From the very concept of "atmospheric pressure" it follows that air must have weight, otherwise it could not put pressure on anything. But we do not notice this, it seems to us that the air is weightless. Before talking about atmospheric pressure, you need to prove that air has weight, you need to somehow weigh it. How to do it? We will consider air weight and atmospheric pressure in detail in the article, studying them using experiments.
Experience
We will weigh the air in a glass vessel. It enters the container through a rubber tube in the neck. The valve closes the hose so that no air enters it. We remove air from the vessel using a vacuum pump. Interestingly, as the pump is pumped out, the sound of the pump changes. The smaller the volume of air remaining in the flask, the quieter the pump runs. The longer we pump out the air, the lower the pressure in the vessel becomes.
When all the air has been removed, close the tap, pinch the hose to block air access. Weigh the flask without air, then open the tap. The air will go inside with a characteristic whistle, and its weight will be added to the weight of the bulb.
First, place an empty vessel with a closed tap on the scale. There is a vacuum inside the container, weigh it. We open the tap, the air goes inside, and again weighed the contents of the flask. The difference in weight of the filled and empty flasks will be the mass of air. Everything is simple.
Air weight and atmospheric pressure
Now let's move on to solving the following problem. To calculate the density of air, you need to divide its mass by volume. The volume of the flask is known because it is indicated on its wall. ฯ = m air / V. I must say that to obtain the so-called high vacuum, that is, the complete absence of air in the vessel, it takes a lot of time. If the flask is 1.2 l, it is about half an hour.
We found that air has a mass. Earth attracts him, and therefore gravity acts on him. Air presses the earth with a force equal to the weight of the air. Atmospheric pressure therefore exists. It manifests itself in various experiments. We will carry out one of these.
Syringe experiment
Take an empty syringe to which a flexible tube is attached. Lower the syringe plunger and immerse the hose in a container of water. Pull the piston up and the water will begin to rise through the tube, filling the syringe. Why, then, the water, which gravity pulls down, still rises behind the piston up?
In a vessel, atmospheric pressure acts on it from top to bottom. Denote it by P atm . According to Pascal's law, the pressure that the atmosphere produces on the surface of a liquid is transmitted unchanged. It spreads to all points, which means that there is also atmospheric pressure inside the tube, and in the syringe above the water layer there is a vacuum (airless space), i.e. P = 0. So it turns out that atmospheric pressure presses on the water below, but there is no pressure above the piston, because there is a void. Due to the pressure difference, water enters the syringe.
Experience with mercury
Air weight and atmospheric pressure - how big are they? Maybe this is something that can be neglected? After all, one cubic meter of iron has a mass of 7600 kg, and one cubic meter of air - only 1.3 kg. To understand, we will modify the experiment just carried out. Instead of a syringe, take a bottle closed by a cork with a tube. Connect the tube to the pump and start pumping air.
Unlike previous experience, we create a vacuum not under the piston, but in the entire volume of the bottle. Turn off the pump and simultaneously lower the bottle tube into a container of water. We will see how the water literally in a few seconds with a characteristic sound filled the bottle through the tube. The high speed with which it "burst" into the bottle, suggests that atmospheric pressure is a rather large value. Experience proves this.
First measured the atmospheric pressure, the weight of the air Italian scientist Torricelli. He had such an experience. I took a glass tube a little more than 1 m long, sealed at one end. Filled it with mercury to the brim. After that, he took the jar of mercury, pressed his open end with his finger, turned the tube over and immersed it in a container. If there were no atmospheric pressure, then all the mercury would have spilled, but this did not happen. It partially poured out; the level of mercury was established at a height of 760 mm.
This happened because the atmosphere pressed on the mercury in the tank. It is for this reason that in our previous experiments, water was driven into a tube, which is why water followed the syringe. But in these two experiments, we took water, whose density is low. Mercury has a high density, so atmospheric pressure could raise mercury, but not to the very top, but only by 760 mm.
According to Pascal's law, the pressure produced on mercury is transmitted to all its points in an unchanged form. So, inside the tube is also atmospheric pressure. But on the other hand, this pressure is balanced by the pressure of the liquid column. Denote the height of the mercury column h. We can say that atmospheric pressure acts on mercury from bottom to top, and hydrostatic pressure acts from top to bottom. There is a vacuum in the remaining unfilled 240 mm. By the way, this vacuum is also called the Torricellian void.
Formula and calculations
Atmospheric pressure P atm is hydrostatic and is calculated by the formula ฯ rt * g * h. ฯ rt = 13600 kg / m 3 . g = 9.8 N / kg. h = 0.76 m. P atm = 101.3 kPa. This is a fairly large value. A sheet of paper lying on the table produces a pressure of 1 Pa, and atmospheric pressure - 100 thousand pascals. It turns out that you need to put one on another 100 thousand sheets of paper so that they produce such pressure. Curious, isn't it? Atmospheric pressure and air weight are very large, so they pushed water into the bottle with such force during the experiment.