Quantities and their measurements table. Basic physical quantities and units of their measurement

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1 Pa = 1 N/m2 = 1 kg/(m s2)

The unit of pressure closest to the SI is the bar (bar), a size that is very convenient for practice (1 bar = 1,105 Pa).

In the liquid manometers used so far, the measure of the measured pressure is the height of the liquid column. Therefore, it is natural to use units of pressure determined by the height of the liquid column, i.e., based on units of length. In countries with metric systems of measures, the units of pressure are millimeter and meter of water column (mm of water column and m of water column) and millimeter of mercury (mm of mercury).

The dimensions of these pressure units are converted to SI units based on the formula

where H is the height of the liquid column, m, p is the density of the liquid, kg/m3, g is the free fall acceleration, m/s2.

1) Vacuum gauges are often called pressure gauges designed to measure low absolute pressures, significantly lower than atmospheric pressure (in vacuum technology).

Methods and means of measuring pressure

Methods for measuring pressure largely predetermine both the principles of operation and the design features of measuring instruments. In this regard, first of all, we should dwell on the most general methodological issues of pressure measurement technology.

Pressure, based on the most general positions, can be determined both by its direct measurement, and by measuring another physical quantity that is functionally related to the measured pressure.

In the first case, the measured pressure acts directly on the sensitive element of the device, which transmits information about the pressure value to the subsequent links of the measuring chain, which converts it into the required form. This method of determining pressure is a method of direct measurements, and is most widely used in pressure measurement technology. It is the basis for the operation of most pressure gauges and pressure transmitters.

In the second case, other physical quantities or parameters are directly measured that characterize the physical properties of the measured medium, the values ​​of which are naturally related to pressure (the boiling point of a liquid, the speed of propagation of ultrasound, the thermal conductivity of a gas, etc.). This method is a method of indirect pressure measurements and is used, as a rule, in cases where the direct method is not applicable for one reason or another, for example, when measuring ultra-low pressure (vacuum technique) or when measuring high and ultra-high pressures.

Pressure is a derivative physical quantity determined by three basic physical quantities - mass, length and time. The specific implementation of the pressure value depends on how the unit of pressure is represented. When measured by formula (1), pressure is determined by force and area, and by formula (2) - by length, density and acceleration. Methods for determining pressure, based on the measurement of these quantities, are absolute (fundamental) methods and are used when reproducing the unit of pressure by weight-piston and liquid-type standards, and also allow, if necessary, to certify exemplary measuring instruments.

The relative measurement method, in contrast to the absolute one, is based on a preliminary study of the pressure dependence of the physical properties and parameters of the sensitive elements of pressure measuring instruments using direct methods, measurements or other physical quantities and properties of the measured medium - using indirect measurement methods. For example, strain gauges, before being used to measure pressure, must first be calibrated to standard measuring instruments of appropriate accuracy.

In addition to classification according to the main measurement methods and types of pressure, pressure measuring instruments are classified according to the principle of operation, functionality, range and accuracy of measurements.

The most significant classification feature is the principle of operation of a pressure measuring instrument, in accordance with which the further presentation is built.

Modern pressure measuring instruments are measuring systems, the links of which have different functional purposes. Generalized block diagrams of pressure gauges and pressure transducers are shown in fig. 1, a and b. The most important link of any pressure measuring instrument is its sensitive element (SE), which perceives the measured pressure and converts it into the primary signal entering the instrument's measuring circuit. With the help of intermediate converters, the signal from the SE is converted into pressure gauge readings or recorded by it, and in measuring converters (IND) - into a unified output signal that enters the measurement, control, regulation and control systems. At the same time, intermediate converters and secondary devices are in many cases unified and can be used in combination with various types of SE. Therefore, the fundamental features of pressure gauges and IPD depend, first of all, on the type of SE.

This lesson will not be new for beginners. We all heard from school such things as a centimeter, a meter, a kilometer. And when it came to mass, they usually said grams, kilograms, tons.

Centimeters, meters and kilometers; grams, kilograms and tons have one common name - units of measurement of physical quantities.

