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.