Pressure is a physical quantity that is calculated as follows: divide the pressure force by the area on which this force acts. The force of pressure is determined by the weight. Any physical object exerts pressure because it has at least some weight. The article will discuss in detail the pressure in gases. Examples will illustrate what it depends on and how it changes.
The difference in pressure mechanisms of solid, liquid and gaseous substances
What is the difference between liquids, solids and gases? The first two have volume. Solid bodies retain their shape. A gas placed in a vessel occupies all of its space. This is due to the fact that gas molecules practically do not interact with each other. Therefore, the mechanism of gas pressure is significantly different from the mechanism of pressure of liquids and solids.
Let's put the weight down 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 are approaching the molecules fromwhich 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, let's introduce a concept without which further explanations are impossible - atmospheric pressure. This is the effect that the air (atmosphere) around us has. Air only seems weightless to us, in fact it has weight, and to prove this, let's conduct an experiment.
We will weigh the air in a glass vessel. It enters there through a rubber tube in the neck. Remove air with a vacuum pump. Let's 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.
Pressure in vessel
Let's figure out how gases act on the walls of vessels. Gas molecules practically do not interact with each other, but they do not scatter from each other. This means that they still reach the walls of the vessel, and then return. When a molecule hits the wall, its impact acts on the vessel with some force. This power is short-lived.
Another example. Let's throw a ball at a sheet of cardboard, the ball will bounce, and the cardboard will deviate a little. Let's replace the ball with sand. The impacts will be tiny, we won't even hear them, but their power will build up. The sheet will constantly be rejected.
Now let's take the smallest particles, for example air particles that we have in our lungs. We blow on the cardboard, and it will deviate. We forceair molecules hit the cardboard, as a result, a force acts on it. What is this power? This is the force of pressure.
Let's conclude: gas pressure is caused by impacts 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, it is in the gas, that is, in the air. Because the impacts of air molecules on one and the other side of the sheet balance each other. How to check if air molecules really hit the wall? This can be done by removing the impacts of molecules on one side, for example, by pumping out air.
Experiment
There is a special device - a vacuum pump. This is a glass jar 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 to each other. A manometer is attached to the vacuum unit, which measures the difference in air pressure outside and under the hood. The faucet allows the hose leading to the pump to be connected to the space under the hood.
Place a slightly inflated balloon under the cap. Due to the fact that it is slightly inflated, the impacts 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. What about a balloon? It increases in size. Pressure, that is, impacts of moleculesoutside the ball, getting smaller. Air particles inside the ball remain, the compensation of shocks from the outside and from the inside is violated. The volume of the ball grows due to the fact that the force of pressure of the air molecules from the outside is partially taken over by the elastic force of the rubber.
Now close the faucet, turn off the pump, open the faucet again, disconnect the hose to let air under the cap. The ball will begin to shrink in size. When the pressure difference outside and under the cap is zero, it will be the same size as it was before the start of the experiment. This experience proves that you can see the pressure with your own eyes if it is greater on one side than on the other, i.e. if the gas is removed from one side and left on the other.
The conclusion is this: pressure is a quantity that is determined by the impacts of molecules, but the impacts can be more numerous and less numerous. The more hits 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.
Dependence of pressure on volume
Let's say we have a certain mass of the eye, that is, a certain number of molecules. In the course of the experiments that we will consider, this quantity does not change. The gas is in a cylinder with a piston. The piston can be moved up and down. The upper part of the cylinder is open, we will put an elastic rubber film on it. The gas particles 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 hits will increase because the molecule has to travel a shorter distance to reach the wall. As a result, the pressure should increase, and the film should bend outward. Therefore, with a decrease in volume, the pressure of a gas increases, but this is provided that the mass of the gas and the temperature remain unchanged.
If you move the piston down, the distance between the molecules will increase, which means that the time it will take them to reach the walls of the cylinder and the film will also increase. Hits will become rarer. The gas outside has a higher pressure than the one inside the cylinder. Therefore, the film will bend inwards. Conclusion: pressure is a quantity that depends on volume.
Dependence of pressure on temperature
Suppose we have a vessel with a gas at a low temperature and there is a vessel with the same gas in the same quantity at a high temperature. At any temperature, the pressure of a gas is due to the impacts of molecules. The number of gas molecules in both vessels is the same. The volume is the same, so the distance between the molecules remains the same.
As the temperature rises, the particles begin to move faster. Consequently, the number and strength of their impacts on the walls of the vessel increases.
The following experiment helps to verify the correctness of the statement that as the temperature of a gas increases, its pressure increases.
Takebottle, the neck of which is closed 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 cold and place a bottle there, the balloon will deflate and even be pulled in.