In this lesson, we will look at the most popular units of measurement, but we will not go deep into this topic, since units of measurement go into the realm of physics. Today we are forced to study part of physics, as we need it for further study of mathematics.

Lesson content

Length units

The following units of measurement are used to measure length:

• millimeters;
• centimeters;
• decimeters;
• meters;
• kilometers.

millimeter(mm). You can even see millimeters with your own eyes if you take the ruler that we used at school every day.

Small lines that follow each other in a row are millimeters. More precisely, the distance between these lines is one millimeter (1 mm):

centimeter(cm). On the ruler, each centimeter is indicated by a number. For example, our ruler, which was in the first figure, had a length of 15 centimeters. The last centimeter on this ruler is marked with the number 15.

There are 10 millimeters in one centimeter. You can put an equal sign between one centimeter and ten millimeters, since they denote the same length:

1cm=10mm

You can see for yourself if you count the number of millimeters in the previous figure. You will find that the number of millimeters (distance between lines) is 10.

The next unit of length is decimeter(dm). There are ten centimeters in one decimeter. Between one decimeter and ten centimeters, you can put an equal sign, since they denote the same length:

1 dm = 10 cm

You can verify this if you count the number of centimeters in the following figure:

You will find that the number of centimeters is 10.

The next unit of measure is meter(m). There are ten decimeters in one meter. Between one meter and ten decimeters, you can put an equal sign, since they denote the same length:

1 m = 10 dm

Unfortunately, the meter cannot be illustrated in the figure, because it is rather large. If you want to see the meter live, take a tape measure. Everyone has it in the house. On a tape measure, one meter will be designated as 100 cm. This is because there are ten decimeters in one meter, and one hundred centimeters in ten decimeters:

1 m = 10 dm = 100 cm

100 is obtained by converting one meter to centimeters. This is a separate topic, which we will consider a little later. In the meantime, let's move on to the next unit of length, which is called a kilometer.

The kilometer is considered the largest unit of measurement for length. Of course, there are other older units, such as a megameter, a gigameter, a terameter, but we will not consider them, since a kilometer is enough for us to further study mathematics.

There are a thousand meters in one kilometer. You can put an equal sign between one kilometer and a thousand meters, since they denote the same length:

1 km = 1000 m

Distances between cities and countries are measured in kilometers. For example, the distance from Moscow to St. Petersburg is about 714 kilometers.

International system of units SI

The international system of units SI is a certain set of generally accepted physical quantities.

The main purpose of the international system of SI units is to reach agreements between countries.

We know that the languages ​​and traditions of the countries of the world are different. There's nothing to be done about it. But the laws of mathematics and physics work the same everywhere. If in one country “twice two is four”, then in another country “twice two is four”.

The main problem was that for each physical quantity there are several units of measurement. For example, we have just learned that there are millimeters, centimeters, decimeters, meters and kilometers for measuring length. If several scientists speaking different languages ​​gather in one place to solve some problem, then such a large variety of length units can give rise to contradictions between these scientists.

One scientist will claim that in their country length is measured in meters. The second might say that in their country, length is measured in kilometers. The third one can offer his own unit of measure.

Therefore, the international system of units SI was created. SI is an abbreviation for the French phrase Le Système International d'Unités, SI (which in Russian means - the international system of units SI).

The SI lists the most popular physical quantities and each of them has its own generally accepted unit of measurement. For example, in all countries, when solving problems, it was agreed that the length would be measured in meters. Therefore, when solving problems, if the length is given in another unit of measurement (for example, in kilometers), then it must be converted to meters. We will talk about how to convert one unit of measure to another a little later. In the meantime, let's draw our international system of units SI.

Our drawing will be a table of physical quantities. We will include each studied physical quantity in our table and indicate the unit of measurement that is accepted in all countries. Now we have studied the units of measurement of length and learned that meters are defined in the SI system for measuring length. So our table will look like this:

Mass units

Mass is a measure of the amount of matter in a body. In the people, body weight is called weight. Usually, when something is weighed, they say "it weighs so many kilograms" , although we are not talking about weight, but about the mass of this body.

However, mass and weight are different concepts. Weight is the force with which a body acts on a horizontal support. Weight is measured in newtons. And mass is a quantity that shows the amount of matter in this body.

