Today we will try to find the answer to the question “Heat transfer is?..”. 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. The exchange takes place between two bodies or their system. In this case, a prerequisite will be the transfer of heat from more heated bodies to less heated ones.
Process Features
Heat transfer is the same type of phenomenon that can occur both with direct contact and with separating 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 consisting 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. This is where the transfer of heat energy takes place. It is logical to assume that it will end whenwhen the system reaches a state of thermodynamic or thermal equilibrium. The process occurs spontaneously, as the second law of thermodynamics can tell us.
Views
Heat transfer is a process that can be divided into three ways. They will have a basic nature, since within them real subcategories can be distinguished, having their own characteristic features along with general patterns. To date, it is customary to distinguish three types of heat transfer. These are conduction, convection and radiation. Let's start with the first, perhaps.
Methods of heat transfer. Thermal conductivity
This is the name of the property of a material body to carry out the transfer of energy. At the same time, it is transferred from the hotter part to the colder one. This phenomenon is based on the principle of chaotic 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 more kinetic energy. Electrons, molecules, atoms participate in the process of heat conduction. It is carried out in bodies, different parts of which have different temperatures.
If a substance is capable of conducting 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 will pass through unit indicators of length and area per unit of time. In this case, the body temperature will change exactly by 1 K.
Previously it was believed that heat exchange invarious bodies (including the heat transfer of enclosing structures) is due to the fact that the so-called caloric flows from one part of the body to another. However, no one found signs of its actual existence, and when the molecular-kinetic theory developed to a certain level, everyone forgot to think about caloric, since the hypothesis turned out to be untenable.
Convection. Water heat transfer
This method of heat energy exchange is understood as transfer by means of internal flows. Let's imagine a kettle of water. As you know, hotter air currents rise to the top. And cold, heavier ones sink down. So why should water be any different? It's exactly the same with her. And in the process of such a cycle, all layers of water, no matter how many there are, will heat up until a state of thermal equilibrium occurs. Under certain conditions, of course.
Radiation
This method is based on the principle of electromagnetic radiation. It comes from internal energy. We will not go into the theory of thermal radiation much, we will simply note that the reason here lies in the arrangement of charged particles, atoms and molecules.
Simple heat conduction problems
Now let's talk about how the calculation of heat transfer looks in practice. Let's solve a simple problem related to the amount of heat. Let's say we have a mass of water equal to half a kilogram. Initial water temperature - 0 degreesCelsius, final - 100. Let's find the amount of heat spent by us to heat this mass of matter.
For this we need the formula Q=cm(t2-t1), where Q is the amount of heat, c is the specific heat capacity of water, m is the mass of the substance, t1 is the initial temperature, t2 is the final temperature. For water, the value of c is tabular. The specific heat capacity will be equal to 4200 J / kgC. Now we substitute these values into the formula. We get that the amount of heat will be equal to 210000 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 in two ways. The first is mechanical work. The second is the communication of a certain amount of heat. By the way, the first law of thermodynamics is based on this principle. Here is its formulation: if a certain amount of heat was imparted to the system, it will be spent on doing work on external bodies or on increasing its internal energy. Mathematical notation: dQ=dU + dA.
Pros or cons?
Absolutely all the quantities that are included in the mathematical notation of the first law of thermodynamics can be written both with a “plus” sign and with a “minus” sign. Moreover, their choice will be dictated by the conditions of the process. Assume that the system receives some amount of heat. In this case, the bodies in it heat up. Therefore, there is an expansion of the gas, which means thatwork is being done. As a result, the values will be positive. If the amount of heat is taken away, the gas cools, and work is done on it. The values will be reversed.
Alternative formulation of the first law of thermodynamics
Suppose we have some intermittent engine. In it, the working body (or system) performs 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 will be equal to zero. It turns out that the amount of heat will be equal to the work done. These provisions allow us to formulate the first law of thermodynamics in a different way.
From it we can understand that a perpetual motion machine of the first kind cannot exist in nature. That is, a device that does work in a larger amount compared to the energy received from outside. In this case, actions must be performed periodically.
First law of thermodynamics for isoprocesses
Let's start with the isochoric process. It keeps the volume constant. This means that the change in volume will be zero. Therefore, the work will also be equal to zero. Let us discard this term from the first law of thermodynamics, after which we obtain the formula dQ=dU. This means that in an isochoric process, all the heat supplied to the system goes to increase the internal energy of the gas or mixture.
Now let's talk about the isobaric process. The pressure remains constant. In this case, the internal energy will change in parallel with the work. Here is the original formula: dQ=dU + pdV. We can easily calculate the work done. It will be equal to the expression uR(T2-T1). 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 done in an isobaric process.
