Physics as a science studying natural phenomena uses a standard research technique. The main stages can be called: observation, hypothesis, experiment, justification of the theory. During the observation, the distinguishing features of the phenomenon, the course of its course, possible causes and consequences are established. The hypothesis allows us to explain the course of the phenomenon, to establish its laws. The experiment confirms (or does not confirm) the validity of the hypothesis. Allows you to establish a quantitative ratio of values during the experiment, which leads to the exact establishment of dependencies. The hypothesis confirmed in the course of the experiment forms the basis of the scientific theory.
No theory can claim to be reliable if it has not received full and unconditional confirmation during the experiment. Carrying out the latter is associated with measurements of physical quantities characterizing the process. Physical quantity is the basis of measurements.
What it is
The measurement concerns those values that confirm the validity of the hypothesis of patterns. Physical quantity is a scientific characteristic of a physical body, the qualitative relationship of which is common to many similar bodies. For each body, such a quantitative characteristic is purely individual.
If we turn to the specialized literature, then in the reference book of M. Yudin et al. (1989 edition) we read that physical quantity is: “a characteristic of one of the properties of a physical object (physical system, phenomenon or process), common in qualitative terms for many physical objects, but quantitatively individual for each object ”.
Ozhegov's Dictionary (1990 edition) states that the physical quantity is "the size, volume, length of the subject."
For example, length is a physical quantity. The mechanics interpret the length as the distance traveled, the electrodynamics use the length of the wire, in thermodynamics a similar value determines the thickness of the walls of the vessels. The essence of the concept does not change: units of quantities can be the same, and the value can be different.
A distinctive feature of a physical quantity, say, from a mathematical one, is the presence of a unit of measure. Meter, foot, arshin - examples of length units.
Units
In order to measure a physical quantity, it should be compared with a quantity accepted as a unit. Remember the wonderful cartoon "Forty-eight Parrots." To establish the length of a boa constrictor, the heroes measured its length either in parrots, then in elephants, or in monkeys. In this case, the length of the boa constrictor was compared with the growth of other cartoon characters. The result quantitatively depended on the standard.
A unit of a physical quantity is a measure of its measurement in a certain system of units. The confusion in these measures arises not only due to imperfection, heterogeneity of measures, but sometimes because of the relativity of units.
Russian measure of length - arshin - the distance between the index and thumb. However, the hands of all people are different, and the arshin measured by the hand of an adult male is different from the arshin on the hand of a child or woman. The same discrepancy between the measures of length concerns the fathom (the distance between the tips of the fingers of the arms extended apart) and the elbow (the distance from the middle finger to the elbow of the hand).
Interestingly, the clerks took men of small stature to the shops. Tricky merchants saved fabric with the help of several smaller measures: arshin, elbow, and fathom.
Systems of measures
Such a variety of measures existed not only in Russia, but also in other countries. The introduction of units of measurement was often arbitrary, sometimes these units were introduced only because of the convenience of measuring them. For example, mm of mercury was introduced to measure atmospheric pressure. The well-known experience of Torricelli, in which a tube filled with mercury was used, made it possible to introduce such an unusual value.
Engine power was compared with
horsepower (which is practiced in our time).
Different physical quantities made measuring physical quantities not only complex and unreliable, but also complicating the development of science.
Unified system of measures
A unified system of physical quantities, convenient and optimized in every industrialized country, has become an urgent need. The idea of choosing the smallest possible number of units with the help of which other quantities could be expressed in mathematical relations was taken as the basis. Such basic values should not be related to each other, their value is determined unambiguously and clearly in any economic system.
They tried to solve this problem in various countries. The creation of a unified system of measures (Metric, GHS, ISS and others) has been undertaken repeatedly, but these systems were inconvenient either from a scientific point of view or in domestic, industrial applications.
The task set at the end of the 19th century was solved only in 1958. At a meeting of the International Committee of Legal Metrology, a unified system was introduced.
Unified system of measures
The year 1960 was marked by a historic meeting of the General Conference on Weights and Measures. A unique system called the “Systeme internationale d'unites” (abbreviated SI) was adopted by the decision of this honorable meeting. In the Russian version, this system is called the International System (SI abbreviation).