But there is nothing wrong with calling the mass of the body weight. Even in medicine they say "human weight" , although we are talking about the mass of a person. The main thing is to be aware that these are different concepts.

The following units of measure are used to measure mass:

• milligrams;
• grams;
• kilograms;
• centners;
• tons.

The smallest unit of measurement is milligram(mg). Milligram most likely you will never put into practice. They are used by chemists and other scientists who work with small substances. It is enough for you to know that such a unit of mass measurement exists.

The next unit of measure is gram(G). In grams, it is customary to measure the amount of a product when compiling a recipe.

There are a thousand milligrams in one gram. You can put an equal sign between one gram and a thousand milligrams, since they denote the same mass:

1 g = 1000 mg

The next unit of measure is kilogram(kg). The kilogram is a common unit of measure. It measures everything. The kilogram is included in the SI system. Let's also include one more physical quantity in our SI table. We will call it "mass":

There are a thousand grams in one kilogram. Between one kilogram and a thousand grams, you can put an equal sign, since they denote the same mass:

1 kg = 1000 g

The next unit of measure is centner(c). In centners, it is convenient to measure the mass of a crop harvested from a small area or the mass of some kind of cargo.

There are one hundred kilograms in one centner. An equal sign can be put between one centner and one hundred kilograms, since they denote the same mass:

1 q = 100 kg

The next unit of measure is ton(t). In tons, large loads and masses of large bodies are usually measured. For example, the mass of a spaceship or a car.

There are a thousand kilograms in one ton. You can put an equal sign between one ton and a thousand kilograms, since they denote the same mass:

1 t = 1000 kg

Time units

We don't need to explain what time is. Everyone knows what time is and why it is needed. If we open the discussion to what time is and try to define it, then we will begin to delve into philosophy, and this is not what we need now. Let's start with time units.

The following units of measurement are used to measure time:

• seconds;
• minutes;
• clock;
• day.

The smallest unit of measurement is second(with). Of course, there are also smaller units such as milliseconds, microseconds, nanoseconds, but we will not consider them, since at the moment there is no point in this.

In seconds, various indicators are measured. For example, how many seconds does it take an athlete to run 100 meters. The second is included in the international SI system of units for measuring time and is denoted as "s". Let's also include one more physical quantity in our SI table. We will call it "time":

minute(m). There are 60 seconds in one minute. You can put an equal sign between one minute and sixty seconds, since they represent the same time:

1 m = 60 s

The next unit of measure is hour(h). There are 60 minutes in one hour. You can put an equal sign between one hour and sixty minutes, since they represent the same time:

1 h = 60 m

For example, if we studied this lesson for one hour and we are asked how much time we spent studying it, we can answer in two ways: "we studied the lesson for one hour" or so "we studied the lesson for sixty minutes" . In both cases, we will answer correctly.

The next unit of time is day. There are 24 hours in a day. Between one day and twenty-four hours you can put an equal sign, since they denote the same time:

1 day = 24 hours

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Since 1963, in the USSR (GOST 9867-61 "International System of Units"), in order to unify units of measurement in all fields of science and technology, the international (international) system of units (SI, SI) has been recommended for practical use - this is a system of units for measuring physical quantities , adopted by the XI General Conference on Weights and Measures in 1960. It is based on 6 basic units (length, mass, time, electric current, thermodynamic temperature and light intensity), as well as 2 additional units (flat angle, solid angle) ; all other units given in the table are their derivatives. The adoption of a single international system of units for all countries is intended to eliminate the difficulties associated with translating the numerical values ​​of physical quantities, as well as various constants from any one currently operating system (CGS, MKGSS, ISS A, etc.), into another.

