الديناميكا الحرارية - الحرارة .. الفيزياء

Thermodynamics - Heat .. Physics

Thermodynamics

Thermodynamics is the study of the laws of heat exchange and transformation. The first law of thermodynamics states that heat is a form of energy and therefore cannot be lost.

(Opposite page) When the work KJ caused by a falling body (top image) is converted into heat, the temperature of water rises by KJ F degrees Celsius (bottom image) The temperature scale is determined by fixed points, then the distance between them is divided into equal parts. (Left) A gas-powered thermometer used to measure small changes with great accuracy.

Adding an amount of heat K to a gas in a combustion chamber increases its internal energy from Q1 to Q2 and expands it from H1 to H2 at atmospheric pressure D.

Thermal energy is generated by the combustion of oxygen with acetylene on its own - but it must be converted into another form of energy. When an amount of heat 5 is added to a certain body and an amount of work is applied to it, this results in a change in its internal energy or thermodynamic energy Q according to the following equation: Q2 - Q1 = 5 + Q. A heat engine is nothing but a device to which an amount of heat is added, resulting in an amount of work being generated which is considered a negative amount in this case. If this process occurs at a constant temperature (K zero), it is called an adiabatic process, and the equation then becomes: Q2 - Q1 = Q.

It is usually difficult to know the absolute value of the internal energy Q, but it is easy to calculate its changes by measuring the quantities 5 and H. If we assume that the volume of the body has changed from H1 to H, the resulting work is equal to the quantity D (H2 - H1), where D represents the pressure of the medium surrounding the body itself. This quantity is positive if the body is given a quantity of work instead of taking it from it.

The quantity 5 is used, in the case of a chemical reaction, to express the heat added to the reaction or released from it. If we assume that H1 represents the initial heat content of the body, and H2 represents its final heat content, we have an equation of the type 5 = H2 - H1. The quantity H is called the total heat content of the body (enthalpy), and this results in the following equation: H2 - H1 = (Q2 - Q1) + D (H2 - H1).

It is worth noting that the change in heat content H2 - H1 is considered negative if the reaction is a source of heat (exothermic reaction) and positive if it absorbs heat (endothermic reaction). It has been generally agreed that the quantities of heat and work are considered positive if they enter the body and negative if they are emitted from it. When a substance changes from a liquid to a gaseous state, an adiabatic energy change occurs (i.e. a change in energy that is not accompanied by any change in temperature) usually accompanied by an expansion in the volume of the substance that leads to giving the surrounding medium a certain amount of work, and thus the equation becomes H G - H S = Q E - Q S + D (H G - H S) where the quantity H E - H S represents the heat content of the evaporation process (or the specific latent heat). For this reason, if we want to convert an amount of water equivalent to 2 kg from a temperature of zero C to steam at a temperature of one hundred C, two quantities of heat must be provided:
(1) The heat (which is equal to K N D) required to raise the temperature of the water from zero to one hundred and is equal here to 2 × 100 × N J (N is the specific heat capacity of water).
(2) The heat (which is equal to K (H G - H S) required to convert the water to steam at a constant temperature and is equal to 2 (H G - H S) ) Joel

A similar change in the amount of heat content is called the heat content of fusion H S - H S (i.e. the latent heat of fusion) and occurs when a solid body turns into a liquid body.

The mutual reactions between two gases present in two chambers with equal temperatures result.

If a certain body moves from a state of energy S to a state of energy P, the condition Q S - Q S = 5 + H remains applicable according to the first law of thermodynamics without taking into account the path of the body's transformation from S to P. However, all heat transfer processes are characterized by being subject to one unchanging pattern, which is that they are unable, during any stage of their transition from S to P, to move from one heat source to another source with a higher temperature without an external amount of work being applied to them - i.e. heat always flows from a hot body to a cold body.

This was the second law of thermodynamics.

Let us imagine a vessel isolated from the outside and composed of two chambers of equal volume containing oxygen, and let us assume that all the molecules present in the first chamber | They move at the same speed, let's say it is equal to 250 m/s. Since the temperature in units (Kelvin K) is directly proportional to the kinetic energy of the molecules and in our case is equal to 4.8 × 10 kinetic energy, it is possible to know it, and it is exactly 81 K.

As for the second chamber called B, where the molecules move in all directions and at different speeds with an average value of 300 m/s, the temperature is equal to 118 K. As a result, according to the second law of thermodynamics, heat will flow from chamber A to chamber B across the gap between them, which leads to an increase in the average speed of the molecules in the first chamber and in the number of collisions occurring between them, i.e. an increase in their irregularity (Figure 20 B).
This interpretation is another way of expressing the second law: Heat flows in the group always in the direction that leads to an increase in its irregularity.

The irregularity of the body is measured by a quantity called entropy. The increase in entropy B2 - B1 is defined as equal to the sum of the heat increases transferred at a certain degree according to the following equation: B2 - B1 = 5/d For example, when a liquid turns into a gas at its boiling point, the disorder of its molecules increases - that is, the value of entropy inside it increases - and the equation becomes as follows: B2 - B1 = (H G - H S) / D The quantity H G - H S represents the heat content of the vaporization of the body (see page 25) and represents its boiling point.

This tendency represented by the increase in the value of entropy in all natural processes means that the energy of the universe continues to lose its order more and more - that is, a decrease in its available quantity, and this ultimately leads to the disappearance of energy - that is, to the universe reaching a state of maximum entropy, a state sometimes called the heat death of the universe.

At the opposite end of the entropy scale is the temperature of absolute zero. It represents the temperature at which all atoms and molecules of matter stop vibrating and moving, all collisions between them cease, and the system becomes completely in a state of zero entropy. This is the third law of thermodynamics, which states as follows: The value of entropy in a certain group is zero if its temperature reaches absolute zero.