Pascal's law for liquids and gases states that pressure, propagating in a substance, does not change its strength and is transmitted in all directions equally. Liquid and gaseous substances behave under pressure with some differences. The difference is due to the behavior of particles and the weight of gases and liquids. In the article we will consider in detail all this with the help of visual experiments.
Is fluid pressure transmitted
Take a cylindrical vessel, which is sealed from above by a piston. There is fluid inside and a weight on the piston. She exerts pressure with a force equal to her weight. This pressure is transmitted to the fluid. Its molecules, unlike solid particles, can freely move relative to each other. There is no strict order in their arrangement; they are scattered randomly.
Knowledge of the peculiarities of the motion of particles of different substances in the future will help us understand Pascal's law for liquids and gases. How will the liquid molecules behave if we act on them with the force of pressure of a weight? Experience will help us answer this question.
How fluid behaves under pressure
The model of the liquid will be glass balls, and the model of the vessel will be a box without a lid. Balls, as well as particles of liquid matter, move freely in containers. Take any item matching the width of the box. It will simulate a piston.
Push the piston onto the fluid. How do its molecules behave? We see that they press both on the bottom of the tank and on its walls. They push each other and tend to fall out of the box. If it was a real liquid, then it would try to splash out of the vessel. Later, when we study Pascal's law for liquids and gases, we will see this in experience. Due to the fact that the molecules move freely, the pressure exerted by the weight is transmitted both to the sides and down. And what will happen if you replace the liquid with gas?
How air behaves under pressure
Let's say we have a cylinder with a piston filled with air. Put a load on top of the piston. How is gas pressure transmitted? When the piston moves down, the distance between the molecules in the upper part of the gas decreases, but this is not for long. The speed of gas molecules is hundreds of meters per second. The distance between them is much larger than their size. They move in random directions and collide with each other.
When the piston lowers, the particles simply lock in a smaller volume. As a result, they more often hit the walls of the vessel, and with a decrease in gas volume its pressure increases. This postulate must be remembered so that later it will be easier to understand Pascal's law for liquids and gases. The number of beats per second for every square centimeter is almost the same. This means that the pressure that the piston produces is transmitted in all directions without change.
Pressure transfer in different directions
Pascal’s law, the transmission of pressure by liquids and gases, cannot be understood unless one understands one oddity: how is it that we push down, and the pressure is transferred down and to the sides? But what if a tube is connected to the cylinder, will pressure be transmitted upward along it? Let's make an experiment.
Take two syringes filled with water and connect them with a tube. We observe how pressure will be transmitted by the fluid that is in the syringes. Press the piston of one syringe. The pressure on the piston, and therefore on the fluid, is directed downward. However, we see that the piston of the second syringe rises. It turns out that the pressure, transmitted through the tube, changes the direction of the force. Interestingly, syringes can be placed not only vertically, but also at right angles to each other. The result will be the same.
Pour out water and air will remain in the syringes. Repeat the experience. During the experiment, we will see that the gas also transmits pressure in all directions. There is only one difference with the liquid. If the piston of one syringe is lowered to the maximum and locked with a finger, then when the piston of another syringe is pressed, the gas will be compressed. Its volume will decrease approximately two times, and the piston will strive to bounce up. This gas, which seeks to increase its volume, causes the piston to move up. It would have been different with the liquid; it would not have been possible to compress it so easily.
Pascal's Law
The transmission of pressure by liquids and gases will be studied through experience. He was invented by the French physicist Blaise Pascal. Take a hollow ball to which a glass tube is attached. In different parts of the ball (top, side, bottom) there are small holes. A piston is placed inside the tube. This is a special device to demonstrate Pascal's law.
Fill the ball through the tube with water to see how it will behave. Although gravity acts on the ball from top to bottom, trickles of water flow from the holes of the ball at an angle, to the side and even up. Of course, they deviate slightly from their original direction, because gravity acts on them. We see that the pressure produced by the water is transmitted in all directions.
If instead of water we take smoke and do this experiment, we will observe the transfer of pressure in the gas with our own eyes, because smoke is a gas painted with small particles of soot or tar. Due to the fact that it is very light, gravity will not act on it so much, it will not deviate as much as trickles of water from its original position. We can draw the following conclusion: the pressure exerted on a liquid or gas is transmitted, without changing the force, to any point of the liquid and gas in all directions. This is Pascal's law for liquids and gases. Formula: P = F / S, where P is the pressure. It is equal to the ratio of the force F to the area S, on which it acts perpendicularly.