Value name	Units; SI values	Notation
		Russian	international
I. Length, mass, volume, pressure, temperature
	Meter - a measure of length, numerically equal to the length of the international standard of the meter; 1 m=100 cm (1 10 2 cm)=1000 mm (1 10 3 mm)	m	m
	Centimeter \u003d 0.01 m (1 10 -2 m) \u003d 10 mm	cm	cm
	Millimeter \u003d 0.001 m (1 10 -3 m) \u003d 0.1 cm \u003d 1000 microns (1 10 3 microns)	mm	mm
	Micron (micrometer) = 0.001 mm (1 10 -3 mm) = 0.0001 cm (1 10 -4 cm) = 10,000	mk	μ
	Angstrom = one ten billionth of a meter (1 10 -10 m) or one hundred millionth of a centimeter (1 10 -8 cm)	Å	Å
Weight	Kilogram - the basic unit of mass in the metric system of measures and the SI system, numerically equal to the mass of the international standard of the kilogram; 1 kg=1000 g	kg	kg
	Gram \u003d 0.001 kg (1 10 -3 kg)	G	g
	Ton = 1000 kg (1 10 3 kg)	t	t
	Centner \u003d 100 kg (1 10 2 kg)	c
	Carat - non-systemic unit of mass, numerically equal to 0.2 g		ct
	Gamma=one millionth of a gram (1 10 -6 g)		γ
Volume	Liter \u003d 1.000028 dm 3 \u003d 1.000028 10 -3 m 3	l	l
Pressure	Physical, or normal, atmosphere - pressure balanced by a mercury column 760 mm high at a temperature of 0 ° = 1.033 at = = 1.01 10 -5 n / m 2 = 1.01325 bar = 760 torr = 1.033 kgf / cm 2	atm	atm
	Technical atmosphere - pressure equal to 1 kgf / cmg \u003d 9.81 10 4 n / m 2 \u003d 0.980655 bar \u003d 0.980655 10 6 dynes / cm 2 \u003d 0.968 atm \u003d 735 torr	at	at
	Millimeter of mercury column \u003d 133.32 n / m 2	mmHg Art.	mm Hg
	Tor - the name of an off-system unit of pressure measurement, equal to 1 mm Hg. Art.; given in honor of the Italian scientist E. Torricelli	torus
	Bar - unit of atmospheric pressure \u003d 1 10 5 n / m 2 \u003d 1 10 6 dynes / cm 2	bar	bar
Pressure (sound)	Bar-unit of sound pressure (in acoustics): bar - 1 dyne / cm 2; at present, a unit with a value of 1 n / m 2 \u003d 10 dynes / cm 2 is recommended as a unit of sound pressure	bar	bar
	The decibel is a logarithmic unit of measurement of the level of excess sound pressure, equal to 1/10 of the unit of measurement of excess pressure - white	dB	db
Temperature	Degree Celsius; temperature in °K (Kelvin scale), equal to temperature in °C (Celsius scale) + 273.15 °C	°C	°C
II. Force, power, energy, work, amount of heat, viscosity
Force	Dyna - a unit of force in the CGS system (cm-g-sec.), At which an acceleration equal to 1 cm / sec 2 is reported to a body with a mass of 1 g; 1 din - 1 10 -5 n	din	dyn
	Kilogram-force is a force imparting to a body with a mass of 1 kg an acceleration equal to 9.81 m / s 2; 1kg \u003d 9.81 n \u003d 9.81 10 5 din	kg, kgf
Power	Horsepower=735.5W	l. with.	HP
Energy	Electron-volt - the energy that an electron acquires when moving in an electric field in vacuum between points with a potential difference of 1 V; 1 ev \u003d 1.6 10 -19 j. Multiple units are allowed: kiloelectron-volt (Kv) = 10 3 eV and megaelectron-volt (MeV) = 10 6 eV. In modern particles, the energy is measured in Bev - billions (billions) eV; 1 Bzv=10 9 ev	ev	eV
	Erg=1 10 -7 J; erg is also used as a unit of work, numerically equal to the work done by a force of 1 dyne in a path of 1 cm	erg	erg
Work	Kilogram-force-meter (kilogrammeter) - a unit of work numerically equal to the work done by a constant force of 1 kg when the point of application of this force moves a distance of 1 m in its direction; 1kGm = 9.81 J (at the same time, kGm is a measure of energy)	kgm, kgf m	kgm
Quantity of heat	Calorie - an off-system unit for measuring the amount of heat equal to the amount of heat required to heat 1 g of water from 19.5 ° C to 20.5 ° C. 1 cal = 4.187 J; common multiple unit kilocalorie (kcal, kcal), equal to 1000 cal	feces	cal
Viscosity (dynamic)	Poise is a unit of viscosity in the CGS system of units; the viscosity at which a 1 dyne viscous force acts in a layered flow with a velocity gradient of 1 sec -1 per 1 cm 2 of the layer surface; 1 pz \u003d 0.1 n s / m 2	pz	P
Viscosity (kinematic)	Stokes is the unit of kinematic viscosity in the CGS system; equal to the viscosity of a liquid having a density of 1 g / cm 3, resisting a force of 1 dyne to the mutual movement of two layers of liquid with an area of ​​\u200b\u200b1 cm 2 located at a distance of 1 cm from each other and moving relative to each other at a speed of 1 cm per second	st	St
