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# Physical quantities

Perception of the physical quantities

The experiment is the basis of every objective perception of nature. The aim of the experiment is to determine parameters on which the given natural concept depends. Those parameters we call physical quantities which can be understood as abstractions, we use in order to describe the concepts. The first step to understand physical concepts is determining parameters on which those concepts depend on. After that we can perform the measuring, comparing with given values of quantities which we call units.

The further step in the initiation with the physical concepts is to connect the derived physical quantities into mathematic formulas, which show the physical laws. So, for example, to describe the drop we use the equation v=g*t so the different physical quantities are connected into one physical law. It is understandable that the number of physical quantities can be very big. Because of that physical quantities are divided into basic physical quantities and derived physical quantities.

The basic physical quantities

Today's development of physics shows that the whole range of concepts researched by modern physics can be shown through the following basic physical quantities:

3 mechanical quantities

1 electric quantity

1 thermic quantity

1 photometric quantity

1 atomistic quantity

In mechanics we have the following physical quantities: length, time, mass and sometimes force.

In electric measurements there is a fourth physical quantity introduced in 1901. by Giorgi and is called the strength of electricity. All other quantities are derived and can be expressed by means of three mechanical quantities and strength of electricity.

For the field of thermodynamics it is necessary to introduce another physical quantity: temperature.

For the photometric measurements 3 basic quantities are necessary: two kinetic,  the length and time and the forth the light flow. The other possibility is to define the analogue strength in electrodynamics. That quantity will be called light strength.

When performing measurements in the nuclear physics we except for the mechanical quantities also need another specific quantity, a number of equal for the given problem equivalent units, for example atom, molecule, radicals, electrons etc.

It is necessary to know the number of individual particles. As that number can be determined only in the exceptional cases, we instead of that take the quantity of material , proportional to the number of particles. As the consequence of discontinuity of  structure of the material, the factor of the proportionality among the quantity of material and number of particles is the universal constant (Avangard's number AO = NO/m). So, as the basic quantity that is based on the number of the particles is introduced the quantity of material, which will be defined as 1 mol.

The 11th General conference for the weights and measures, held in October 1960. accepted the following basic quantities as quantities which will form the International system of measures (Systeme International d' Unites, symbol SI):

-         length (mechanics)

-         time (mechanics)

-         mass (mechanics)

-         electricity strength (electrodynamics)

-         temperature (thermodynamics)

-         light strength (photometry)

The 14th General international conference for weights and measures added in 1971. the seventh basic physical quantity:

-         the quantity of material (nuclear physic)

The measuring systems; units

The measuring systems

The set of units of basic physical quantities we call the measuring system. The measuring system generally comprises the units of basic physical quantities determined by the given field of physics. So, we are talking about the measuring system in mechanics or in thermodynamics; each of those systems includes certain number of units.

The history of metric measuring systems

The necessity to measure and compare the quantities is above all the practical problems that appeared already in the beginning of civilization. It is natural that the fist quantities for length, weight and time were connected to objects or time intervals for example parts of the body. With the broader development of international exchange there appeared the necessity to standardize measuring units. The social revolution was necessary in order to throw away the traditional units and accept that what is today called the international unit system.

On the 23rd of  August 1790., the French constituent assembly, which aroused during the French revolution, gives a task to the French Academy of Science to prepare the unified system of measures for length (as well as surface and volume). On the 9th of March, next year the Academy proposes that the system is decimal and that it's basis be the tenmilionth part of the squate of the Earth's meridian. Delambre and Mchain have measured that the square of the Parisian meridian between Dunquerque and Barcelona is approx. 9°30′, it was by Law accepted on the 7th of April 1975, by the French government that the measure for length is meter, defined as tenmilionth part of the square of the Earth's meridian. At the same time the units for surface and volume were defined, as well as for mass (kilogram). The samples of the ancient kilogram and ancient meter were kept in the French national archive in 1799. The metric system has spread into most European countries in the 19th century. In the meantime the unit for measuring time still stay in the old system. At the same time in other branches of physics (electricity and magnetism) the international systems of units are accepted.

The international system of measuring units (SI)

The international system of measuring units consists of the following units:

-         for length - meter (m)

-         for mass - kilogram (kg)

-         for time - second (s)

-         for the electricity strength - ampere (A)

-         for thermodynamic temperature - Calvin (K)

-         for the light strength - Candel (cd)

-         for the quantity of material - mol (mol)

The definition of the SI units

1 meter is the length equal to 1 650763,73 wavelength in the vacuum radiation which is correspondent to the transforming of the atom nuklid 86  Kr from the state 5 d5 in the state 2p10.

1 kilogram is the mass of the international ancient kilogram which is kept in Sevres near Paris.

1 second lasts 9 192 631 770 periods of radiation which corresponds to transforming between two superfine level of basic atom nuklid 133Cs.

1 ampere is the strength of the stabile electricity which passes through two straight line unmeasured long conductors of very small intersection, distanced for 1 meter in vacuum, cause among them electro- dynamic force 2*10-7 N/m.

1 Calvin is 273,16th part of thermo-dynamic temperature of the inertial point of the chemically pure water in the natural isotopic mixture.

1 Candel is the light strength in the direction vertical to the leak of the surface of the 1/600 000 m2 if the black body on the temperature of melting point of platinum under the pressure of 101 325 m-1-kgs-2 (later that unit was abandoned as being imprecise).

1 mol is the quantity of material in the system which withholds as many equal individual particles as there are atoms in the 0,012 kg of isotope of carbon 12C.

Other measuring systems in physics and technical sciences

CGS system was once generally accepted and is still often used today. This system is based on three metric units: centimeter for length, gram for mass and second for time.

Technical or M Kp S system. In the original definition of the metric system kilogram was defined as the unit for mass. The concept difference , important in physics, that exists among mass and weight was not always noticed in practice. That's why kilogram is often used as unit for weight - force. At that case we call the unit for force kilopond (kp) - 1 kilopond is the weight of the mass 1 kg.  On the place of normal Earth acceleration - 1 kp = 9,806 66 N = 1000 ponds.

Some physical units out of measuring systems

Although the unit of SI system are accepted in most countries it is often that in general use are some units out of this system. The reason for this is tradition, but also the fact that some of SI units being rather unpractical.

Pressure 1 physical (normal) atmosphere (atm) is defined as the pressure 0,76 m high tower of liquid of 13 595,1 kg m-3 density (approx. mercury at 0°C) on the place of the normal Earth acceleration - 1 atm= 101 325 Pa

Work and energy 1 kilowatt hour is the work done on the constant power of 1 kW in one hour time. - 1kWh = 3,6*10 on 6J.

The quantity of heat 1 thermo-chemical calorie is defined as 4,1840 J. 1 international calorie was originally defined as 1/880 international Wh - 1 caliT= 4,1868 J. 1 water calorie is defined as the quantity of heat needed to heat 1 kg of pure water from 14,5° to 15.5 °C under the normal pressure of p0 = 1 atm. - 1 cal= 4,1855 J.

cal = 4,1855 J.

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