Symbol of strength. Basic physical quantities, their letter designations in physics

It's no secret that there are special notations for quantities in any science. Letter designations in physics prove that this science is no exception in terms of identifying quantities using special symbols. There are quite a lot of basic quantities, as well as their derivatives, each of which has its own symbol. So, letter designations in physics are discussed in detail in this article.

Physics and basic physical quantities

Thanks to Aristotle, the word physics began to be used, since it was he who first used this term, which at that time was considered synonymous with the term philosophy. This is due to the commonality of the object of study - the laws of the Universe, more specifically - how it functions. As you know, the first scientific revolution took place in the 16th-17th centuries, and it was thanks to it that physics was singled out as an independent science.

Mikhail Vasilyevich Lomonosov introduced the word physics into the Russian language by publishing a textbook translated from German - the first physics textbook in Russia.

So, physics is a branch of natural science devoted to the study of the general laws of nature, as well as matter, its movement and structure. There are not as many basic physical quantities as it might seem at first glance - there are only 7 of them:

• length,
• weight,
• time,
• current strength,
• temperature,
• amount of substance
• the power of light.

Of course, they have their own letter designations in physics. For example, the symbol chosen for mass is m, and for temperature - T. Also, all quantities have their own unit of measurement: the luminous intensity is candela (cd), and the unit of measurement for the amount of substance is mole.

Derived physical quantities

There are much more derivative physical quantities than basic ones. There are 26 of them, and often some of them are attributed to the main ones.

So, area is a derivative of length, volume is also a derivative of length, speed is a derivative of time, length, and acceleration, in turn, characterizes the rate of change in speed. Momentum is expressed through mass and speed, force is the product of mass and acceleration, mechanical work depends on force and length, energy is proportional to mass. Power, pressure, density, surface density, linear density, amount of heat, voltage, electrical resistance, magnetic flux, moment of inertia, moment of impulse, moment of force - they all depend on mass. Frequency, angular velocity, angular acceleration are inversely proportional to time, and electric charge has a direct dependence on time. Angle and solid angle are derived quantities from length.

What letter represents voltage in physics? Voltage, which is a scalar quantity, is denoted by the letter U. For speed, the designation is the letter v, for mechanical work - A, and for energy - E. Electric charge is usually denoted by the letter q, and magnetic flux - F.

SI: general information

The International System of Units (SI) is a system physical units, which is based on the International System of Quantities, including names and designations of physical quantities. It was adopted by the General Conference on Weights and Measures. It is this system that regulates letter designations in physics, as well as their dimensions and units of measurement. Letters of the Latin alphabet are used for designation, and in some cases - of the Greek alphabet. It is also possible to use special characters as a designation.

Conclusion

So, at any scientific discipline There are special designations for various kinds of quantities. Naturally, physics is no exception. There are quite a lot of letter symbols: force, area, mass, acceleration, voltage, etc. They have their own symbols. There is a special system called the International System of Units. It is believed that basic units cannot be mathematically derived from others. Derivative quantities are obtained by multiplying and dividing from basic quantities.

STATE SECURITY SYSTEM
UNITS OF MEASUREMENT

UNITS OF PHYSICAL QUANTITIES

GOST 8.417-81

(ST SEV 1052-78)

USSR STATE COMMITTEE ON STANDARDS

Moscow

DEVELOPED USSR State Committee for Standards PERFORMERSYu.V. Tarbeev,Dr.Tech. sciences; K.P. Shirokov,Dr.Tech. sciences; P.N. Selivanov, Ph.D. tech. sciences; ON THE. EryukhinaINTRODUCED USSR State Committee for Standards Member of Gosstandart OK. IsaevAPPROVED AND PUT INTO EFFECT Resolution State Committee USSR according to standards of March 19, 1981 No. 1449

STATE STANDARD OF THE USSR UNION

State system for ensuring the uniformity of measurements UNITSPHYSICALSIZE State system for ensuring the uniformity of measurements. Units of physical quantities

