The physical quantity is ... Measuring physical quantities. System of physical quantities

Physics as a science that studies natural phenomena uses the standard research methodology. The main stages can be called: observation, promotion of the hypothesis, conducting an experiment, justifying the theory. During the observation, distinctive features of the phenomenon, the course of its course, possible causes and consequences are established. The hypothesis makes it possible to explain the course of the phenomenon, to establish its regularities. The experiment confirms (or does not confirm) the validity of the hypothesis. Allows you to establish a quantitative relationship between the values in the course of the experiment, which leads to an accurate determination of the dependencies. The hypothesis, confirmed in the course of the experiment, lies at the basis of the scientific theory.

No theory can claim validity unless it has been fully and unconditionally confirmed in the course of the experiment. The latter is associated with measurements of physical quantities characterizing the process. The physical quantity is the basis of the measurements.

What it is

The measurement concerns those quantities that confirm the validity of the hypothesis of regularities. A physical quantity is a scientific characteristic of a physical body whose qualitative relation is common to a multitude of similar bodies. For each body, this quantitative characteristic is highly individual.

If we turn to the special literature, then in M. Yudin et al. (1989 edition) we read that the physical quantity is: "the characteristic of one of the properties of a physical object (physical system, phenomenon or process), qualitatively common for many physical Objects, but quantitatively individual for each object. "

Dictionary Ozhegova (1990 edition) argues that the physical magnitude is - "size, volume, length of the subject."

For example, length is a physical quantity. Mechanics length is interpreted as the distance traveled, electrodynamics uses the length of the wire, in thermodynamics an analogous value determines the thickness of the walls of the vessels. The essence of the concept does not change: units of values can be the same, and the value - different.

A distinctive feature of a physical quantity, say, from a mathematical one, is the presence of a unit of measurement. Meter, foot, arshin are examples of length units.

Units

To measure a physical quantity, it should be compared with the value taken as a unit. Remember the wonderful cartoon "Forty-eight parrots". To establish the length of the boa constrictor, the heroes measured his length in parrots, then in elephants, then in monkeys. In this case, the length of the boa constrictor was compared with the growth of other cartoon characters. The result was quantitatively dependent on the standard.

The unit of physical quantity is the measure of its measurement in a certain system of units. The confusion in these measures arises not only due to imperfections, the heterogeneity of measures, but sometimes also because of the relativity of units.

Russian measure of length - arshin - the distance between the forefinger and the thumb. However, the hands of all people are different, and arshin, measured by the hand of an adult man, differs from the arshin on the hand of a child or a woman. The same inconsistency of the measures of length concerns the sazhen (the distance between the tips of the fingers placed at the hands) and the elbow (the distance from the middle finger to the elbow of the hand).

It is interesting that men were employed as stewards in stalls. Sly merchants saved cloth with a few smaller measures: arshin, elbow, sazhen.

Measure systems

Such a variety of measures existed not only in Russia, but also in other countries. The introduction of units of measure was often arbitrary, sometimes these units were introduced only because of the convenience of measuring them. For example, to measure atmospheric pressure, mm of mercury was injected. The famous experience of Torricelli, in which a tube filled with mercury was used, made it possible to introduce such an unusual value.

The power of the engines was compared with the horsepower (which is practiced in our time).

Different physical quantities measured physical quantities made not only complex and unreliable, but also complicating the development of science.

Unified system of measures

A single system of physical quantities, convenient and optimized in each industrialized country, has become an urgent necessity. As a basis, the idea of choosing the fewest possible units was adopted, with the help of which other quantities could be expressed in mathematical relationships. Such basic values should not be related to each other, their significance is determined unambiguously and understandably in any economic system.

This problem was solved in different countries. The creation of a unified system of measures (Metric, GHS, ISS, and others) has been undertaken many times, but these systems have been inconvenient either from the scientific point of view or in domestic, industrial applications.

The task, set at the end of the 19th century, was decided only in 1958. At the meeting of the International Committee of Legal Metrology a unified system was presented.

Unified system of measures

1960 was marked by the historic meeting of the General Conference on Measures and Weights. A unique system called "Systeme internationale d'unites" (abbreviated SI) was adopted by the decision of this honorary meeting. In the Russian version, this system is called the International System (abbreviation SI).

