The study of the density of substances begins in a high school physics course. This concept is considered fundamental in the further presentation of the fundamentals of molecular kinetic theory in physics and chemistry courses. The purpose of studying the structure of matter, research methods, we can assume the formation of scientific ideas about the world.
The initial idea of a single picture of the world gives physics. Grade 7 studies the density of a substance based on the simplest ideas about research methods, the practical application of physical concepts and formulas.
Physical research methods
As is known, observation and experiment are distinguished among the methods of studying natural phenomena. Observe natural phenomena taught in elementary school: carry out the simplest measurements, often keep a "calendar of nature." These forms of learning can lead the child to the need to study the world, compare observed phenomena, and identify causal relationships.
However, only a fully conducted experiment will give the young researcher the tools to uncover the secrets of nature. The development of experimental, research skills is carried out in practical classes and in the course of laboratory work.
An experiment in a physics course begins with the definitions of such physical quantities as length, area, volume. In this case, a connection is established between mathematical (quite abstract for a child) and physical knowledge. Turning to the experience of the child, considering the facts long known to him from a scientific point of view, contributes to the formation of the necessary competence. The purpose of training in this case is the desire for independent comprehension of the new.
Density study
In accordance with the problematic teaching method, at the beginning of the lesson you can ask a well-known riddle: “What is heavier: a kilogram of fluff or a kilogram of cast iron?” Of course, 11-12-year-olds easily give an answer to a question they know. But appeal to the essence of the issue, the ability to reveal its feature, leads to the concept 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 assess the differences between substances, as well as the aggregative states of a substance. An indication of the difference in the physical properties of solids, liquids, and gases, discussed earlier, an explanation of this difference not only in the structure and relative position of particles, but also in the mathematical expression of the characteristics of matter, takes the study of physics to a different level.
To consolidate knowledge about the physical meaning of the studied concept allows the table of the density of substances. The child, giving the answer to the question: “What does the density of a certain substance mean?”, Understands that this is a mass of 1 cm 3 (or 1 m 3 ) substances.
The question of density units can be raised already at this stage. It is necessary to consider ways of converting units of measurement in various reference systems. This makes it possible to get rid of the static thinking, to adopt other systems of calculus in other matters.
Density determination
Naturally, the study of physics cannot be complete without solving problems. At this stage, calculation formulas are introduced. The density formula in class 7 physics is probably the first physical ratio of values for children. She pays special attention not only due to the study of the concepts of density, but also upon the fact of teaching methods for solving problems.
It is at this stage that the algorithm for solving the physical computational problem, the ideology of applying the basic formulas, definitions, and laws, are laid. The teacher tries to teach the analysis of the problem, the way to search for the unknown, the features of using units of measure, by applying a relation such as the density formula in physics.
Problem Solving Example
Example 1
Determine what substance a cube weighing 540 g and a volume of 0.2 dm 3 is made of .
ρ -? m = 540 g, V = 0.2 dm 3 = 200 cm 3
Analysis
Based on 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 the substance. In the tables, this value is given in g / cm 3 , so the volume of dm 3 is converted to cm 3 .
Decision
By definition: ρ = m: V.
We are given: volume, mass. The density of the substance 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.
Determination of other quantities
Using the density calculation formula, one can also determine other physical quantities. The mass, volume, linear dimensions of bodies associated with the volume are easily calculated in tasks. Knowledge of mathematical formulas for determining the area and volume of geometric figures is used in problems, which allows us to clarify the need to study mathematics.
Example 2
Determine the thickness of the copper layer with which the part is coated with a surface area of 500 cm 2 , if it is known that 5 g of copper is used up on the coating.
h -? S = 500 cm 2 , m = 5 g, ρ = 8.92 g / cm 3 .
Analysis
The table of the density of substances allows you to determine the density of copper.
We use the density calculation formula. In this formula there is a volume of substance, from which linear dimensions can be determined.
Decision
By definition: ρ = m: V, but in this formula there is no desired quantity, therefore we use:
V = S x h.
Substituting into the main formula, we obtain: ρ = m: Sh, whence:
h = m: S x ρ.
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 microns.
Experimental determination of density
The experimental nature of physical science is demonstrated during laboratory experiments. At this stage, skills are acquired to conduct the experiment, explaining its results.
The practical task of determining the density of a substance includes:
- Determination of fluid density. At this stage, the guys who have already used the previously measured cylinder, easily determine the density of the liquid using the formula.
- Determination of the density of the substance of a solid body of regular shape. This task is also not in doubt, since similar calculation problems have already been considered and experience has been gained in measuring volumes by linear dimensions of bodies.
- Determination of the density of a solid body of irregular shape. When completing this task, we use the method of determining the body volume of an irregular shape using a beaker. It is worth repeating again the features of this method: the ability of a solid to displace a fluid whose volume is equal to the volume of the body. Further, the task is solved as standard.
Advanced Challenges
You can complicate the task by inviting the children to determine the substance of which the body is made. The table of density of substances used in this allows you to pay attention to the need for the ability to work with background information.
In solving experimental problems, students are required to have the necessary amount of knowledge in the field of using physical devices and converting units of measurement. Often this is what causes the largest 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 the study.
Bulk density
The study of a pure substance is, of course, interesting, but are pure substances often found? 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 mistake and use the average density of substances.
It is extremely necessary to clarify this issue, to give an opportunity to see, feel the difference between the density of a substance and bulk density at an early stage. 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.) 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 a substance is one of such quantities.
Typically, the relative density of a 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 using a pycnometer. But this information is not used in the school course in natural sciences; it is interesting for in-depth study (most often optional).
The olympiad level of studying physics and chemistry can also affect the concept of "relative density of matter in hydrogen." It is usually applied to gases. To determine the relative density of the gas, find the ratio of the molar mass of the test gas to the molar mass of hydrogen. The use of relative molecular weight is not excluded.