GOST 8.417-81 (ST SEV 1052-78)

By Decree of the USSR State Committee on Standards dated March 19, 1981 No. 1449, the introduction date was established

from 01/01/1982

This standard establishes units of physical quantities (hereinafter referred to as units) used in the USSR, their names, designations and rules for the use of these units. The standard does not apply to units used in scientific research and when publishing their results, if they do not consider and use the results of measurements of specific physical quantities, as well as units of quantities assessed on conventional scales*. * Conventional scales mean, for example, the Rockwell and Vickers hardness scales and the photosensitivity of photographic materials. The standard complies with ST SEV 1052-78 in terms of general provisions, units of the International System, units not included in SI, rules for the formation of decimal multiples and submultiples, as well as their names and designations, rules for writing unit designations, rules for the formation of coherent derived SI units (see reference appendix 4).

1. GENERAL PROVISIONS

1.1. The units of the International System of Units*, as well as decimal multiples and submultiples of them, are subject to mandatory use (see Section 2 of this standard). * International System of Units (international abbreviated name - SI, in Russian transcription - SI), adopted in 1960 by the XI General Conference on Weights and Measures (GCPM) and refined at subsequent CGPM. 1.2. It is allowed to use, along with the units according to clause 1.1, units that are not included in the SI, in accordance with clauses. 3.1 and 3.2, their combinations with SI units, as well as some decimal multiples and submultiples of the above units that are widely used in practice. 1.3. It is temporarily allowed to use, along with the units under clause 1.1, units that are not included in SI, in accordance with clause 3.3, as well as some multiples and submultiples of them that have become widespread in practice, combinations of these units with SI units, decimal multiples and submultiples of them them and with units according to clause 3.1. 1.4. In newly developed or revised documentation, as well as publications, the values ​​of quantities must be expressed in SI units, decimal multiples and fractions of them and (or) in units allowed for use in accordance with clause 1.2. It is also allowed in the specified documentation to use units according to clause 3.3, the withdrawal period of which will be established in accordance with international agreements. 1.5. The newly approved regulatory and technical documentation for measuring instruments must provide for their calibration in SI units, decimal multiples and submultiples of them, or in units allowed for use in accordance with clause 1.2. 1.6. Newly developed regulatory and technical documentation on verification methods and means must provide for verification of measuring instruments calibrated in newly introduced units. 1.7. SI units established by this standard and units allowed for use in paragraphs. 3.1 and 3.2 shall apply in educational processes all educational institutions, in textbooks and textbooks. 1.8. Revision of regulatory, technical, design, technological and other technical documentation in which units not provided for by this standard are used, as well as bringing into compliance with paragraphs. 1.1 and 1.2 of this standard for measuring instruments, graduated in units subject to withdrawal, are carried out in accordance with clause 3.4 of this standard. 1.9. In contractual-legal relations for cooperation with foreign countries, with participation in the activities of international organizations, as well as in technical and other documentation supplied abroad along with export products (including transport and consumer packaging), international designations of units are used. In documentation for export products, if this documentation is not sent abroad, it is allowed to use Russian unit designations. (New edition, Amendment No. 1). 1.10. In regulatory and technical design, technological and other technical documentation for various types of products and products used only in the USSR, Russian unit designations are preferably used. At the same time, regardless of what unit designations are used in the documentation for measuring instruments, when indicating units of physical quantities on plates, scales and shields of these measuring instruments, international unit designations are used. (New edition, Amendment No. 2). 1.11. In printed publications it is allowed to use either international or Russian designations of units. The simultaneous use of both types of symbols in the same publication is not allowed, with the exception of publications on units of physical quantities.

