The focus of our article is the amount of heat. We consider the concept of internal energy, which is transformed by changing this value. And also we will show some examples of the use of calculations in human activity.
With any word of the native language, each person has their own associations. They are determined by personal experience and irrational feelings. What does the word “warmth” usually represent? A soft blanket, a working central heating battery in the winter, the first sunlight in the spring, a cat. Or the look of the mother, the comforting word of a friend, the attention shown in time.
Physicists mean by this quite specific term. And very important, especially in some sections of this complex, but fascinating science.
Considering the amount of heat in isolation from the simplest processes that rely on the law of conservation of energy is not worth it - nothing will be clear. Therefore, to begin with, let us remind their readers.
Thermodynamics considers any thing or object as a compound of a very large number of elementary parts - atoms, ions, molecules. Its equations describe any change in the collective state of the system as a whole and as part of a whole when the macroparameters change. The latter are understood as temperature (denoted as T), pressure (P), concentration of components (as a rule, C).
Internal energy is a rather complicated term, in the sense of which it is worth sorting out before talking about the amount of heat. It denotes the energy that changes with increasing or decreasing value of the macroparameters of the object and does not depend on the reference system. It is part of the total energy. It coincides with it in conditions when the center of mass of the investigated thing is at rest (that is, there is no kinetic component).
When a person feels that a certain object (say, a bicycle) has become hot or cold, this indicates that all the molecules and atoms that make up this system have experienced a change in internal energy. However, the unchanged temperature does not mean the preservation of this indicator.
Work and warmth
The internal energy of any thermodynamic system can be transformed in two ways:
- by doing work on it;
- during heat exchange with the environment.
The formula for this process is:
dU = Q-А, where U is internal energy, Q is heat, and A is work.
Let the reader not be seduced by the simplicity of expression. The permutation shows that Q = dU + A, but the introduction of entropy (S) leads the formula to the form dQ = dSxT.
Since in this case the equation becomes a differential, then the first expression requires the same. Further, depending on the forces acting on the examined object and the parameter, which is calculated, displays the necessary ratio.
Take as an example of a thermodynamic system a metal ball. If you put pressure on him, throw him up, drop him in a deep well, then that means doing work on him. From the outside, all these harmless actions will not cause any harm to the ball, but its internal energy will change, albeit very slightly.
The second method is heat exchange. Now we come to the main goal of this article: the description of what is the amount of heat. This is the change in the internal energy of the thermodynamic system that occurs during heat transfer (see the formula above). It is measured in joules or calories. Obviously, if you hold the ball over a cigarette lighter, in the sun, or just in a warm hand, it will heat up. And then you can by the temperature change to find the amount of heat, which he was at the same time reported.
Why gas is the best example of changing the internal energy, and why because of this, students do not like physics
Above, we described changes in the thermodynamic parameters of a metal ball. Without special devices, they are not very noticeable, and the reader can only believe a word about the processes taking place with the object. Another thing, if the system - gas. Press down on it - it will be seen, heat up - pressure will rise, lower it under the ground - and this can be easily fixed. Therefore, in textbooks, it is often the gas that is most often taken as a visual thermodynamic system.
But, alas, in modern education not much attention is paid to real experiments. The scientist who writes the methodical manual perfectly understands what is at stake. It seems to him that with the example of gas molecules, all thermodynamic parameters will be properly demonstrated. But for a student who is just discovering this world, it’s boring to hear about the ideal flask with a theoretical piston. If real research laboratories existed in the school and hours were allocated for work, everything would be different. So far, unfortunately, the experiments are only on paper. And, most likely, this is what causes people to consider this section of physics as something purely theoretical, far from life and unnecessary.
An example of changing thermodynamic parameters
Therefore, we decided to cite the bicycle already mentioned above as an example. A man puts pressure on the pedals - makes work on them. In addition to telling the whole mechanism of torque (thanks to which the bicycle moves in space), the internal energy of the materials from which the levers are made changes. The cyclist clicks on the handles to turn - and again does the work.
The internal energy of the outer coating (plastic or metal) increases. A person goes to the clearing under the bright sun - the bike heats up, its amount of heat changes. Stops to rest in the shade of an old oak, and the system cools down, losing calories or joules. Increases speed - the exchange of energy grows. However, the calculation of the amount of heat in all these cases will show a very small, imperceptible value. Therefore, it seems that there are no manifestations of thermodynamic physics in real life.
Application of calculations to change the amount of heat
Probably, the reader will say that all this is very informative, but why are we so tormented at school with these formulas. And now we will give examples in which areas of human activity they are needed directly and how it applies to anyone in his everyday life.
First, look around you and count: how many metal objects surround you? Surely more than ten. But before you become a clip, car, ring or flash drive, any metal is smelted. Each plant that processes, say, iron ore, must understand how much fuel is needed to optimize costs. And counting on this, it is necessary to know the heat capacity of the metal-containing raw material and the amount of heat that it needs to communicate to all the technological processes. Since the energy released by a unit of fuel is calculated in joules or calories, the formulas are needed directly.
Or another example: in most supermarkets there is a department with frozen goods - fish, meat, fruit. Where raw materials from meat of animals or seafood turns into a semi-finished product, they should know how much electricity refrigerators and freezers per ton or unit of finished product will consume. To do this, calculate how much heat loses a kilogram of strawberries or squid when cooled by one degree Celsius. But in the end it will show how much electricity the freezer spends of a certain power.
Airplanes, steamboats, trains
Above, we have shown examples of relatively fixed, static objects that are reported or from which, on the contrary, take a certain amount of heat. For objects in the process of moving in conditions of constantly changing temperature, calculations of the amount of heat are important for another reason.
There is such a thing as “metal fatigue”. It also includes the maximum permissible loads at a certain rate of temperature change. Imagine a plane taking off from the humid tropics to the frozen upper layers of the atmosphere. Engineers have to work a lot so that it does not fall apart due to cracks in the metal that appear when the temperature drops. They are looking for a composition of the alloy that can withstand real loads and will have a large margin of safety. And in order not to search blindly, hoping to accidentally stumble upon the desired composition, one has to do many calculations, including including changes in the amount of heat.