Open lesson in physics on the topic: "Density of matter." How is the density of a material measured? Density of various materials Why is it necessary to study the topic density

In many industries, as well as in construction and agriculture The concept of "material density" is used. This is a calculated quantity that is the ratio of the mass of a substance to the volume it occupies. Knowing this parameter, for example, for concrete, builders can calculate the required quantity when pouring various reinforced concrete structures: building blocks, ceilings, monolithic walls, columns, protective sarcophagi, swimming pools, sluices and other objects.

How to determine density

It is important to note that when determining the density of building materials, you can use special reference tables that give these values ​​for various substances. Calculation methods and algorithms have also been developed that make it possible to obtain such data in practice if there is no access to reference materials.

Density is determined by:

• liquid bodies with a hydrometer device (for example, the well-known process of measuring the parameters of the electrolyte of a car battery);
• solid and liquid substances using a formula with known initial data of mass and volume.

All independent calculations, of course, will have inaccuracies, because it is difficult to reliably determine the volume if the body has an irregular shape.

Errors in density measurements

• The error is systematic. It appears constantly or can change according to a certain law in the process of several measurements of the same parameter. Associated with the error of the instrument scale, low sensitivity of the device or the degree of accuracy of the calculation formulas. So, for example, by determining body mass using weights and ignoring the effect of buoyant force, the data is obtained approximate.
• The error is random. It is caused by incoming reasons and has a different impact on the reliability of the data being determined. Temperature change environment, atmospheric pressure, room vibrations, invisible radiation and air vibrations - all this is reflected in the measurements. It is impossible to completely avoid such influence.

• Error in rounding values. When obtaining intermediate data in the calculation of formulas, numbers often have many significant figures after the decimal point. The need to limit the number of these characters implies the appearance of an error. This inaccuracy can be partially reduced by leaving in intermediate calculations several orders of magnitude more numbers than required by the final result.
• Negligence errors (misses) arise due to erroneous calculations, incorrect inclusion of measurement limits or the device as a whole, and illegibility of control records. The data obtained in this way may differ sharply from similarly performed calculations. Therefore, they should be removed and the work done again.

True Density Measurement

When considering the density of a building material, you need to take into account its true value. That is, when the structure of a substance of a unit volume does not contain shells, voids and foreign inclusions. In practice, there is no absolute uniformity when, for example, concrete is poured into a mold. To determine its real strength, which directly depends on the density of the material, the following operations are carried out:

• The structure is crushed to a powder state. At this stage, the pores are removed.
• Dry at temperatures above 100 degrees, and remove any remaining moisture from the sample.
• Cool to room temperature and pass through a fine sieve with a mesh size of 0.20 x 0.20 mm, giving uniformity to the powder.
• The resulting sample is weighed on a high-precision electronic scale. The volume is calculated in a volume meter by immersion into a liquid structure and measuring the displaced liquid (pycnometric analysis).

The calculation is carried out according to the formula:

where m is the mass of the sample in g;

V is the volume value in cm3.

Density measurement in kg/m 3 is often applicable.

Average material density

To determine how building materials behave under real operating conditions under the influence of moisture, positive and negative temperatures, and mechanical loads, you need to use average density. It characterizes the physical state of materials.

If the true density is a constant value and depends only on chemical composition and structures crystal lattice substance, then the average density is determined by the porosity of the structure. It represents the ratio of the mass of a material in a homogeneous state to the volume of occupied space under natural conditions.

The average density gives the engineer an idea of ​​the mechanical strength, moisture absorption rate, thermal conductivity coefficient and other important factors used in the construction of the elements.

The concept of bulk density

Introduced for the analysis of bulk building materials (sand, gravel, expanded clay, etc.). The indicator is important for calculating the cost-effective use of certain components of a building mixture. It shows the ratio of the mass of a substance to the volume it occupies in a state of loose structure.

For example, if the granular shape of the material and the average density of the grains are known, then it is easy to determine the voidness parameter. When producing concrete, it is more advisable to use a filler (gravel, crushed stone, sand) that has less porosity of the dry substance, since the base cement material will be used to fill it, which will increase the cost.

Density indicators of some materials

If we take the calculated data from some tables, then in them:

• materials containing calcium, silicon and aluminum oxides vary from 2400 to 3100 kg per m 3.
• Wood species with a cellulose base - 1550 kg per m 3.
• Organics (carbon, oxygen, hydrogen) - 800-1400 kg per m 3.
• Metals: steel - 7850, aluminum - 2700, lead - 11300 kg per m 3.

At modern technologies In the construction of buildings, the material density indicator is important from the point of view of the strength of load-bearing structures. All heat-insulating and moisture-proofing functions are performed by low-density materials with a closed-cell structure.

The purpose of the lesson:

• consolidate students' knowledge about density
• show how to determine the mass and volume of a body by its density
• solve calculation problems

Lesson structure:

1. Org. moment
2. Repetition of material
3. Learning new material
4. Fixing the material
5. Homework

1. Org. moment
2. Repetition of material

The lesson uses a presentation (Appendix 1)

• Do all bodies have the same volume and the same mass? (slide 2)
• Why?
• Do all bodies of the same mass have the same volume? (slide 2)
• Why?
• what does it depend on?
• What is the density of a substance? (slide 3)
• Density is a physical quantity equal to the ratio of the mass of a body to its volume.
• write down the density formula. (slide 4)

• get the density name in SI (slide 4)

Today in the lesson we will calculate the mass and volume of bodies based on density, write down the topic of the lesson in your notebook.

