Liquids and gases

1997/03/01 Bandres Unanue, Luis Iturria: Elhuyar aldizkaria

• Fisika

Well, in this article we have to question that “wisdom”, because from the hydraulic point of view we can not say so much of architecture.

Why were these great aqueducts made? Of course, for the transport of water, but was it not easier – as is done today, to introduce and pass under the ground some tubes? Of course! But the engineers of this time did not know the law of communicated containers, that is, they did not know that in a container with two entirely long tubes the water levels of both pipes are always the same. However, at this time it was thought that if the pipes were placed on the ground, the water drops, but as there are many peaks, for example, the water could not be driven up. Therefore, the water should always be conducted on a slope; for example, to transport the water from the reservoirs in the mountains, at all points of the route, buildings with low slopes had to be installed.

Push up?

If we say that liquids put pressure on the bottom and walls inside a container, those who do not know the rays of physics will also say so. However, can a liquid raise pressure? The answer, as the reader can perceive, is yes, if not, why ask?

But you have to check the answer given and for this we propose a simple attempt. Take a glass tube wide enough. Cut a circular piece of hard cardboard slightly larger than the diameter of the tube.

As you can see in figure 1, once closed one end of the tube with the circular piece, we put it in a container with water. So that the piece does not fall, let's pass through the center a small coil for the grip. If we dive into the tube, we will see that the circular piece does not sink; without pulling the thread, we will see that the piece has remained or has stuck to the tube. Witchcraft? No, of course, water is what pushes the piece from bottom to top.

Below we can measure the rising pressure described. For this, we will pay special attention to the water of the tube, as indicated by the figure. When the water level inside the tube equals the water inside the container, the circular piece falls. In other words, the pressure from the bottom to the top exerted by the external water on the part is equal to that from top to bottom inside the tube: if the heights are equal, the pressures are also equal. Surprising? No, it is more than a law of pressure that appears on the submerged bodies. And pulling this thread we will quickly come to understand the weight loss of submerged bodies, which is known as the “Archimides principle”.

To see better and clearer what was said, we could do the same test we described above, but this time we can replace water with lighter liquid like alcohol in ink. So, and if we accept what has been said so far, the level of liquid inside the tube should be higher than that of the outside to prevent the circular piece from falling. And also, once done, you can easily calculate the density of the liquid inside the tube. Do you dare, reader?

If in these trials we used tubes in different ways, on the other hand, it would be relatively easy to accept that another aspect of the pressure law of physics, that is, that the pressure on bodies submerged in liquids has to do with the depth, not with the shape of the tube or the width of its mouth.

But, of course, the reader will ask us to verify it (Figure 2). To do this, we will go back and perform the same test that we have described with different tubes, so that we dive to the same depth (for this, the best option is to place paper marks at the same height on each tube).

There is no doubt that, in all cases, when the liquid from the tubes reaches the same height, the circular piece falls. Although the shapes of the tubes differ from the width of the mouths, the height is the pressure factor.

However, we must make a small warning so that the experiment is truly valuable. As mentioned above, the factor that influences the pressure is the height, not the length, so if instead of putting vertically the tube we do it in a transversal way we should add more liquid to the interior to reach that first level. Understand? the preiso on the part has nothing to do with the amount of liquid inside the tube, but with its height.

What has the most weight?

It is not any question. In one of the plates of the balance we will place a tresca full of water up to the top, in the other an identical tresca and also full of water that floats the piece of wood. What does it weigh more?

If you ask a single question to several readers, you will receive different answers. Some will answer that wooden clothes have more weight, as in addition to taking into account the weight of water, also wood. Others will lean down a path with only water, arguing that water weighs more than wood.

What is the correct answer? Both weigh the same. It is true that the second tresca has water and wood, but we have said that the piece of wood is floating and, therefore, the weight of the amount of water that has expelled the piece is equal to that of itself, we have not invented it, that is what Archimedes has said. So the balance will be in balance, it has to be!

Before we finish, we would like to pose a new problem. For us to place on a plate of a balance a container with water but not completely full and one of those small weights that are used to measure it, on the other, to balance the balance, we will place only weights. Once balanced, pour the pisito into the container of the first dish. Will the balance remain balanced?

Due to the principle of Archimedes, to “weigh” less than on the outside, the small weight inside the container, we could think that the dish will rise. However, we will see that the balance will remain in balance. How can this be understood?

The pisito, by blowing in the container, drives part of the water and, therefore, increases the level of water that had previously in the container. Therefore, this small piece will cause lower downforce, but due to the higher water level, it will generate greater pressure on the bottom of the container. These two adverse effects are compensated and, consequently, the balance remains in balance. Of course, we can give a simpler explanation: we have not touched everything we have put on two plates, we have simply changed place, so if at first it was in balance, why will it change later?