2. UNITS OF THE INTERNATIONAL SYSTEM

2.1. The main SI units are given in table. 1.

Table 1

Magnitude
Name	Dimension	Name	Designation		Definition
			international
Length					A meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 S [XVII CGPM (1983), Resolution 1].
Weight		kilogram			The kilogram is a unit of mass equal to the mass of the international prototype of the kilogram [I CGPM (1889) and III CGPM (1901)]
Time					A second is a time equal to 9192631770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom [XIII CGPM (1967), Resolution 1]
Force electric current					Ampere is force equal to strength constant current, which, when passing through two parallel straight conductors of infinite length and negligibly small circular cross-sectional area, located in a vacuum at a distance of 1 m from each other, would cause on each section of a conductor 1 m long an interaction force equal to 2 × 10 -7 N [CIPM (1946), Resolution 2 , approved by the IX CGPM (1948)]
Thermodynamic temperature					Kelvin is a unit of thermodynamic temperature equal to 1/273.16 of the thermodynamic temperature of the triple point of water [XIII CGPM (1967), Resolution 4]
Quantity of substance					A mole is the amount of substance in a system containing the same number of structural elements as there are atoms in carbon-12 weighing 0.012 kg. When using mole structural elements must be specified and may be atoms, molecules, ions, electrons and other particles or specified groups of particles [XIV CGPM (1971), Resolution 3]
The power of light					Candela is the intensity equal to the luminous intensity in a given direction of a source emitting monochromatic radiation with a frequency of 540 × 10 12 Hz, the energetic luminous intensity of which in that direction is 1/683 W/sr [XVI CGPM (1979), Resolution 3]
Notes: 1. In addition to the Kelvin temperature (symbol T) it is also possible to use Celsius temperature (designation t), defined by the expression t = T - T 0 , where T 0 = 273.15 K, by definition. Kelvin temperature is expressed in Kelvin, Celsius temperature - in degrees Celsius (international and Russian designation °C). The size of a degree Celsius is equal to a kelvin. 2. Kelvin temperature interval or difference is expressed in kelvins. The Celsius temperature interval or difference can be expressed in both kelvins and degrees Celsius. 3. The designation of International Practical Temperature in the 1968 International Practical Temperature Scale, if it is necessary to distinguish it from thermodynamic temperature, is formed by adding the index “68” to the designation of thermodynamic temperature (for example, T 68 or t 68). 4. The uniformity of light measurements is ensured in accordance with GOST 8.023-83.

(Changed edition, Amendment No. 2, 3). 2.2. Additional SI units are given in table. 2.

table 2

Name of quantity
	Name	Designation		Definition
		international
Flat angle				A radian is the angle between two radii of a circle, the length of the arc between which is equal to the radius
Solid angle	steradian			A steradian is a solid angle with a vertex at the center of the sphere, cutting out an area on the surface of the sphere, equal to the area square with side equal to the radius of the sphere

(Changed edition, Amendment No. 3). 2.3. Derived SI units should be formed from basic and additional SI units according to the rules for the formation of coherent derived units (see mandatory Appendix 1). Derived SI units that have special names can also be used to form other derived SI units. Derived units with special names and examples of other derived units are given in Table. 3 - 5. Note. SI electrical and magnetic units should be formed according to the rationalized form of the electromagnetic field equations.

Table 3

Examples of derived SI units, the names of which are formed from the names of basic and additional units

Magnitude
Name	Dimension	Name	Designation
			international
Square		square meter
Volume, capacity		cubic meter
Speed		meter per second
Angular velocity		radians per second
Acceleration		meters per second squared
Angular acceleration		radian per second squared
Wave number		meter to the minus first power
Density		kilogram per cubic meter
Specific volume		cubic meter per kilogram
		ampere per square meter
		ampere per meter
Molar concentration		mole per cubic meter
Flow of ionizing particles		second to the minus first power
Particle flux density		second to the minus first power - meter to the minus second power
Brightness		candela per square meter

