Paper pot

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

• Fisika

But in the field of science, and physics is only a branch of science, there is only one way to accept or reject propositions, that is, experimentation. Therefore, taking a solid parchment paper, we will make a kind of cucurucho and place it on a wire stand as shown in the picture.

After pouring water and egg, we will place it on a lamp. The fire will not damage the paper. How is it possible? In the open pot water cannot exceed one hundred degrees. When the water is heated it will reach 100 degrees and will remain at that temperature while the heat is curled and boiled, so the paper will also be a hundred degrees and will not be lit. Note: it is more convenient to use a cardboard box instead of paper.

A test in this matter, which often happens involuntarily, is that if we start heating a coffee maker with a series of welds and forget to put water, the welds melt and the coffee machine breaks. This is easy to understand, the welding metal is of those that melts with relative ease and since there is no water for heat sorption, the heat generated is only to heat the pot and the temperature is increasing. In this sense, the old Maxim machine guns required water so that the heat of the shooting did not melt the weapons.

To finish with these essays take a thick and a little large nail or a copper stick and a thin paper tape, which is placed in the flames, the paper is staggered or charred, but it does not burn, at least until the stick is put up.

The cause is the thermal conductivity of the metal. If instead of performing this test with a metal wedge with a glass wedge it would burn the paper immediately.

Ice lubrication

As we all know, in a well-grooved soil it slips more easily than in an unwaxed soil. The same should happen with ice, that is, when it is soft it would be more slippery than when it is rough and rough.

However, the northern inhabitants know that carrying a trench on icy soil, easier than carrying it on soft soil, is more slippery than rough ice! How can this be understood?

The main cause of the slippery ice is not its softness or roughness, but the decrease of its melting temperature with pressure.

Thus, when we stand on the skates all the weight is on a very small surface, so the pressure that supports the ice is very high. However, when the pressure is high, the melting point of the ice decreases, so that, for example, if the ice temperature is -3 °C, the melting point of the ice under the skates decreases by about 5 °C, so it melts. For this reason, a thin layer of water will appear between the blades of the skates. The same happens when your feet move. Therefore, the slide does not slide on the ice but on a layer of water. Ice is the only body with this peculiarity, so a physicist could say that “ice is the only sliding body of nature”, the rest will be smooth but not slippery.

So what does it matter if the ice is soft or toothed? As we know, the pressure exerted by a weight is higher the lower the retention surface. When will a higher pressure occur when the ice is soft or rough? Undoubtedly, in the second case, the weight will be on a few points of the ground. The higher the pressure the higher the melting point and the more slippery soil.

Candalos

Who hasn't seen the long ice cubes suspended from the rooftops on a very cold morning?

But when are these candalos made? In frosts or thaw? If it is thawing, how does water freeze above zero degrees? On the other hand, during ice, how is water produced?

What happens is that the sun ray or heat of the house makes the roof snow rise more than zero degrees and melts to the edge of the roof, but here the temperature is low from zero degrees and the water freezes again.

Leaving aside the heat inside the house, let's look at the image above.

We are on a clear day and the air temperature is -1°C. The Sun spreads its rays everywhere, but those that reach the ground are so transversally that they do not give enough heat to melt the snow. On the contrary, those who touch on the roof are placed with a greater inclination, that is, closer to the value of the right angle.

As is known, the light or heat provided by the rays is greater as the angle formed by these rays and the plane of attack increases. The capacity of the rays is directly proportional to the sine of this angle; in the example that appears in the figure, the snow on the roof takes 2.5 times more heat than the soil (sine 60° = 2.5 x sine 20°). Therefore, while the snow melts from the roof, the same does not happen with that of the ground. The water from the roof is poured and the drops reach the edge of the roof, cool through the environment and evaporation and freeze, being suspended. On the first drop the second falls and little by little the candalo becomes.

By the angle of the rays there are other surprising phenomena such as the climatic differences from one place to another or the different seasons of the year in one place. The sun, in principle, is at the same distance from us in summer or in winter, even from the poles or the equator (the difference is so small that you can stop counting). However, since the inclination of the rays on the earth's surface is greater in the equator than in the poles, the temperature differences between these two places are very high, even between summer and winter. These differences are due to the change of temperature and the richness of nature.