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 with the help of 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 the air from the vessel using a vacuum pump. Interestingly, as pumping progresses, the sound of the pump changes. The less air remains 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 is removed,close the faucet, pinch the hose to block the air supply. Weigh the flask without air, then open the tap. The air will enter with a characteristic whistle, and its weight will be added to the weight of the flask.
First place an empty vessel with a closed tap on the balance. There is a vacuum inside the container, let's weigh it. Let's open the tap, the air will go inside, and weigh the contents of the flask again. The difference between the weight of the filled and empty flask will be the mass of air. It's simple.
Air weight and atmospheric pressure
Now let's move on to solving the next 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 marked on the side of the flask. ρ=mair /V. I must say that to obtain the so-called high vacuum, that is, the complete absence of air in the vessel, you need a lot of time. If the flask is 1.2L, it's about half an hour.
We found out that air has mass. The earth pulls it, and therefore the force of gravity acts on it. The air pushes down on the ground with a force equal to the weight of the air. Atmospheric pressure, therefore, exists. It manifests itself in various experiments. Let's do one of these.
Syringe experiment
Take an empty syringe to which a flexible tube is attached. Lower the plunger of the syringe and immerse the hose in a container of water. Pull the plunger up, and the water will begin to rise through the tube, filling the syringe. Why does water, which is pulled down by gravity, still rise behind the piston?
In the vessel, it is affected from top to bottomAtmosphere pressure. Let's denote it Patm. According to Pascal's law, the pressure exerted by the atmosphere on the surface of a liquid is transmitted unchanged. It spreads to all points, which means that atmospheric pressure is also inside the tube, and there is a vacuum (airless space) in the syringe above the water layer, i.e. P=0. So it turns out that atmospheric pressure presses on the water from below, but there is no pressure above the piston, because there is emptiness there. Due to the pressure difference, water enters the syringe.
Experiment with mercury
Air weight and barometric pressure - how big are they? Maybe it's 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, let's modify the experiment we just conducted. Instead of a syringe, take a bottle closed with a cork with a tube. Connect the tube to the pump and start pumping air.
Unlike the previous experience, we create a vacuum not under the piston, but in the entire volume of the bottle. Turn off the pump and at the same time lower the tube of the bottle into a container of water. We will see how water filled the bottle through the tube in just a few seconds with a characteristic sound. The high speed with which she "burst" into the bottle indicates that atmospheric pressure is a rather large value. Experience proves it.
For the first time measured the atmospheric pressure, the weight of the air Italian scientist Torricelli. He had such an experience. I took a glass tube a little over 1 m long, sealed at one end. Filled it with mercury to the brim. AfterThen he took a vessel with mercury, pinched its open end with his finger, turned the tube over and immersed it in a container. If there was no atmospheric pressure, then all the mercury would have poured out, but this did not happen. It poured out partially, the mercury level settled at a height of 760 mm.
It happened because the atmosphere pressed on the mercury in the container. It is for this reason that in our previous experiments, water was driven into the tube, which is why water followed the syringe. But in these two experiments, we took water, the density of which is low. Mercury has a high density, so atmospheric pressure was able to 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 unchanged. This means that there is also atmospheric pressure inside the tube. But on the other hand, this pressure is balanced by the pressure of the liquid column. Let's denote the height of the mercury column as h. We can say that atmospheric pressure acts from bottom to top, and hydrostatic pressure acts from top to bottom. The remaining 240 mm is empty. By the way, this vacuum is also called the Torricelli void.
Formula and calculations
Atmospheric pressure Patm is equal to hydrostatic pressure and is calculated by the formula ρrtgh. ρrt=13600 kg/m3. g=9.8 N/kg. h=0.76 m. Patm=101.3 kPa. This is a fairly large amount. A sheet of paper lying on a table produces a pressure of 1 Pa, and atmospheric pressure is 100,000 pascals. It turns out that you need to put100,000 sheets of paper one on top of the other to produce such pressure. Curious, isn't it? Atmospheric pressure and air weight are very high, so water was pushed into the bottle with such force during the experiment.
