Today we will try to find the answer to the question “Heat transfer - is this? ..”. In the article, we will consider what the process is, what types of it exist in nature, and also find out what is the relationship between heat transfer and thermodynamics.
Definition
Heat transfer is a physical process, the essence of which is the transfer of thermal energy. Exchange occurs between two bodies or their system. In this case, the transfer of heat from more heated bodies to less heated ones will be a prerequisite.
Process features
Heat transfer is the same type of phenomenon that can occur both in direct contact and in the presence of dividing partitions. In the first case, everything is clear; in the second, bodies, materials, and media can be used as barriers. Heat transfer will occur in cases where a system of two or more bodies is not in a state of thermal equilibrium. That is, one of the objects has a higher or lower temperature compared to the other. Then there is the transfer of thermal energy. It is logical to assume that it will end when the system comes into a state of thermodynamic or thermal equilibrium. The process occurs spontaneously, as the second law of thermodynamics can tell us about .
Kinds
Heat transfer is a process that can be divided into three ways. They will have a basic nature, since within them one can distinguish real subcategories having their own characteristic features along with general laws. To date, it is customary to distinguish three types of heat transfer. These are thermal conductivity, convection and radiation. Let's start with the first, perhaps.
Heat transfer methods . Thermal conductivity.
This is the name of the property of a material body to carry out energy transfer. At the same time, it is transferred from the warmer part to the colder one. The basis of this phenomenon is the principle of random motion of molecules. This is the so-called Brownian motion. The higher the temperature of the body, the more actively the molecules move in it, since they have greater kinetic energy. In the process of heat conduction, electrons, molecules, atoms participate. It is carried out in bodies, different parts of which have different temperatures.
If a substance is able to conduct heat, we can talk about the presence of a quantitative characteristic. In this case, its role is played by the coefficient of thermal conductivity. This characteristic shows how much heat passes through unit indicators of length and area per unit time. In this case, the body temperature will change exactly by 1 K.
It was previously believed that heat exchange in various bodies (including heat transfer of building envelopes) is due to the fact that the so-called calorific flow flows from one part of the body to another. However, no one ever found signs of its real existence, and when the molecular kinetic theory developed to a certain level, everyone forgot to think about the calorific value, since the hypothesis turned out to be untenable.
Convection. Heat transfer water
Under this method of exchange of thermal energy refers to the transfer using internal flows. Let's imagine a kettle with water. As you know, more heated air currents rise up. A cold, heavier, fall down. So why should everything be different with water? Everything is exactly the same with her. And in the process of such a cycle, all layers of water, no matter how many there are, are heated until the onset of thermal equilibrium. Under certain conditions, of course.
Radiation
This method consists in the principle of electromagnetic radiation. It arises due to internal energy. We will not go into the theory of thermal radiation much, just note that the reason here is the arrangement of charged particles, atoms and molecules.
Simple Heat Conductivity Tasks
Now let's talk about how the calculation of heat transfer looks in practice. Let's solve a simple problem associated with the amount of heat. Suppose we have a mass of water equal to half a kilogram. The initial water temperature is 0 degrees Celsius, the final one is 100. We find the amount of heat spent by us to heat this mass of substance.
To do this, we need the formula Q = cm (t 2 -t 1 ), where Q is the amount of heat, c is the specific heat of water, m is the mass of the substance, t 1 is the initial, t 2 is the final temperature. For water, the value of c is tabular. The specific heat will be equal to 4200 J / kg * Ts. Now we substitute these values in the formula. We get that the amount of heat will be 210,000 J, or 210 kJ.
The first law of thermodynamics
Thermodynamics and heat transfer are interconnected by some laws. They are based on the knowledge that changes in internal energy within a system can be achieved using two methods. The first is the completion of mechanical work. The second is the message of a certain amount of heat. Incidentally, the first law of thermodynamics is based on this principle. Here is its formulation: if a certain amount of heat was communicated to the system, it will be spent on doing work on external bodies or on incrementing its internal energy. Mathematical notation: dQ = dU + dA.
Pros or cons?
Absolutely all quantities that are included in the mathematical notation of the first law of thermodynamics can be written with a plus sign or with a minus sign. Moreover, their choice will be dictated by the conditions of the process. Suppose a system receives a certain amount of heat. In this case, the bodies in it are heated. Consequently, gas expansion occurs, which means that work is being done. As a result, the values will be positive. If the amount of heat is taken away, the gas is cooled, work is done on it. Values will be inverse values.
An alternative formulation of the first law of thermodynamics
Suppose we have a certain intermittent engine. In it, the working fluid (or the system) perform a circular process. It is commonly called a cycle. As a result, the system will return to its original state. It would be logical to assume that in this case the change in internal energy would be equal to zero. It turns out that the amount of heat becomes equal to perfect work. These provisions allow us to formulate the first law of thermodynamics in a different way.
From it we can understand that in nature there cannot exist a perpetual motion machine of the first kind. That is, a device that does more work than the energy received from outside. In this case, actions must be performed periodically.
The first law of thermodynamics for isoprocesses
Let's start with an isochoric process. With it, the volume remains constant. So, the change in volume will be zero. Therefore, the work will also be equal to zero. We remove this term from the first law of thermodynamics, after which we obtain the formula dQ = dU. Therefore, in the isochoric process, all the heat supplied to the system is spent on increasing the internal energy of the gas or mixture.
Now let's talk about the isobaric process. A constant value in it remains pressure. In this case, the internal energy will change in parallel with the completion of work. Here is the original formula: dQ = dU + pdV. We can easily calculate the work being done. It will be equal to the expression uR (T 2 -T 1 ). By the way, this is the physical meaning of the universal gas constant. In the presence of one mole of gas and a temperature difference of one Kelvin, the universal gas constant will be equal to the work performed during the isobaric process.