Based on 7 basic units and 2 additional ones. Their numerical value is determined in the form of a standard

Table of physical quantities SI

Name of the main unit	Measured value	Designation
		International	Russian
Basic units
kilogram	Weight	Kg	Kg
meter	Length	M	M
second	Time	S	from
ampere	Current strength	A	A
Kelvin	Temperature	TO	TO
Mole	Amount of substance	Mol	Mole
Candela	The power of light	Cd	Cd
Additional units
Radian	Flat angle	Rad	glad
Steradian	Solid angle	Sr	Wed

The system itself can not consist of only seven units, since the variety of physical processes in nature requires the introduction of more and more new quantities. In the structure itself, not only the introduction of new units is envisaged, but also their interrelation in the form of mathematical relationships (they are often called dimensional formulas).

A unit of physical quantity is obtained with the use of multiplication, exponentiation, and division of basic units in the dimension formula. The absence of numerical coefficients in such equations makes the system not only convenient in all respects, but also coherent.

Derived units

Units of measurement, which are formed from seven basic, are called derivatives. In addition to the basic and derived units, there was a need to introduce additional (radians and steradians). Their dimension is considered to be zero. The absence of measuring devices for their determination makes it impossible to measure them. Their introduction is due to the use in theoretical studies. For example, the physical quantity "force" in this system is measured in newtons. Since force is a measure of the mutual action of bodies on each other, which is the reason for the variation of the velocity of a body of a certain mass, it can be determined as the product of a unit of mass per unit velocity divided by the unit of time:

F = k0M0v / T, where k - coefficient of proportionality, M - unit of mass, v - unit of speed, T - unit of time.

SI gives the following formula of dimensions: H = kg0m / s ² , where three units are used. And a kilogram, and a meter, and a second are referred to the main ones. The coefficient of proportionality is 1.

It is possible to introduce dimensionless quantities, which are defined as a ratio of homogeneous quantities. The friction coefficient, as is known, is equal to the ratio of the friction force to the force of normal pressure.

Table of physical quantities derived from the main

Unit name	Measured value	Dimension formula
Joule	energy	Kg0m 2 0s -2
Pascal	pressure	Kg0 m -1 0s -2
Tesla	magnetic induction	Kg 0A -1 0s -2
Volt	Electrical stress	Kg 0m 2 0s -3 0A -1
Om	Electrical resistance	Kg 0m 2 0s -3 0A -2
pendant	Electric charge	A0 s
Watt	power	Kg 0m 2 0s -3
Farad	Electrical capacity	M -2 0kg -1 0c 4 0A 2
Joule to Calvin	Heat capacity	Kg 0m 2 0s -2 0K -1
Becquerel	Activity of radioactive substance	C -1
Weber	Magnetic flow	M 2 0kg 0s -2 0A -1
Henry	Inductance	M 2 0kg 0s -2 0A -2
Hertz	Frequency	S -1
Gray	The absorbed dose	M 2 0s -1
Sievert	Equivalent radiation dose	M 2 0s -2
Suite	Illumination	M -2 0кд 0ср -2
Lumen	Light flow	Cd 0sp
Newton	Strength, weight	M 0kg 0s -2
Siemens	Electrical conductivity	M -2 0кг -1 0с 3 0А 2
Farad	Electrical capacity	M -2 0kg -1 0c 4 0A 2

Extrasystem units

The use of historically developed quantities that are not included in the SI or differ only in a numerical coefficient is allowed in the measurement of quantities. These are extrasystem units. For example, mm of mercury, X-ray and others.

Numerical coefficients are used to introduce the lobes and multiples. Attachments correspond to a certain number. Examples include centi-, kilo-, deca-, mega-, and many others.

1 kilometer = 1000 meters,

1 centimeter = 0.01 meters.

Typology of quantities

Let's try to specify a few basic features that allow you to set the type of value.

1. Direction. If the action of a physical quantity is directly related to a direction, it is called a vector, and some are scalar.

2. Presence of dimension. The existence of a formula of physical quantities makes it possible to call them dimensional. If in the formula all units have zero degree, then they are called dimensionless. It would be more correct to call them quantities with a dimension equal to 1. In fact, the concept of a dimensionless quantity is illogical. The main property is the dimension - nobody canceled it!

3. If possible additions. An additive value whose value can be added, subtracted, multiplied by a coefficient, etc. (for example, mass) is a physical quantity that is summable.

4. In relation to the physical system. Extensive - if its value can be made up from the values of the subsystem. An example is the area measured in square meters. Intensive - a value whose value does not depend on the system. These include temperature.