III. Magnetic flux, magnetic induction, magnetic field strength, inductance, capacitance
magnetic flux	Maxwell - a unit of measurement of magnetic flux in the cgs system; 1 μs is equal to the magnetic flux passing through the area of ​​1 cm 2 located perpendicular to the lines of induction of the magnetic field, with an induction equal to 1 gauss; 1 μs = 10 -8 wb (Weber) - units of magnetic current in the SI system	ms	Mx
Magnetic induction	Gauss is a unit of measure in the cgs system; 1 gauss is the induction of such a field in which a rectilinear conductor 1 cm long, located perpendicular to the field vector, experiences a force of 1 dyne if a current of 3 × 10 10 CGS units flows through this conductor; 1 gs \u003d 1 10 -4 t (tesla)	gs	Gs
Magnetic field strength	Oersted - unit of magnetic field strength in the CGS system; for one oersted (1 e) the intensity at such a point of the field is taken, in which a force of 1 dyne (dyne) acts on 1 electromagnetic unit of the amount of magnetism; 1 e \u003d 1 / 4π 10 3 a / m	uh	Oe
Inductance	Centimeter - a unit of inductance in the CGS system; 1 cm = 1 10 -9 gn (henry)	cm	cm
Electrical capacitance	Centimeter - unit of capacitance in the CGS system = 1 10 -12 f (farads)	cm	cm
IV. Light intensity, luminous flux, brightness, illumination
The power of light	A candle is a unit of luminous intensity, the value of which is taken so that the brightness of a full emitter at the solidification temperature of platinum is 60 sv per 1 cm 2	St.	cd
Light flow	Lumen - a unit of luminous flux; 1 lumen (lm) is radiated within a solid angle of 1 stere by a point source of light that has a luminous intensity of 1 St in all directions.	lm	lm
	Lumen-second - corresponds to the light energy generated by a luminous flux of 1 lm, emitted or perceived in 1 second	lm s	lm sec
	Lumen hour equals 3600 lumen seconds	lm h	lm h
Brightness	Stilb is a unit of brightness in the CGS system; corresponds to the brightness of a flat surface, 1 cm 2 of which gives in the direction perpendicular to this surface, a luminous intensity equal to 1 ce; 1 sb \u003d 1 10 4 nt (nit) (unit of brightness in the SI system)	Sat	sb
	Lambert is an off-system unit of brightness, derived from the stilb; 1 lambert = 1/π st = 3193 nt
	Apostille = 1 / π St / m 2
illumination	Fot - unit of illumination in the SGSL system (cm-g-sec-lm); 1 ph corresponds to the surface illumination of 1 cm 2 with a uniformly distributed luminous flux of 1 lm; 1 f \u003d 1 10 4 lux (lux)	f	ph
V. Radiation intensity and doses
Intensity	Curie is the basic unit for measuring the intensity of radioactive radiation, curie corresponding to 3.7·10 10 decays in 1 sec. any radioactive isotope	curie	C or Cu
	millicurie \u003d 10 -3 curie, or 3.7 10 7 acts of radioactive decay in 1 sec.	mcurie	mc or mCu
	microcurie = 10 -6 curie	microcurie	μC or μCu
Dose	X-ray - the amount (dose) of X-ray or γ-rays, which in 0.001293 g of air (i.e., in 1 cm 3 of dry air at t ° 0 ° and 760 mm Hg) causes the formation of ions that carry one electrostatic a unit of the amount of electricity of each sign; 1 p causes the formation of 2.08 10 9 pairs of ions in 1 cm 3 of air	R	r
	milliroentgen \u003d 10 -3 p	mr	mr
	microroentgen = 10 -6 p	microdistrict	µr
	Rad - the unit of the absorbed dose of any ionizing radiation is equal to rad 100 erg per 1 g of the irradiated medium; when air is ionized by X-rays or γ-rays, 1 p is equal to 0.88 rad, and when tissues are ionized, practically 1 p is equal to 1 rad	glad	rad
	Rem (X-ray biological equivalent) - the amount (dose) of any type of ionizing radiation that causes the same biological effect as 1 p (or 1 rad) of hard X-rays. The unequal biological effect with equal ionization by different types of radiation led to the need to introduce another concept: the relative biological effectiveness of radiation -RBE; the relationship between doses (D) and the dimensionless coefficient (RBE) is expressed as Drem =D rad RBE, where RBE=1 for x-rays, γ-rays and β-rays and RBE=10 for protons up to 10 MeV, fast neutrons and α - natural particles (on the recommendation of the International Congress of Radiologists in Copenhagen, 1953)	reb, reb	rem