Based on 7 basic units and 2 additional. Their numerical value is determined as a standard
SI physical quantity table
Name of the main unit | Measured value | Designation |
International | Russian |
Basic units |
kilogram | Weight | kg | kg |
meter | Length | m | m |
second | Time | s | with |
ampere | Amperage | 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 cannot consist of only seven units, since the diversity of physical processes in nature requires the introduction of ever new quantities. The structure itself provides not only the introduction of new units, but also their relationship in the form of mathematical relationships (they are often called dimensional formulas).
A unit of physical quantity is obtained using multiplication, exponentiation, and division of the 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 (consistent).
Derivative Units
Units of measurement, which are formed from seven basic units, are called derivatives. In addition to the basic and derived units, it became necessary to introduce additional (radian and steradian). Their dimension is considered to be zero. The absence of measuring instruments for their determination makes their measurement impossible. 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 one another, which is the reason for varying the speed of a body of a certain mass, it can be defined as the product of a unit of mass by a unit of speed divided by a unit of time:
F = k٠M٠v / T, where k is the proportionality coefficient, M is the unit of mass, v is the unit of speed, T is the unit of time.
SI gives the following dimensional formula: N = kg٠m / s 2 , where three units are used. And a kilogram, and a meter, and a second are assigned to the main ones. The proportionality coefficient is 1.
It is possible to introduce dimensionless quantities, which are defined as the ratio of homogeneous quantities. These include the coefficient of friction, as is known, equal to the ratio of the friction force to the force of normal pressure.
Table of physical quantities derived from basic
Unit Name | Measured value | Dimension Formula |
Joule | energy | kg٠m 2 ٠s -2 |
Pascal | pressure | kg٠ m -1 ٠s -2 |
Tesla | magnetic induction | kg ٠A -1 ٠s -2 |
Volt | electrical voltage | kg ٠m 2 ٠s -3 ٠A -1 |
Ohm | Electrical resistance | kg ٠m 2 ٠s -3 ٠A -2 |
pendant | Electric charge | A s |
Watt | power | kg ٠m 2 ٠s -3 |
Farad | Electric capacity | m -2 ٠kg -1 ٠c 4 ٠A 2 |
Joule on kelvin | Heat capacity | kg ٠m 2 ٠s -2 ٠K -1 |
Becquerel | Activity of a radioactive substance | S -1 |
Weber | Magnetic flux | m 2 ٠kg ٠s -2 ٠A -1 |
Henry | Inductance | m 2 ٠kg ٠s -2 ٠A -2 |
Hertz | Frequency | s -1 |
Gray | Absorbed dose | m 2 ٠s -1 |
Sievert | Equivalent dose of radiation | m 2 ٠s -2 |
Suite | Illumination | m -2 ٠cd ٠avr -2 |
Lumen | Light flow | cd ٠av |
Newton | Strength, Weight | m ٠kg ٠s -2 |
Siemens | Electrical conductivity | m -2 ٠kg -1 ٠s 3 ٠A 2 |
Farad | Electric capacity | m -2 ٠kg -1 ٠c 4 ٠A 2 |
Extra system units
The use of historically established values that are not included in SI or differ only in a numerical coefficient is allowed when measuring quantities. These are off-system units. For example, mmHg, X-ray and others.
Numeric coefficients are used to enter fractional and multiple values. Prefixes correspond to a certain number. Examples include Santi, Kilo, Deck, Mega, and many others.
1 kilometer = 1000 meters
1 centimeter = 0.01 meters.
Typology of quantities
We will try to indicate several basic features that allow us to establish the type of quantity.
1. Direction. If the action of a physical quantity is directly related to the direction, it is called vector, others are scalar.
2. The presence of dimension. The existence of the formula of physical quantities makes it possible to call them dimensional. If in the formula all units have a zero degree, then they are called dimensionless. It would be more correct to call them quantities with a dimension equal to 1. After all, the concept of a dimensionless quantity is illogical. The main property - dimension - has not been canceled!
3. If possible addition. An additive quantity whose value can be added, subtracted, multiplied by a coefficient, etc. (for example, mass) is a physical quantity that is cumulative.
4. In relation to the physical system. Extensive - if its value can be composed of the values of the subsystem. An example is the area measured in square meters. Intensive - a value whose value is independent of the system. These include temperature.