In physics, the concept of "heat" is associated with the transfer of thermal energy between different bodies. Due to these processes, heating and cooling of bodies, as well as a change in their states of aggregation, occurs. Let us consider in more detail the question of what is heat.
Concept concept
What is heat? Each person can answer this question from an everyday point of view, meaning under the concept under consideration the sensations that he has when the ambient temperature rises. In physics, this phenomenon is understood as the process of energy transfer associated with a change in the intensity of the chaotic movement of molecules and atoms that form the body.
In general, we can say that the higher the body temperature, 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 may think that this concept is similar to the concept of "temperature", but it is not. Heat is kinetic energy, temperature is a measure of thisenergy. 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 filled. Putting both vessels on the fire, one can observe that the one in which there is less water begins to boil first. In order for the second vessel to boil, it will need some more heat from the fire. When both vessels are boiling, you can measure their temperature, it will be the same (100 oC), but more heat was needed for a full vessel to boil water in it.
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 main unit of heat, in everyday life you can often hear about calories (kcal). This concept is understood as 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 equal to 4.184 J. You can also hear about big and small calories, which are 1 kcal and 1 cal, respectively.
The concept of heat capacity
Knowing what heat is, let's consider a physical quantity that directly characterizes it - heat capacity. Under this concept,physics mean the amount of heat that must be given to or taken from a body in order for its temperature to change by 1 kelvin (K).
The heat capacity of a particular body depends on 2 main factors:
- on the chemical composition and state of aggregation in which the body is presented;
- of his mass.
To make this characteristic independent of the mass of an object, in the physics of heat another quantity was introduced - the specific heat capacity, which determines the amount of heat transferred or taken in by a given body per 1 kg of its mass when the temperature changes by 1 K.
To clearly show the difference in specific heat capacities for different substances, for example, take 1 g of water, 1 g of iron and 1 g of sunflower oil and heat them. The temperature will change fastest for the iron sample, then for the oil drop, and last for the water.
Note that the specific heat capacity 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 of the heat transfer process
Having de alt with the question of what heat is, one should give the main mathematical expression that characterizes the process of its transfer for absolutely any bodies in any state of aggregation. This expression has the form: Q=cmΔT, where Q is the amount of transferred (received) heat, c is the specific heat of the object in question, m -its mass, ΔT 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, the object retains 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 matter
As you know, there are 3 main aggregate states in which matter can be:
- gas;
- liquid;
- solid body.
In order for a transition from one state to another to occur, it is necessary for the body to inform or take away heat from it. For such processes in physics, the concepts of specific heats of melting (crystallization) and boiling (condensation) were introduced. All these quantities determine the amount of heat required to change the state of aggregation, which releases or absorbs 1 kg of body weight. For these processes, the equation is valid: Q=Lm, where L is the specific heat of the corresponding transition between the states of matter.
Below are the main features of the processes of changing the state of aggregation:
- These processes take place at a constant temperature, such as boiling or melting.
- They are reversible. For example, the amount of heat that a given body absorbed in order to melt will be exactly equal to the amount of heat that will be released into the environment if this body passes againto a solid state.
Thermal equilibrium
This is another important issue related to the concept of "warmth" that needs to be addressed. If two bodies with different temperatures are brought into contact, then after a while the temperature in the entire system will even out and become the same. To achieve thermal equilibrium, a body with a higher temperature must give off heat to the system, and a body with a lower temperature must accept this heat. The laws of heat physics that describe this process can be expressed as a combination of the main heat transfer equation and the equation that determines the change in the aggregate state of matter (if any).
A striking example of the process of spontaneous establishment of thermal equilibrium is a red-hot iron bar that is thrown into the water. In this case, the hot iron will give off heat to the water until its temperature becomes equal to the temperature of the liquid.
Basic 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 exchange to occur in this way, contact between 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 bodies involved to conduct heat. A striking example of thermal conductivity ishuman 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 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 of electromagnetic radiation of various frequencies by a heated body. Sunlight is a prime example of radiation.