Note. Multiple and submultiple units of measurement, with the exception of units of time and angle, are formed by multiplying them by the corresponding power of 10, and their names are attached to the names of units of measurement. It is not allowed to use two prefixes to the name of the unit. For example, you cannot write millimicrowatts (mmkw) or micromicrofarads (mmf), but you need to write nanowatts (nw) or picofarads (pf). You should not use prefixes to the names of such units that denote a multiple or submultiple unit of measurement (for example, micron). Multiple units of time may be used to express the duration of processes and designate calendar dates of events.

The most important units of the International System of Units (SI)

Basic units
(length, mass, temperature, time, electric current, light intensity)

Value name		Notation
		Russian	international
Length	A meter is a length equal to 1650763.73 wavelengths of radiation in vacuum, corresponding to the transition between levels 2p 10 and 5d 5 krypton 86 *	m	m
Weight	Kilogram - mass corresponding to the mass of the international standard of the kilogram	kg	kg
Time	Second - 1/31556925.9747 part of a tropical year (1900) **	sec	S, s
The strength of the electric current	Ampere - the strength of an unchanging current, which, passing through two parallel rectilinear conductors of infinite length and negligible circular cross section, located at a distance of 1 m from one another in a vacuum, would cause a force between these conductors equal to 2 10 -7 n for each meter length	a	A
The power of light	A candle is a unit of luminous intensity, the value of which is taken so that the brightness of a full (absolutely black) emitter at the solidification temperature of platinum is 60 ce per 1 cm 2 ***	St.	cd
Temperature (thermodynamic)	Degree Kelvin (Kelvin scale) - a unit of temperature measurement according to the thermodynamic temperature scale, in which the temperature of the triple point of water **** is set to 273.16 ° K	°K	°K

* That is, the meter is equal to the indicated number of radiation waves with a wavelength of 0.6057 microns, obtained from a special lamp and corresponding to the orange line of the spectrum of the neutral gas of krypton. This definition of the unit of length allows you to reproduce the meter with the greatest accuracy, and most importantly, in any laboratory that has the appropriate equipment. This eliminates the need to periodically check the standard meter with its international standard stored in Paris.
** That is, a second is equal to the specified part of the time interval between two successive passages of the Earth in orbit around the Sun of the point corresponding to the vernal equinox. This gives greater accuracy in determining the second than defining it as part of a day, since the length of the day varies.
*** That is, the luminous intensity of a certain reference source emitting light at the melting temperature of platinum is taken as a unit. The old International Candlestick Standard is 1.005 of the new Candlestick Standard. Thus, within the limits of usual practical accuracy, their values ​​can be considered as coinciding.
**** Triple point - melting temperature of ice in the presence of saturated water vapor above it.