Lesson topic: determining the mass and volume of a body based on the density of the substance. (slide 8)

Write the formula for calculating density in your notebook

what will the mass of the body be equal to if we know the density of the body and its volume?, who will write it down?

– to calculate mass from its density and volume, you need to multiply the density by its volume. (slide 9)

No. 1 - everyone solves the problem themselves

We check:
Task
Answer: m=2825g. (slide 12)

Attention! What will the volume of the body be equal to if we know the mass and density of the substance?

; - to calculate the volume of a body from its density, you need to divide the mass of the body by its density (slide 13)

The problem is solved on the board and remains as a sample solution.

No. 2 - everyone solves the problem themselves

1. A steel part of a machine has a mass of 7890 g. Determine its volume if the density of steel is 7.8. (Appendix 3)

We check:
Task
Answer: V=1023.077 cm 3 (slide 16)

• Homework §22 ex. No. 8 (slide 17)
• Open task 3 (slide 18)

No. 3 - everyone solves problems themselves

Task report form No. 3 (slide 19)– please note that you must fill in the field with your last name

Last name student name_____________________________________________

fill out the calculated physical quantity with its units of measurement in the corresponding cells (slide 19)

Date of completion______________________________________

• Everyone has such a form on their desk; you fill out the corresponding cells in which you write only numerical values.
• Close Excel. Save the change to the document. (slide 21)

Lesson over thanks! (slide 22)

Used Books:

1. Physics textbook 7, author – A.V. Peryshkin – Moscow – “Enlightenment”
2. Methodological materials 7,- L.A.Kirik – Moscow – “ILEKS”
3. Multi-level independent and control work 7, - L.A. Kirik - Moscow - “ILEKS”
4. Collection of problems in physics - V.I.Lukashik - Moscow - “Enlightenment”

The bodies around us consist of various substances: iron, wood, rubber, etc. The mass of any body depends not only on its size, but also on the substance of which it consists. Bodies of the same volume, consisting of different substances, have different masses. For example, having weighed two cylinders made of different substances - aluminum and lead, we will see that the mass of the aluminum cylinder is less than the mass of the lead cylinder.

At the same time, bodies with the same masses, consisting of different substances, have different volumes. Thus, an iron bar weighing 1 ton occupies a volume of 0.13 m 3, and ice weighing 1 ton occupies a volume of 1.1 m 3. The volume of ice is almost 9 times greater than the volume of an iron bar. That is, different substances can have different densities.

It follows that bodies with the same volume, consisting of different substances, have different masses.

Density shows the mass of a substance taken in a certain volume. That is, if the mass of a body and its volume are known, the density can be determined. To find the density of a substance, you need to divide the mass of the body by its volume.

The density of the same substance in solid, liquid and gaseous states is different.

Density of some solids, liquids and gases are given in tables.

Densities of some solids (at normal atmospheric pressure, t = 20 ° C).

Densities of some liquids (at normal atmospheric pressure t = 20 ° C).

Answer from Flush[newbie]
Density of matter: formula, calculation.
Everything around us consists of different substances. Ships and bathhouses are built from wood, irons and cots are made from iron, tires on wheels and erasers on pencils are made from rubber. AND various items have different weights - any of us can easily carry a juicy ripe melon from the market, but a weight of the same size will have to work hard.
Everyone remembers the famous joke: “Which is heavier? A kilogram of nails or a kilogram of fluff? We will no longer fall for this childish trick, we know that the weight of both will be the same, but the volume will be significantly different. So why is this happening? Why do different bodies and substances have different weights with the same size? Or vice versa, the same weight with different sizes? Obviously, there is some characteristic due to which substances are so different from each other. In physics, this characteristic is called the density of matter and is taught in the seventh grade.
Density of a substance: definition and formula
The definition of density of a substance is as follows: density shows what the mass of a substance is in a unit volume, for example, in one cubic meter. Thus, the density of water is 1000 kg/m3, and ice is 900 kg/m3, which is why ice is lighter and is located on top of reservoirs in winter. That is, what shows us the density of matter in in this case? An ice density of 900 kg/m3 means that an ice cube with sides of 1 meter weighs 900 kg. And the formula for determining the density of a substance is as follows: density = mass/volume. The quantities included in this expression are designated as follows: mass – m, body volume – V, and density is designated by the letter? (Greek letter "rho"). And the formula can be written as follows:
?=m/V
How to find the density of a substance
How to find or calculate the density of a substance? To do this, you need to know body volume and body weight. That is, we measure the substance, weigh it, and then simply substitute the obtained data into the formula and find the value we need. And how the density of a substance is measured is clear from the formula. It is measured in kilograms per cubic meter. Sometimes they also use a value such as grams per cubic centimeter. Converting one value to another is very simple. 1 g = 0.001 kg, and 1 cm3 = 0.000001 m3. Accordingly, 1 g/(cm) ^3 = 1000 kg/m^3. It should also be remembered that the density of a substance is different in different states of aggregation. That is, in solid, liquid or gaseous form. The density of solids is most often higher than the density of liquids and much higher than the density of gases. Perhaps a very useful exception for us is water, which, as we have already considered, weighs less in the solid state than in the liquid state. It is because of this strange feature of water that life is possible on Earth. Life on our planet, as we know, originated from the oceans. And if water behaved like all other substances, then the water in the seas and oceans would freeze through, the ice, being heavier than water, would sink to the bottom and lie there without melting. And only at the equator, in a small column of water, would life exist in the form of several species of bacteria. So we can say thank you to the water for our existence.