The amount of heat: concept, calculations, application
The focus of our article is the amount of heat. We will consider the concept of internal energy, which is transformed when this value changes. We will also show some examples of the application of calculations in human activity.
Heat
Each person has their own associations with any word in their native language. They are determined by personal experience and irrational feelings. What is usually represented by the word "warmth"? Soft blanket, running central heating battery in winter, first sunshine in spring, cat. Or a mother's glance, a comforting word from a friend, timely attention.
Physicists mean by this a very specific term. And very important, especially in some areas of this complex but fascinating science.
Thermodynamics
It is not worth considering the amount of heat in isolation from the simplest processes on which the law of conservation of energy is based - nothing will be clear. Therefore, to begin with, let us remind their readers.
Thermodynamics considers any thing or object as a combination 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 a part of the 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
Internal energy is a rather complex term, in the sense of which it is worth understanding before talking about the amount of heat. It denotes the energy that changes with an increase or decrease in the value of the object's macroparameters and does not depend on the frame of reference. It is part of the total energy. It coincides with it in conditions when the center of mass of the thing under study is at rest (that is, there is no kinetic component).
When a person feels that some object (say, a bicycle) has warmed up or cooled down, this shows that all the molecules and atoms that make up this system have experienced a change in internal energy. However, the invariability of the 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 looks like this:
dU = Q-A, where U is internal energy, Q is heat, A is work.
Let the reader not be deceived by the simplicity of expression. The permutation shows that Q = dU + A, but the introduction of the entropy (S) brings the formula to the form dQ = dSxT.
Since in this case the equation takes the form of a differential, the first expression requires the same. Further, depending on the forces acting in the investigated object, and the parameter that is calculated, the required ratio is derived.
Take a metal ball as an example of a thermodynamic system. If you press on it, throw it up, drop it into a deep well, then this means doing work on it. Outwardly, all these harmless actions will not cause any harm to the ball, but its internal energy will change, albeit very slightly.
The second way is heat exchange. Now we come to the main goal of this article: a description of what the amount of heat is. This is such a change in the internal energy of a thermodynamic system that occurs during heat exchange (see the formula above). It is measured in joules or calories. Obviously, if you hold the ball over a lighter, in the sun, or just in a warm hand, it will heat up. And then, by changing the temperature, you can find the amount of heat that was communicated to him at the same time.
Why gas is the best example of a change in internal energy, and why, because of this, schoolchildren do not like physics
Above, we described changes in the thermodynamic parameters of a metal ball. They are not very noticeable without special devices, and the reader can only take a word about the processes taking place with the object. It's another matter if the system is gas. Press on it - it will be visible, heat it up - the pressure will rise, lower it under the ground - and this can be easily fixed. Therefore, in textbooks, it is gas that is most often taken as a visual thermodynamic system.
But, alas, in modern education not much attention is paid to real experiences. The scientist who writes the methodological manual perfectly understands what is at stake. It seems to him that by the example of gas molecules, all thermodynamic parameters will be properly demonstrated. But a student who is just discovering this world is bored with hearing about an ideal flask with a theoretical piston. If there were real research laboratories in the school and hours were allocated to work in them, everything would be different. So far, unfortunately, the experiments are only on paper. And, most likely, this is the reason that people consider this branch of physics to be something purely theoretical, far from life and unnecessary.
Therefore, we decided to cite the bicycle already mentioned above as an example. A person presses on the pedals - he does work on them. In addition to imparting torque to the entire mechanism (thanks to which the bicycle moves in space), the internal energy of the materials from which the levers are made changes. The cyclist presses the handles to turn, and again does the job.
The internal energy of the outer coating (plastic or metal) increases. A person drives out into a clearing under the bright sun - the bike heats up, its amount of heat changes. Stops to rest in the shade of an old oak tree and the system cools down, losing calories or joules. Increases speed - energy exchange increases. 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 for the change in 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 of in which areas of human activity they are needed directly and how this applies to anyone in his everyday life.
First, look around you and count: how many metal objects surround you? Probably more than ten. But before it becomes a paper clip, wagon, ring or flash drive, any metal is smelted. Every mill that processes, say, iron ore must understand how much fuel is required in order to optimize costs. And when calculating this, it is necessary to know the heat capacity of the metal-containing raw material and the amount of heat that must be reported to it in order for all technological processes to occur. 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, fruits. Where raw materials from animal meat or seafood are converted into a semi-finished product, you should know how much electricity refrigerators and freezers will use per ton or unit of finished product. To do this, calculate how much heat a kilogram of strawberries or squid loses when cooled by one degree Celsius. And in the end, this will show how much electricity a freezer of a certain power will spend.
Airplanes, steamers, trains
Above, we showed examples of relatively motionless, static objects that are reported or from which, on the contrary, a certain amount of heat is taken away. For objects moving in the process of operation under 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 into the frozen upper atmosphere. Engineers have to work hard to keep it from falling apart due to cracks in the metal that appear when the temperature drops. They are looking for an alloy composition 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, you have to do a lot of calculations, including those involving changes in the amount of heat.