Complementary and derived units

Value name	Units; their definition	Notation
		Russian	international
I. Flat angle, solid angle, force, work, energy, amount of heat, power
flat corner	Radian - the angle between two radii of a circle, cutting an arc on a circle rad, the length of which is equal to the radius	glad	rad
Solid angle	A steradian is a solid angle whose vertex is located in the center of the sphere ster and which cuts out on the surface of the sphere an area equal to the area of ​​a square with a side equal to the radius of the sphere	erased	sr
Force	Newton force, under the influence of which a body with a mass of 1 kg acquires an acceleration equal to 1 m / s 2	n	N
Work, energy, amount of heat	Joule - the work done by a constant force of 1 n acting on the body on a path of 1 m traveled by the body in the direction of the force	j	J
Power	Watt - the power at which for 1 sec. work done in 1 j	Tue	W
II. Quantity of electricity, electrical voltage, electrical resistance, electrical capacitance
Quantity of electricity, electric charge	Pendant - the amount of electricity flowing through the cross section of the conductor for 1 second. at a direct current of 1 a	to	C
Electrical voltage, electrical potential difference, electromotive force (EMF)	Volt - the voltage in the section of the electrical circuit, when passing through which the amount of electricity in 1 k, work is done in 1 j	in	V
Electrical resistance	Ohm - the resistance of the conductor, through which, at a constant voltage at the ends of 1 V, a direct current of 1 A passes	ohm	Ω
Electrical capacitance	Farad is the capacitance of a capacitor, the voltage between the plates of which changes by 1 V when it is charged with an amount of electricity of 1 kV.	f	F
III. Magnetic induction, magnetic flux, inductance, frequency
Magnetic induction	Tesla is the induction of a homogeneous magnetic field, which acts on a section of a rectilinear conductor 1 m long, placed perpendicular to the direction of the field, with a force of 1 n when a direct current of 1 a passes through the conductor	tl	T
Flux of magnetic induction	Weber - magnetic flux created by a uniform field with a magnetic induction of 1 t through an area of ​​1 m 2 perpendicular to the direction of the magnetic induction vector	wb	wb
Inductance	Henry is the inductance of a conductor (coil) in which an EMF of 1 V is induced when the current in it changes by 1 A in 1 sec.	Mr	H
Frequency	Hertz - the frequency of a periodic process, in which for 1 sec. one oscillation occurs (cycle, period)	Hz	Hz
IV. Luminous flux, light energy, brightness, illumination
Light flow	Lumen - the luminous flux that gives inside a solid angle of 1 ster a point source of light of 1 s, radiating equally in all directions	lm	lm
light energy	Lumen second	lm s	lm s
Brightness	Nit - the brightness of a luminous plane, each square meter of which gives in a direction perpendicular to the plane, a luminous intensity of 1 sv	nt	nt
illumination	Lux - illumination created by a luminous flux of 1 lm with its uniform distribution over an area of ​​1 m 2	OK	lx
Light quantity	lux second	lx sec	lx s

Consider a physical record m=4kg. In this formula "m"- designation of physical quantity (mass), "4" - numerical value or magnitude, "kg"- unit of measurement of a given physical quantity.

The values ​​are of different kinds. Here are two examples:
1) The distance between points, the lengths of segments, broken lines - these are quantities of the same kind. They are expressed in centimeters, meters, kilometers, etc.
2) The durations of time intervals are also quantities of the same kind. They are expressed in seconds, minutes, hours, etc.

Quantities of the same kind can be compared and added:

BUT! It is pointless to ask which is greater: 1 meter or 1 hour, and you cannot add 1 meter to 30 seconds. The duration of time intervals and distance are quantities of various kinds. They cannot be compared or combined.

Values ​​can be multiplied by positive numbers and zero.

Taking any value e per unit of measurement, it can be used to measure any other quantity a the same kind. As a result of the measurement, we get that a=x e, where x is a number. This number x is called the numerical value of the quantity a with unit of measure e.

There are dimensionless physical quantities. They do not have units of measurement, that is, they are not measured in anything. For example, the coefficient of friction.

What is SI?