Table 4

Derived SI units with special names

Magnitude
Name	Dimension	Name	Designation		Expression in terms of major and minor, SI units
			international
Frequency
Strength, weight
Pressure, mechanical stress, elastic modulus
Energy, work, amount of heat					m 2 × kg × s -2
Power, energy flow					m 2 × kg × s -3
Electric charge (amount of electricity)
Electrical voltage, electrical potential, electrical potential difference, electromotive force					m 2 × kg × s -3 × A -1
Electrical capacity	L -2 M -1 T 4 I 2				m -2 × kg -1 × s 4 × A 2
					m 2 × kg × s -3 × A -2
Electrical conductivity	L -2 M -1 T 3 I 2				m -2 × kg -1 × s 3 × A 2
Magnetic induction flux, magnetic flux					m 2 × kg × s -2 × A -1
Magnetic flux density, magnetic induction					kg × s -2 × A -1
Inductance, mutual inductance					m 2 × kg × s -2 × A -2
Light flow
Illumination					m -2 × cd × sr
Activity of a nuclide in a radioactive source (radionuclide activity)		becquerel
Absorbed radiation dose, kerma, absorbed dose indicator (absorbed dose ionizing radiation)
Equivalent radiation dose

(Changed edition, Amendment No. 3).

Table 5

Examples of derived SI units, the names of which are formed using the special names given in table. 4

Magnitude
Name	Dimension	Name	Designation		Expression in terms of SI major and supplementary units
			international
Moment of power		newton meter			m 2 × kg × s -2
Surface tension		Newton per meter
Dynamic viscosity		pascal second			m -1 × kg × s -1
		pendant per cubic meter
Electrical bias		pendant per square meter
		volt per meter			m × kg × s -3 × A -1
Absolute dielectric constant	L -3 M -1 × T 4 I 2	farad per meter			m -3 × kg -1 × s 4 × A 2
Absolute magnetic permeability		henry per meter			m × kg × s -2 × A -2
Specific energy		joule per kilogram
Heat capacity of the system, entropy of the system		joule per kelvin			m 2 × kg × s -2 × K -1
Specific heat capacity, specific entropy		joule per kilogram kelvin		J/(kg × K)	m 2 × s -2 × K -1
Surface energy flux density		watt per square meter
Thermal conductivity		watt per meter kelvin			m × kg × s -3 × K -1
		joule per mole			m 2 × kg × s -2 × mol -1
Molar entropy, molar heat capacity	L 2 MT -2 q -1 N -1	joule per mole kelvin		J/(mol × K)	m 2 × kg × s -2 × K -1 × mol -1
		watt per steradian			m 2 × kg × s -3 × sr -1
Exposure dose (X-ray and gamma radiation)		pendant per kilogram
Absorbed dose rate		gray per second

3. UNITS NOT INCLUDED IN SI

3.1. The units listed in table. 6 are allowed for use without a time limit, along with SI units. 3.2. Without a time limit, it is allowed to use relative and logarithmic units with the exception of the neper unit (see clause 3.3). 3.3. The units given in table. 7 may be temporarily applied until relevant international decisions are taken on them. 3.4. Units, the relationships of which with SI units are given in Reference Appendix 2, are withdrawn from circulation within the time limits provided for by the programs of measures for the transition to SI units, developed in accordance with RD 50-160-79. 3.5. In justified cases in industries National economy It is allowed to use units not provided for by this standard by introducing them into industry standards in agreement with Gosstandart.

Table 6

Non-system units allowed for use along with SI units

Name of quantity					Note
	Name	Designation		Relation to SI unit
		international
Weight
	atomic mass unit			1.66057 × 10 -27 × kg (approx.)
Time 1
				86400 s
Flat angle				(p /180) rad = 1.745329… × 10 -2 × rad
				(p /10800) rad = 2.908882… × 10 -4 rad
				(p /648000) rad = 4.848137…10 -6 rad
Volume, capacity
Length	astronomical unit			1.49598 × 10 11 m (approx.)
	light year			9.4605 × 10 15 m (approx.)
				3.0857 × 10 16 m (approx.)
Optical power	diopter
Square
Energy	electron-volt			1.60219 × 10 -19 J (approx.)
Full power	volt-ampere
Reactive power
Mechanical stress	newton per square millimeter
1 It is also possible to use other units that are widely used, for example, week, month, year, century, millennium, etc. 2 It is allowed to use the name “gon” 3 It is not recommended to use for precise measurements. If it is possible to shift the designation l with the number 1, the designation L is allowed. Note. Units of time (minute, hour, day), plane angle (degree, minute, second), astronomical unit, light year, diopter and atomic mass unit are not allowed to be used with prefixes

(Changed edition, Amendment No. 3).

