Heat capacity is the ability to absorb certain amounts of heat during heating or to give off when cooled. The heat capacity of the body is the ratio of an infinitely small number of heat that the 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 value refers to a single amount of a substance. Accordingly, the number of substances can be measured in cubic meters, kilograms or even in moles. What does it depend on? In physics, the heat capacity directly depends on which quantity unit it belongs to, which means that molar, mass, and volume heat capacity are distinguished. In the construction industry, you will not meet with molar measurements, but with others - all the time.
What affects the specific heat?
What is the specific heat, you know, but what values affect the indicator is not yet clear. The specific heat is directly affected by several components: the temperature of the substance, pressure, and other thermodynamic characteristics.
During an increase in the temperature of a product, its specific heat increases, however, certain substances differ in a completely non-linear curve in this dependence. For example, with an increase in temperature indicators from zero to thirty-seven degrees, the specific heat 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 a stable pressure and at a stable volume will differ.
How to calculate the parameter?
Are you interested in what is the heat capacity? The calculation formula is as follows: C = Q / (m · ΔT). What are these meanings? 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. The table below shows the heat capacity of some materials.
What about the calculation of heat capacity?
Calculating the heat capacity is not an easy task, especially if you use exclusively thermodynamic methods, or rather it is impossible to do. Because physicists use methods of statistical physics or knowledge of the microstructure of products. How to make calculations for gas? The heat capacity of the gas is calculated from the calculation of the average energy of the thermal motion of individual molecules in a substance. The movements of the molecules can be of the translational and rotational types, and inside the molecule there can be a whole atom or an atomic vibration. Classical statistics say that for each degree of freedom of rotational and translational movements, the value in the molar heat capacity of gas is equal to R / 2, and for each vibrational degree of freedom the value is R. This rule is also called the law of equidistribution.
In this case, a particle of a monatomic gas differs in only three translational degrees of freedom, and therefore its heat capacity should be equal to 3R / 2, which is in excellent agreement with experience. Each diatomic gas molecule is characterized by three translational, two rotational and one vibrational degrees of freedom, which means that the law of equal distribution will be equal to 7R / 2, and experience has shown that the heat capacity of a mole of diatomic gas at ordinary temperature is 5R / 2. Why was there such a divergence of theory? All this is due to the fact that when establishing the heat capacity, it will be necessary to take into account different quantum effects, in other words, 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 to the heat capacity of the system.
In solids, the thermal motion of atoms is a weak vibration 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 atoms in the molecule. In practice, this value is the limit to which the heat capacity of the body tends at high temperature indicators. The value is achieved with ordinary 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, then how will the value be calculated then? If we are talking about low temperature indicators, then the heat capacity of the solid then turns out to be proportional to T 3 or the so-called Debye heat capacity law. The main criterion that makes it possible to distinguish between high and low temperatures is the usual comparison of them with a parameter characteristic of a particular substance - this may be the characteristic or Debye temperature q D. The presented value is established by the vibrational spectrum of atoms in the production and substantially depends on the crystal structure.
For metals, conduction electrons give a certain contribution to the specific heat. This part of the specific heat is calculated using the Fermi-Dirac statistics, in which electrons are taken into account. The electronic heat capacity of the metal, proportional to the usual heat capacity, is a relatively small value, and it makes a contribution to the heat capacity of the metal only at temperature values close to absolute zero. Then the lattice heat capacity becomes very small, and it 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 a product per unit temperature. This value is denoted by the letter C and is measured in joules divided by kilogram per kelvin - J / (kg · K). That's all for the mass heat capacity.
What is volumetric heat capacity?
Volumetric heat capacity is a certain amount of heat that needs to be brought to a unit volume of production in order to heat it per unit temperature. This indicator is measured in joules divided by cubic meter to kelvin or J / (m³ · K). In many building directories, it is precisely the mass specific heat capacity in operation that is considered.
Practical application of heat capacity in the construction industry
Many heat-intensive materials are used actively in the construction of heat-resistant walls. This is extremely important for houses that are characterized by periodic heating. For example, stove. The heat-intensive products and walls erected from them perfectly accumulate heat, store it during heating periods and gradually release heat after turning off the system, thus allowing maintaining an acceptable temperature throughout the day.
So, the more heat is 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 house building have a significantly lower heat capacity than expanded polystyrene. If you take ecowool, it is three times more heat-resistant than concrete. It should be noted that in the formula for calculating the heat capacity there is no reason that mass is present. Due to the large mass of concrete or brick in comparison with ecowool, it is possible to accumulate huge amounts of heat in the stone walls of structures and smooth out all daily temperature fluctuations. Only a small mass of insulation in all frame houses, despite the good heat capacity, is the weakest zone in all frame technologies. To solve this problem, impressive heat accumulators are installed in all houses. What it is? These are structural parts that are distinguished by their large mass with a fairly good heat capacity index.
Examples of heat accumulators in life
What could it be? For example, some internal brick walls, a large stove or fireplace, screed made of concrete.
Furniture in any house or apartment is an excellent heat accumulator, because plywood, chipboard and wood can actually be stored in heat three times as much only a kilogram of weight than the notorious brick.
Are there any shortcomings in the heat accumulators? Of course, the main disadvantage of this approach is that the heat accumulator needs to be designed at the stage of creating a model of a frame house. All due to the fact that it has a large weight, 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 not only the mass will have to be taken into account, 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 accumulator, then the products will function as needed only twenty-three percent better than a concrete cube, whose weight is two and a half tons.
Which substance is most suitable for a heat accumulator?
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 the helium gases (5190 J / (kg · K) and hydrogen (14300 J / (kg · K)) have greater heat capacity, but they are problematic to put into practice. If desired and necessary, see the heat capacity table for the substances you need.