According to Professor Peter Kampson and Dr. Naoko Sano of Newcastle University, published in the journal Metrology (Metrology), the kilogram standard adds an average of about 50 micrograms per hundred years, which can ultimately affect very many physical quantities.

The kilogram is the only SI unit that is still defined using a standard. All other measures (meter, second, degree, ampere, etc.) can be determined with the required accuracy in a physical laboratory. The kilogram is included in the definition of other quantities, for example, the unit of force is the newton, which is defined as the force that changes the speed of a 1 kg body by 1 m/s in the direction of the force in 1 second. Other physical quantities depend on the Newton value, so that in the end the chain can lead to a change in the value of many physical units.

The most important kilogram is a cylinder with a diameter and height of 39 mm, consisting of an alloy of platinum and iridium (90% platinum and 10% iridium). It was cast in 1889 and is stored in a safe at the International Bureau of Weights and Measures in the city of Sèvres near Paris. The kilogram was originally defined as the mass of one cubic decimeter (liter) of pure water at 4°C and standard atmospheric pressure at sea level.

Initially, 40 exact copies were made from the kilogram standard, which were sold all over the world. Two of them are located in Russia, at the All-Russian Research Institute of Metrology. Mendeleev. Later, another series of replicas was cast. Platinum was chosen as the base material for the reference because of its high oxidation resistance, high density, and low magnetic susceptibility. The standard and its replicas are used to standardize the mass in a wide variety of industries. Including where micrograms are essential.

Physicists believe that the fluctuations in weight were the result of atmospheric pollution and changes in the chemical composition in the surface of the cylinders. Despite the fact that the standard and its replicas are stored in special conditions, this does not save the metal from interacting with the environment. The exact weight of a kilogram was determined using X-ray photoelectron spectroscopy. It turned out that the kilogram “recovered” by almost 100 mcg.

At the same time, copies of the standard from the very beginning differed from the original and their weight also changes in different ways. So, the main American kilogram initially weighed 39 micrograms less than the standard, and a check in 1948 showed that it had increased by 20 micrograms. Another American copy, on the contrary, is losing weight. In 1889, the kilogram number 4 (K4) weighed 75 micrograms less than the standard, and in 1989 already 106.

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Electric current is characterized by such quantities as current strength, voltage and resistance, interconnected. Before considering the question of what voltage is measured in, it is necessary to find out exactly what this value is and what its role in the formation of current is.

How voltage works

The general concept of electric current is the directed movement of charged particles. These particles are electrons, the movement of which occurs under the influence of an electric field. The more charges you need to move, the more work is done by the field. This work is affected not only by the current strength, but also by the voltage.

The physical meaning of this value lies in the fact that the work of the current in any section of the circuit is correlated with the amount of charge that passes through this section. In the process of this work, a positive charge moves from a point where there is a small potential to a point with a large potential value. Thus, voltage is defined as or electromotive force, and work itself is energy.

The work of an electric current is measured in joules (J), and the amount of electric charge is a pendant (C). As a result, the voltage is a ratio of 1 J/C. The resulting unit of voltage is called the volt.

To clearly explain the physical meaning of stress, you need to refer to the example of a hose filled with water. In this case, the volume of water will play the role of current, and its pressure will be equivalent to voltage. When water moves without a tip, it moves freely and in large quantities through the hose, creating low pressure. If you press the end of the hose with your finger, then there will be a decrease in volume while increasing water pressure. The jet itself will travel a much greater distance.

The same thing happens in electricity. The strength of the current is determined by the number or volume of electrons moving through the conductor. The voltage value, in fact, is the force with which these electrons are pushed. It follows that, under the condition of the same voltage, a conductor that conducts a larger amount of current must also have a larger diameter.

Voltage unit

The voltage can be constant or variable, depending on the current. This value can be denoted as the letter B (Russian designation) or V, corresponding to the international designation. To indicate alternating voltage, the symbol "~" is used, which is placed in front of the letter. For constant voltage, there is a “-” sign, but in practice it is almost never used.

When considering the question of what voltage is measured in, it should be remembered that for this there are not only volts. Larger values ​​are measured in kilovolts (kV) and megavolts (mV), which means 1 thousand and 1 million volts, respectively.

How to measure voltage and current