel, and then return. When a molecule hits a wall, its impact acts on the vessel with some force. This power is short-lived.
Another example. Throw the ball into a sheet of cardboard, the ball will bounce, and the cardboard will deviate slightly. Replace the ball with sand. The blows will be tiny, we won’t even hear them, but their strength will build up. The sheet will constantly decline.
Now take the smallest particles, for example the particles of air that we have in our lungs. We’ll give it to the cardboard, and it will deviate. We force air molecules to hit the cardboard; as a result, force acts on it. What is this power? This is the force of pressure.
We conclude: the gas pressure is due to the impact of gas molecules on the walls of the vessel. The microscopic forces that act on the walls add up, and we get what is called the pressure force. The result of dividing force by area is pressure.
The question arises: why, if you take a sheet of cardboard in your hand, it does not deviate? After all, he is in the gas, that is, in the air. Because the blows of air molecules from one and the other side of the sheet balance each other. How to check if air molecules really hit a wall? This can be done if you remove the impact of molecules on one side, for example, pump out air.
Experiment
There is a special device - a vacuum pump. This is a glass cap on a vacuum plate. It has a rubber gasket so that there is no gap between the cap and the plate, so that they fit tightly against each other. A pressure gauge is attached to the vacuum unit, which measures the difference in air pressure outside and under the hood. The faucet allows you to connect the hose going to the pump with the space under the cap.
Place a slightly inflated balloon under the hood. Due to the fact that it is inflated a little, the blows of the molecules inside the ball and outside it are compensated. We cover the ball with a cap, turn on the vacuum pump, open the tap. On the pressure gauge we will see that the difference between the air inside and outside is growing. But what about a balloon? It is increasing in size. The pressure, that is, the blows of molecules outside the ball, is getting smaller. Air particles inside the ball remain, shock compensation is violated from the outside and from the inside. The volume of the ball grows due to the fact that the pressure force of the air molecules outside is partially borne by the force of elasticity of the rubber.
Now close the tap, turn off the pump, open the tap again, disconnect the hose to let air in under the hood. The ball will begin to decrease in size. When the pressure difference outside and under the cap is zero, it will become the same size as it was before the experiment. This experience proves that you can see the pressure with your own eyes if it is greater on the one hand than on the other, that is, if gas is removed on one side and left on the other.
The conclusion is this: pressure is a quantity that is determined by the impacts of molecules, but impacts can be more numerous and less numerous. The more impacts on the walls of the vessel, the greater the pressure. In addition, the greater the speed of the molecules hitting the walls of the vessel, the greater the pressure produced by this gas.
Pressure versus volume
Suppose we have a certain mass of the eye, that is, a certain number of molecules. During the experiments that we will consider, this quantity does not change. Gas is in the cylinder with the piston. The piston can be moved up and down. The upper part of the cylinder is open, we will put on it an elastic rubber film. Particles of gas hit the walls of the vessel and the film. When the air pressure inside and outside is the same, the film is flat.
If you move the piston up, the number of molecules will remain the same, but the distance between them will decrease. They will move at the same speeds, their mass will not change. However, the number of strokes will become larger, because the molecule needs to travel a smaller distance to reach the wall. As a result, the pressure should increase, and the film should bend out. Consequently, with decreasing volume, the gas pressure increases, but this is provided that the gas mass and temperature remain unchanged.
If you move the piston down, the distance between the molecules will increase, which means that the time they need to reach the walls of the cylinder and the film will also increase. Blows will become more rare. A gas that is outside has a higher pressure than one that is inside the cylinder. Consequently, the film will bend inward. Conclusion: pressure is a quantity that depends on volume.
Pressure versus temperature
Suppose we have a vessel with gas at low temperature and there is a vessel with the same gas in the same amount at high temperature. At any temperature, the gas pressure is due to the impact of molecules. The number of gas molecules in both vessels is the same. The volume is the same, which means that the distance between the molecules remains the same.
With increasing temperature, the particles begin to move faster. Consequently, the number and strength of their impacts on the walls of the vessel increases.
The following experience helps to verify the correctness of the statement that with increasing gas temperature its pressure increases.
Take a bottle whose neck is covered with a balloon. Place it in a container of hot water. We will see that the balloon is inflated. If you change the water in the container to a cold one and place the bottle there, the ball will be blown away and even drawn into.