Let's consider in the article how to find the density, and what it is. In the design of many structures and vehicles, a number of physical characteristics are taken into account that a particular material must have. One of them is density.
Mass and volume
Decipher the meaning of two physical quantities that are directly related to it - this is mass and volume. Before we answer the question how to find density.
Mass is a characteristic that describes the inertial properties of bodies and their ability to show 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 when formulating the laws of mechanics and universal gravitation.
Volume is an exclusively geometric characteristic of the body, which quantitatively reflects the part of the space it occupies. Volume is measured in cubic units of length, for example, in SI it is meters cubed.
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 mass to the volume that it occupies, which is mathematically written as follows:
ρ=m/V.
This equality shows the units of ρ (kg/m3). Thus, density, mass and volume are related by a single equality, and the value of ρ for any material shows the volume concentration of its mass.
Let's give a simple example: if you take plastic and iron balls of the same size in your hand, then the second one will have much more weight than the first one. This fact is due to the high density of iron compared to that of plastic.
One of the main manifestations of the ratio of densities in nature will be the buoyancy of bodies. If the body has a lower density than the liquid, then it will never sink in it.
Density of materials
When talking about the density of certain materials, they mean solids. Gases and liquids also have a certain density, but we will not talk about them here.
Solid materials can be either crystalline or amorphous. 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 haveamorphous structure. Below is a table of the density of different types of wood.
Please note that in this case the average density is given. In real life, each tree has unique features, including voids, pores, and the presence of a certain percentage of moisture in the wood.
Below is another table. In it, in g/cm3densities 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 characteristic under study. 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 weigh the body in the air. Let's assume that the resulting weight was P1. After that, the body is weighed in a liquid with a known density ρl. Let the weight of the body in the liquid be P2. Then the value of the density ρ of the material under study will be:
ρ=ρlP1/(P1-P 2).
This formula can be obtained by each student on his own if he considers the law of Archimedesfor the described case.
Historically, it is believed that for the first time hydrostatic weighing was used by the Greek philosopher Archimedes to determine the fake gold crown. The first hydrostatic balances were invented by Galileo Galilei at the end of the 16th century. Currently, electronic pycnometers and density meters are widely used to experimentally determine the value of ρ in liquids, solids and gases.
Theoretical definition of density
The question of how to find the density experimentally was discussed above. However, this ρ of an unknown material can be found theoretically. To do 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, such as metal alloys, then its average density can be determined by the following simple formula:
ρ=∑mi/∑(mi/ρi).
Where mi, ρi are the mass and density of the i-th component, respectively.
If the material has an amorphous structure, then theoretically it will not be possible to accurately determine its density, and experimental techniques must be used.
