Consider in the article how to find the density, and what it is. When designing many structures and vehicles, a number of physical characteristics that a particular material should possess are taken into account. One of them is density.
Mass and volume
We decipher the meaning of two physical quantities that are directly related to it - this is mass and volume. Before we answer the question of how to find the density.
Mass is a characteristic that describes the inertial properties of bodies and their ability to exhibit gravitational attraction to each other. Mass is measured in kilograms in the SI system.
The concepts of inertial and gravitational masses were first introduced into physics by Isaac Newton in the formulation of the laws of mechanics and gravity.
Volume is an exclusively geometric characteristic of the body, which quantitatively reflects part of the space occupied by it. Volume is measured in cubic units of length, for example, in SI it is meters in a cube.
For bodies of known shape (parallelepiped, ball, pyramid), this value can be determined by special formulas, for objects of irregular geometric shape, the volume is determined by immersing them in a liquid.
Physical quantity density
Now you can go directly to the answer to the question of how to find the density. This characteristic is determined by the ratio of body weight to the volume that it occupies, which is mathematically written as follows:
ρ = m / V.
This equality shows the units of measure of ρ (kg / m 3 ). Thus, the density mass and volume are connected by a single equality, and the value ρ for any material shows the volume concentration of its mass.
Let us give a simple example: if you take plastic and iron balls of the same size in your hand, the second will have much more weight than the first. This fact is associated with a high density of iron compared with that for plastic.
One of the main manifestations of the density ratio in nature will be the buoyancy of bodies. If the body has a lower density than the liquid, then it will never drown in it.
Material density
When it comes to the density of certain materials, they mean solids. Gases and liquids also have a certain density, but here we will not talk about them.
Solid materials may have a crystalline or amorphous structure. The value of ρ depends on the structure, interatomic distances, and atomic and molecular masses of materials. For example, all metals are crystals, and glass or wood have an amorphous structure. Below is a table of the density of different varieties of wood.
Please note that in this case the average density is given. In real conditions, each tree has unique features, including voids, pores and the presence of a certain percentage of moisture in the wood.
The following is another table. In it, in g / cm 3 the densities of all pure chemical elements that are at room temperature are given.
It can be seen from the table that all elements have a density greater than that of water. The exception is only three metals - lithium, potassium and sodium, which do not sink, but float on the surface of the water.
How is density measured experimentally?
In fact, there are two techniques for determining the characteristics being studied. The first is to directly weigh the body and measure its linear dimensions.
If the geometric shape of the body is complex, then the so-called hydrostatic method is used.
Its essence is as follows: first, the body is weighed in air. Assume that the weight obtained is P 1 . After that, the body is weighed in a liquid with a known density ρ l . Let the body weight in the liquid be equal to P 2 . Then the density ρ of the investigated material is:
ρ = ρ l * P 1 / (P 1 -P 2 ).
Each student can get this formula independently if he considers the law of Archimedes for the described case.
It is historically believed that for the first time hydrostatic weighing was used by the Greek philosopher Archimedes to determine the fake of the golden crown. The first hydrostatic scales were invented by Galileo Galilei at the end of the 16th century. At present, electronic pycnometers and densitometers are widely used for the experimental determination of ρ of liquids, solids, and gases.
Theoretical determination of density
The question was discussed above how to find the density experimentally. Nevertheless, this ρ of unknown material can be found theoretically. For this, it is necessary to know the type of crystal lattice, the parameters of this lattice, as well as the mass of the atoms forming it. Since any elementary crystal lattice has a certain geometric shape, it is easy to find a formula for determining its volume.
If a crystalline material consists of several chemical elements, for example, metal alloys, then its average density can be determined by the following simple formula:
ρ = ∑m i / ∑ (m i / ρ i ).
Where m i , ρ i are the mass and density of the i-th component, respectively.
If the material has an amorphous structure, then theoretically determining its exact density will not work, and it is necessary to use experimental techniques.