study of the density of matter begins in high school physics course.This concept is considered to be fundamental in what follows the molecular-kinetic theory courses in physics and chemistry.The purpose of studying the structure of matter, methods of research can assume the formation of scientific concepts of the world.
initial idea of a unified picture of the world gives the physicist.Grade 7 studying the density of matter on the basis of the simplest concepts of research methods, practical application of physical concepts and formulas.
Methods of physical examination
It is known that among the methods of the study of natural phenomena isolated observation and experiment.To observe natural phenomena taught in elementary school: a simple measurement is carried out, often leads "Calendar of nature."These forms of training can lead to the need for the child to explore the world, comparing the observed phenomena, identifying cause-and-effect relationships.
However, only fully carried out the experiment will give into the hands of the young explorer tool in uncovering the secrets of nature.The development of experimental research carried out on the skills and practical training in the laboratory work.
experiments in physics course begins with the definition of physical quantities such as length, area, volume.This establishes a connection between the mathematical (abstract enough for a child) and physical knowledge.Appeal to the experience of the child, his consideration of the long-known facts from the scientific point of view contributes to the formation of the required competence.The purpose of training in this case - the desire for a new self-comprehension.
Learning density
In accordance with the problem by training at the beginning of the lesson, you can specify a known riddle: "What is heavier than a kilogram of lint, or a kilo of iron?" Of course, the 11-12-year-olds can easily give them an answer to the famous question.But an appeal to the heart of the matter, the opportunity to discover its peculiarity leads to the concept of density.
matter density - mass per unit of its volume.Table density substances normally present in textbooks or reference books, to evaluate the differences between the substance and physical state of matter.Note the difference in the physical properties of solids, liquids and gases, previously discussed, explanation of this difference is not only in the structure and arrangement of the particles, but in the mathematical expression of the characteristics of the substance, the study of physics takes on a different level.
consolidate the knowledge of the physical meaning of the concept under study allows table-density substances.The child, giving an answer to the question: "What is the value of the density of a given substance?" Understands that a mass of 1 cm3 (or 1 m3) of a substance.
issue of units density can be raised at this stage.It is necessary to consider ways of translation units in different reference frames.This makes it possible to get rid of static thinking, take other system of estimates and other matters.
Density
Naturally, the study of physics can not be complete without addressing the problems.At this point, enter the calculation formula.Formula density physics class 7, probably the first physical magnitude relationship for the children.She pays special attention not only as a result of studying the concepts of density, but also on the fact of training in problem solving.
At this stage lays the physical computing algorithm for solving the problem, the main ideology of the application of the formulas, definitions, laws.To teach the analysis of the problem, a method of searching the unknown, especially the use of units teacher tries on the use of this ratio as the density formula in physics.
Example solving
Example 1
Determine what matter is made of a cube weighing 540 g and the volume of 0.2 dm3.
ρ -?m = 540 g, V = 0,2 dm3 = 200 cm3
analysis
Based on the question of the problem, we understand that identify the material from which the cube is made, we will table density solids.
Therefore, we define the density of matter.In the tables, this value is given in g / cm3, so the volume of transferred dm3 cm3.
decision
By definition: ρ = m: V.
We are: volume, weight.The density of the substance can be calculated:
ρ = 540 g: 200 g = 2.7 cm3 / cm3, which corresponds to aluminum.
answer : cube made of aluminum.
Determination of other variables
Using the formula for calculating density allows you to define and other physical quantities.Mass, volume, linear dimensions of the bodies associated with a volume calculated easily in problems.Knowledge of mathematical formulas determining the area and volume of geometric figures used in tasks that allows you to explain the need to study mathematics.
Example 2
determine the thickness of the copper layer, which is coated with a surface area of item 500 cm2, if you know that to cover the spent 5 g of copper.
h -?S = 500 cm 2, m = 5 g, ρ = 8.92 g / cm3.
analysis
Table density substances to determine the density of copper.
use the formula for calculating density.In this formula, it is the volume of the substance, on the basis of which it is possible to determine the linear dimensions.
decision
By definition: ρ = m: V, but this formula is not the unknown quantity, so we use:
V = S x h.
Substituting the basic formula, we obtain: ρ = m: Sh, where:
h = m: S x ρ.
compute: h = 5 g (500 cm2 x 8.92 g / cm3) = 0.0011 cm = 11 mm.
answer : copper thickness is 11 microns.
Experimental determination of density
experimental nature of physics is demonstrated in laboratory experiments.At this stage of the experience acquired skills, explanations of the results.
Practice for determining the density of matter includes:
- Determining density of liquids.At this stage, the guys have been installed previously graduated cylinder, can easily determine the density of liquids using a formula.
- Density substance of the solid proper shape.This task is also no doubt, as is already considered a similar calculation tasks and the experience acquired by measuring the volume of linear dimensions of bodies.
- density determination of solid bodies of irregular shape.In carrying out this task we use the method of determining the volume of a body by using irregularly shaped beaker.It is useful to recall once again the features of this method: the ability of solid-state drive fluid, the volume of which is equal to the volume of the body.Next task is permitted standard.
jobs increased complexity
can complicate the task by offering children to determine the substance of which the body.As used in this table is the density of substances allows to draw attention to the need for the ability to work with background information.
In solving experimental problems, students are required to have the necessary knowledge in the use of physical devices and transfer units.Often, this is what causes the greatest number of errors and shortcomings.Perhaps this stage of the study of physics is to provide more time, it allows to compare knowledge and research experience.
Bulk density
study pure substance, of course, interesting, but how often do occur clean substances?In everyday life we meet with mixtures and alloys.How to be in this case?The concept of the bulk density does not allow students to make typical mistakes and use the average value of the density of matter.
clarify the matter it is imperative to give the opportunity to see and feel the difference between the density of matter, and bulk density is in the early stages.Understanding this distinction is necessary to further study of physics.
extremely interesting is the difference in the case of bulk materials.Allowing a child study bulk density depending on the sealing material, the size of the individual particles (gravel, sand and the like. D.) May be during initial research.
relative density
Comparison of different substances is quite interesting on the basis of relative values.The relative density of the substance - one of these values.
usually determine the relative density of the material with respect to distilled water.As the ratio of the density of a substance to a reference density, this value is determined using a pycnometer.But natural science school course this information is not used, it is interesting to study in deep (usually optional).
Olympiad level of study of physics and chemistry can affect the concept of "relative density of hydrogen."Usually it is applied to gases.To determine the relative density of the gas are the ratio of the molar mass of the test gas to the molar mass of hydrogen.Using the relative molecular mass is not excluded.
