Table of the density of substances. The density formula in physics. How is density in physics denoted

The study of the density of substances begins in the course of physics of secondary school. This concept is considered fundamental in the further exposition of the foundations of the molecular-kinetic theory in the courses of physics and chemistry. The purpose of studying the structure of matter, research methods, it is possible to assume the formation of scientific ideas about the world.

The initial ideas about a single picture of the world are given by the physicist. Grade 7 studies the density of matter on the basis of the simplest ideas about the methods of investigation, the practical application of physical concepts and formulas.

Methods of physical research

As is known, among the methods of investigating the phenomena of nature, observation and experiment are singled out. Observations of natural phenomena are taught in elementary school: they carry out the simplest measurements, often lead the "Calendar of Nature". These forms of education can lead a child to the need to study the world, to compare the observed phenomena, to identify cause-effect relationships.

However, only a fully-fledged experiment will give the young researcher tools in revealing the secrets of nature. The development of experimental and research skills is carried out in practical classes and during the performance of laboratory work.

The experiments in the physics course begin with definitions of such physical quantities as length, area, and volume. At the same time, a connection is established between mathematical (for a child quite abstract) and physical knowledge. Appeal to the experience of the child, consideration of the facts known to him from the scientific point of view helps to form the necessary competence for him. The goal of learning in this case is the desire to independently comprehend a new one.

The study of density

In accordance with the problematic method of teaching at the beginning of the lesson, you can ask a known riddle: "What is heavier: a kilogram of down or a kilogram of cast iron?" Of course, 11-12 year olds easily answer the question they know. But an appeal to the essence of the question, the opportunity to reveal its peculiarity, leads to the notion of density.

The density of a substance is the mass of a unit of its volume. The table of the density of substances, usually given in textbooks or reference books, makes it possible to estimate the differences between substances, as well as the aggregate states of a substance. Indication of the difference in physical properties of solids, liquids and gases, considered earlier, an explanation of this difference not only in the structure and mutual arrangement of particles, but also in the mathematical expression of the characteristics of matter, translates the study of physics to a different level.

To consolidate knowledge of the physical meaning of the concept under study allows a table of the density of substances. The child, giving an answer to the question: "What does the density of a certain substance mean?", Understands that this mass is 1 cm ³ (or 1 m ³ ) Substance.

The question of density units can be raised already at this stage. It is necessary to consider ways of transferring units of measurement in different frames of reference. This makes it possible to get rid of the static thinking, to accept other systems of calculi in other issues.

Determination of density

Naturally, the study of physics can not be complete without solving problems. At this stage, the calculation formulas are entered. The density formula in class 7 physics is probably the first physical correlation of the values for children. It pays special attention not only to the study of the concepts of density, but also to the fact of teaching methods of solving problems.

It is at this stage that an algorithm for solving a physical computational problem is laid, the ideology of applying the basic formulas, definitions, regularities. To teach the analysis of the problem, the method of searching for the unknown, the peculiarities of using units of measurement, the teacher tries to apply such a relation as the density formula in physics.

Example of problem solving

Example 1

Determine from what substance a cube with a mass of 540 g and a volume of 0.2 dm ^{3 is made} .

Ρ -? M = 540 g, V = 0.2 dm ³ = 200 cm ³

Analysis

Proceeding from the question of the problem, we understand that the table of solid densities will help us determine the material from which the cube is made.

Therefore, we determine the density of matter. In the tables this value is given in g / cm ³ , so the volume of dm ^{3 is} translated in cm ³ .

Decision

By definition: ρ = m: V.

We are given: volume, mass. The density of matter can be calculated:

Ρ = 540 g: 200 cm ³ = 2.7 g / cm ³ , which corresponds to aluminum.

Answer : The cube is made of aluminum.

Definition of other values

Using the density calculation formula allows you to determine other physical quantities. The mass, volume, linear dimensions of the bodies associated with the volume can be easily calculated in the problems. Knowledge of mathematical formulas for determining the area and volume of geometric figures is used in problems, which makes it possible to explain the need for studying mathematics.

Example 2

Determine the thickness of the copper layer to which the surface is coated with a surface area of 500 cm ² , if it is known that 5 g of copper is consumed on the coating.

H -? S = 500 cm ² , m = 5 g, ρ = 8.92 g / cm ³ .

Analysis

Table density of substances allows you to determine the value of the density of copper.

We use the formula for calculating the density. In this formula there is a volume of matter from which it is possible to determine the linear dimensions.

Decision

By definition: ρ = m: V, but this formula does not have the desired value, so we use:

V = S x h.

Substituting in the basic formula, we get: ρ = m: Sh, from which:

H = m: S x ρ.

We calculate: h = 5 g: (500 cm ² x 8.92 g / cm ³ ) = 0.0011 cm = 11 μm.

Answer : The thickness of the copper layer is 11 μm.

Experimental determination of the density

The experimental nature of physical science is demonstrated in the course of laboratory experiments. At this stage skills are acquired for conducting the experiment, explaining its results.

The practical task of determining the density of matter includes:

• Determination of fluid density. At this stage, the guys who have already used the previously measured cylinder can easily determine the density of the liquid using the formula.
• Determination of the density of solid matter in a solid form. This task also does not cause doubts, since similar computational problems have already been considered and the experience of measuring volumes by linear dimensions of bodies has been acquired.
• Determination of the density of a rigid body in an irregular shape. When performing this task, we use the method of determining the volume of a body of irregular shape using a beaker. It is worth recalling once again the peculiarities of this method: the ability of a solid body to displace a fluid whose volume is equal to the volume of the body. Next, the problem is resolved in a standard way.

Tasks of increased complexity

You can complicate the task by inviting the children to determine the substance from which the body is made. The density of substances table used here makes it possible to draw attention to the need to be able to work with reference information.

In solving experimental problems, students are required to have the necessary amount of knowledge in the field of the use of physical instruments and the translation of units of measurement. Often this is what causes the greatest number of errors and shortcomings. Perhaps this stage of the study of physics should be allocated more time, it allows you to compare the knowledge and experience of research.

Bulk density

Investigation of a pure substance is, of course, interesting, but is it often found pure substances? In everyday life we meet with mixtures and alloys. How to be in this case? The concept of bulk density will not allow students to make a typical error and use average values of the density of substances.

It is extremely necessary to explain this question, to give an opportunity to see and feel the difference between the density of matter and the bulk density at the early stages. An understanding of this difference is necessary in the further study of physics.

This difference is extremely interesting in the case of bulk materials. Allow the child to study the bulk density, depending on the compaction of the material, the size of individual particles (gravel, sand, etc.) can be done during the initial research activity.

Relative density of substances

Comparison of the properties of various substances is quite interesting on the basis of relative values. The relative density of matter is one of these quantities.

Usually, the relative density of the substance is determined with respect to distilled water. As the ratio of the density of a given substance to the density of a standard, this value is determined with the aid of a pycnometer. But in the school course of natural science this information is not used, it is interesting for a deep study (most often optional).

The Olympiad level of studying physics and chemistry can also affect the concept of "the relative density of matter over hydrogen." Usually it is applied to gases. To determine the relative density of the gas, the ratio of the molar mass of the test gas to the molar mass of hydrogen is found. The use of relative molecular weight is not excluded.