In physics, the concept of "heat" is associated with the processes of transfer of thermal energy between different bodies. Thanks to these processes, heating and cooling of bodies occurs, as well as a change in their state of aggregation. Let us consider in more detail the question of what is heat.
Concept concept
What is heat? Each person can answer this question from a household point of view, meaning by the concept under consideration the sensations that he has with an increase in the ambient temperature. In physics, this phenomenon is understood as the process of energy transfer associated with a change in the intensity of the random movement of molecules and atoms that form the body.
In the general case, we can say that the higher the temperature of the body, the more internal energy is stored in it, and the more heat it can give to other objects.
Heat and temperature
Knowing the answer to the question of what is heat, many might think that this concept is similar to the concept of "temperature", but it is not. Heat is kinetic energy, but temperature is a measure of this energy. So, the process of heat transfer depends on the mass of the substance, on the number of particles that make it up, as well as on the type of these particles and the average speed of their movement. In turn, the temperature depends only on the last of the listed parameters.
The difference between heat and temperature is easy to understand if you conduct a simple experiment: you need to pour water into two vessels so that one vessel is full and the other is only half full. By putting both vessels on fire, one can observe that the one in which there is less water will begin to boil. To boil the second vessel, he will need some more heat from the fire. When both vessels are boiling, their temperature can be measured, it will turn out to be the same (100 o C), but at the same time more heat was needed for a full vessel to boil water.
Heat units
According to the definition of heat in physics, one can guess that it is measured in the same units as energy or work, that is, in joules (J). In addition to the basic unit of measurement of heat, in everyday life you can often hear about calories (kcal). This concept refers to the amount of heat that needs to be transferred to one gram of water so that its temperature rises by 1 kelvin (K). One calorie is 4.184 J. You can also hear about large and small calories, which are 1 kcal and 1 cal, respectively.
The concept of heat capacity
Knowing what heat is, we consider the physical quantity that directly characterizes it - heat capacity. In physics, this concept means the amount of heat that must be given to the body or taken away from it so that its temperature changes by 1 kelvin (K).
The heat capacity of a particular body depends on 2 main factors:
- from the chemical composition and state of aggregation in which the body is represented;
- from its mass.
In order to make this characteristic independent of the mass of the object, a value was introduced into the physics of heat - specific heat, which determines the amount of heat transferred or taken by a given body per 1 kg of its mass when the temperature changes by 1 K.
To illustrate the difference in specific heat capacities for different substances, for example, you can take 1 g of water, 1 g of iron and 1 g of sunflower oil and heat them. The fastest temperature will change for an iron sample, then for a drop of oil, and last but not least for water.
Note that the specific heat depends not only on the chemical composition of the substance, but also on its state of aggregation, as well as on the external physical conditions under which it is considered (constant pressure or constant volume).
The main equation for the heat transfer process
Having dealt with the question of what heat is, one should give the basic mathematical expression, which is characterized by the process of its transfer for absolutely any bodies in any aggregate states. This expression has the form: Q = c * m * ฮT, where Q is the amount of transferred (received) heat, c is the specific heat of the object in question, m is its mass, ฮT is the change in absolute temperature, which is defined as the difference in body temperatures at the end and at the beginning of the heat transfer process.
It is important to understand that the above formula will always be valid when, during the process under consideration, an object maintains its state of aggregation, that is, it remains a liquid, solid or gas. Otherwise, the equation cannot be used.
Change in the state of aggregation of a substance
As you know, there are 3 main aggregate states in which matter can be:
In order for a transition from one state to another to occur, it is necessary for the body to communicate or take away heat from it. For such processes, the concepts of specific heats of melting (crystallization) and boiling (condensation) were introduced in physics. All these values โโdetermine the amount of heat necessary to change the state of aggregation, which releases or absorbs 1 kg of body weight. For these processes, the equation holds: Q = L * m, where L is the specific heat of the corresponding transition between states of matter.
Below are the main features of the processes of changing the state of aggregation:
- These processes occur at a constant temperature, for example, boiling or melting points.
- They are reversible. For example, the amount of heat that a given body has absorbed in order to melt will be exactly equal to the amount of heat that is released into the environment if this body goes back to solid state.
Thermal equilibrium
This is another important issue related to the concept of โheatโ that needs to be considered. If two bodies with different temperatures are brought into contact, then after a while the temperature in the whole system will equalize and become the same. To achieve thermal equilibrium, a body with a higher temperature must transfer heat to the system, and a body with a lower temperature must accept this heat. The laws of heat physics describing this process can be expressed as a combination of the main heat transfer equation and the equation that determines the change in the state of aggregation of a substance (if any).
A striking example of the process of spontaneous establishment of thermal equilibrium is a hot iron bar, which is thrown into the water. In this case, hot iron will give off heat to the water until its temperature becomes equal to the temperature of the liquid.
The main methods of heat transfer
All processes known to man that go with the exchange of thermal energy occur in three different ways:
- Thermal conductivity. In order for heat transfer to occur in this way, contact of two bodies with different temperatures is necessary. In the contact zone at the local molecular level, kinetic energy is transferred from a hot body to a cold one. The rate of this heat transfer depends on the ability of the participating bodies to conduct heat. A striking example of thermal conductivity is a person touching a metal rod.
- Convection. This process requires the movement of matter, so it is observed only in liquids and gases. The essence of convection is as follows: when the gas or liquid layers are heated, their density decreases, so they tend to rise up. During their rise in the volume of liquid or gas, they transfer heat. An example of convection is the process of boiling water in a kettle.
- Radiation. This process of heat transfer occurs due to the emission by a heated body of electromagnetic radiation of various frequencies. Sunlight is a prime example of radiation.