Heat capacity is the ability to absorb certain amounts of heat during heating or give off when cooled. The heat capacity of a body is the ratio of an infinitesimal amount of heat that a body receives to the corresponding increase in its temperature indicators. The value is measured in J/K. In practice, a slightly different value is used - specific heat.
Definition
What does specific heat mean? This is a quantity related to a single amount of a substance. Accordingly, the amount of a substance can be measured in cubic meters, kilograms, or even in moles. What does it depend on? In physics, the heat capacity depends directly on which quantitative unit it refers to, which means that they distinguish between molar, mass and volumetric heat capacity. In the construction industry, you won't see molar measurements, but you'll see others all the time.
What influences specific heat capacity?
What is heat capacity, you know, but what values affect the indicator is not yet clear. The value of specific heat capacity is directly affected by several components:substance temperature, pressure and other thermodynamic characteristics.
As the temperature of a product rises, its specific heat capacity increases, but certain substances exhibit a completely non-linear curve in this relationship. For example, with an increase in temperature indicators from zero to thirty-seven degrees, the specific heat capacity of water begins to decrease, and if the limit is between thirty-seven and one hundred degrees, then the indicator, on the contrary, will increase.
It is worth noting that the parameter also depends on how the thermodynamic characteristics of the product (pressure, volume, and so on) are allowed to change. For example, the specific heat at stable pressure and at stable volume will be different.
How to calculate the parameter?
Are you interested in what is the heat capacity? The calculation formula is as follows: C \u003d Q / (m ΔT). What are these values? Q is the amount of heat that the product receives when heated (or released by the product during cooling). m is the mass of the product, and ΔT is the difference between the final and initial temperatures of the product. Below is a table of the heat capacity of some materials.
What about the heat capacity calculation?
Calculating the heat capacity is not an easy task, especially if only thermodynamic methods are used, it is impossible to do it more precisely. Therefore, physicists use the methods of statistical physics or knowledge of the microstructure of products. How to calculate for gas? Heat capacity of gasis calculated from the calculation of the average energy of thermal motion of individual molecules in a substance. The movements of molecules can be of a translational and rotational type, and inside a molecule there can be a whole atom or vibration of atoms. Classical statistics says that for each degree of freedom of rotational and translational motions, there is a value in the molar heat capacity of the gas that is equal to R / 2, and for each vibrational degree of freedom the value is equal to R. This rule is also called the equipartition law.
At the same time, a particle of a monatomic gas differs by only three translational degrees of freedom, and therefore its heat capacity should be equal to 3R/2, which is in excellent agreement with experiment. Each diatomic gas molecule has three translational, two rotational and one vibrational degrees of freedom, which means that the equipartition law will be 7R/2, and experience has shown that the heat capacity of a mole of a diatomic gas at ordinary temperature is 5R/2. Why was there such a discrepancy in theory? Everything is due to the fact that when establishing the heat capacity, it will be necessary to take into account various quantum effects, in other words, to use quantum statistics. As you can see, heat capacity is a rather complicated concept.
Quantum mechanics says that any system of particles that oscillate or rotate, including a gas molecule, can have certain discrete energy values. If the energy of thermal motion in the installed system is insufficient to excite oscillations of the required frequency, then these oscillations do not contribute toheat capacity of the system.
In solids, the thermal motion of atoms is a weak oscillation near certain equilibrium positions, this applies to the nodes of the crystal lattice. An atom has three vibrational degrees of freedom and, according to the law, the molar heat capacity of a solid is equal to 3nR, where n is the number of existing atoms in the molecule. In practice, this value is the limit to which the heat capacity of the body tends at high temperatures. The value is achieved with normal temperature changes in many elements, this applies to metals, as well as simple compounds. The heat capacity of lead and other substances is also determined.
What about low temperatures?
We already know what heat capacity is, but if we talk about low temperatures, how will the value be calculated then? If we are talking about low temperature indicators, then the heat capacity of a solid body then turns out to be proportional to T 3 or the so-called Debye heat capacity law. The main criterion for distinguishing high temperatures from low ones is the usual comparison of them with a characteristic parameter for a particular substance - this can be the characteristic or Debye temperature qD. The presented value is set by the vibration spectrum of atoms in the product and depends significantly on the crystal structure.
In metals, conduction electrons make a certain contribution to the heat capacity. This part of the heat capacity is calculated usingFermi-Dirac statistics, which takes electrons into account. The electronic heat capacity of a metal, which is proportional to the usual heat capacity, is a relatively small value, and it contributes to the heat capacity of the metal only at temperatures close to absolute zero. Then the lattice heat capacity becomes very small and can be neglected.
Mass heat capacity
Mass specific heat is the amount of heat that needs to be brought to a unit mass of a substance in order to heat the product per unit temperature. This value is denoted by the letter C and it is measured in joules divided by a kilogram per kelvin - J / (kg K). This is all about the mass heat capacity.
What is volumetric heat capacity?
Volume heat capacity is a certain amount of heat that needs to be added to a unit volume of a product in order to heat it per unit temperature. This indicator is measured in joules divided by a cubic meter per kelvin or J / (m³ K). In many building reference books, it is the mass specific heat capacity at work that is considered.
Practical application of heat capacity in the construction industry
Many heat-intensive materials are actively used in the construction of heat-resistant walls. This is extremely important for houses that are characterized by periodic heating. For example, oven. Heat-intensive products and walls built from them perfectly accumulate heat, store it during heating periods of time and gradually release heat after turning offsystem, thus allowing to maintain an acceptable temperature throughout the day.
So, the more heat stored in the structure, the more comfortable and stable the temperature in the rooms will be.
It is worth noting that ordinary brick and concrete used in housing construction have a much lower heat capacity than expanded polystyrene. If we take ecowool, then it is three times more heat-consuming than concrete. It should be noted that in the formula for calculating the heat capacity, it is not in vain that there is mass. Due to the large huge mass of concrete or brick, in comparison with ecowool, it allows accumulating huge amounts of heat in the stone walls of structures and smoothing out all daily temperature fluctuations. Only a small mass of insulation in all frame houses, despite the good heat capacity, is the weakest area for all frame technologies. To solve this problem, impressive heat accumulators are installed in all houses. What it is? These are structural parts that are characterized by a large mass with a fairly good heat capacity index.
Examples of heat accumulators in life
What could it be? For example, some kind of internal brick walls, a large stove or fireplace, concrete screeds.
Furniture in any house or apartment is an excellent heat accumulator, because plywood, chipboard and wood can actually store heat only per kilogram of weight three times more than the notorious brick.
Are there any disadvantages in heat accumulators? Of course, the main disadvantage of this approach isthe fact that the heat accumulator needs to be designed at the stage of creating a frame house layout. All due to the fact that it is very heavy, and this will need to be taken into account when creating the foundation, and then imagine how this object will be integrated into the interior. It is worth saying that it is necessary to take into account not only the mass, it will be necessary to evaluate both characteristics in the work: mass and heat capacity. For example, if you use gold with an incredible weight of twenty tons per cubic meter as a heat storage, then the product will function as it should only twenty-three percent better than a concrete cube, which weighs two and a half tons.
Which substance is best suited for a heat storage?
The best product for a heat accumulator is not concrete and brick at all! Copper, bronze and iron do a good job of this, but they are very heavy. Oddly enough, but the best heat accumulator is water! The liquid has an impressive heat capacity, the largest among the substances available to us. Only helium gases (5190 J / (kg K) and hydrogen (14300 J / (kg K)) have more heat capacity, but they are problematic to apply in practice. If desired and necessary, see the heat capacity table of the substances you need.