Table 7

Units temporarily approved for use

Name of quantity					Note
	Name	Designation		Relation to SI unit
		international
Length	nautical mile			1852 m (exactly)	In maritime navigation
Acceleration					In gravimetry
Weight				2 × 10 -4 kg (exactly)	For precious stones and pearls
Linear density				10 -6 kg/m (exactly)	IN textile industry
Speed					In maritime navigation
Rotation frequency	revolutions per second
	revolutions per minute			1/60 s -1 = 0.016(6) s -1
Pressure
Natural logarithm of the dimensionless ratio of a physical quantity to the physical quantity of the same name, taken as the original					1 Np = 0.8686…V = = 8.686… dB

(Changed edition, Amendment No. 3).

4. RULES FOR THE FORMATION OF DECIMAL MULTIPLES AND MULTIPLE UNITS, AS WELL AS THEIR NAMES AND DESIGNATIONS

4.1. Decimal multiples and submultiples, as well as their names and designations, should be formed using the factors and prefixes given in Table. 8.

Table 8

Factors and prefixes for the formation of decimal multiples and submultiples and their names

Factor	Console	Prefix designation		Factor	Console	Prefix designation
		international				international

4.2. Attaching two or more prefixes in a row to the name of a unit is not allowed. For example, instead of the name of the unit micromicrofarad, you should write picofarad. Notes: 1 Due to the fact that the name of the basic unit - kilogram - contains the prefix “kilo”, to form multiple and sub-multiple units of mass, the sub-multiple unit of gram (0.001 kg, kg) is used, and the prefixes must be attached to the word “gram”, for example, milligram (mg, mg) instead of microkilogram (m kg, μkg). 2. The submultiple unit of mass - “gram” can be used without attaching a prefix. 4.3. The prefix or its designation should be written together with the name of the unit to which it is attached, or, accordingly, with its designation. 4.4. If a unit is formed as a product or relation of units, the prefix should be attached to the name of the first unit included in the product or relation. It is allowed to use a prefix in the second factor of the product or in the denominator only in justified cases, when such units are widespread and the transition to units formed in accordance with the first part of the paragraph is associated with great difficulties, for example: ton-kilometer (t × km; t × km), watt per square centimeter (W / cm 2; W/cm 2), volt per centimeter (V / cm; V/cm), ampere per square millimeter (A / mm 2; A/mm 2). 4.5. The names of multiples and submultiples of a unit raised to a power should be formed by attaching a prefix to the name of the original unit, for example, to form the names of a multiple or submultiple unit of a unit of area - a square meter, which is the second power of a unit of length - a meter, the prefix should be attached to the name of this last unit: square kilometer, square centimeter, etc. 4.6. Designations of multiples and submultiples of a unit raised to a power should be formed by adding the appropriate exponent to the designation of a multiple or submultiple of that unit, the exponent meaning the exponentiation of a multiple or submultiple unit (together with the prefix). Examples: 1. 5 km 2 = 5(10 3 m) 2 = 5 × 10 6 m 2. 2. 250 cm 3 /s = 250(10 -2 m) 3 /(1 s) = 250 × 10 -6 m 3 /s. 3. 0.002 cm -1 = 0.002(10 -2 m) -1 = 0.002 × 100 m -1 = 0.2 m -1. 4.7. Recommendations for choosing decimal multiples and submultiples are given in Reference Appendix 3.

5. RULES FOR WRITING UNIT DESIGNATIONS

5.1. To write the values ​​of quantities, units should be designated with letters or special signs (...°,... ¢,... ¢ ¢), and two types of letter designations are established: international (using letters of the Latin or Greek alphabet) and Russian (using letters of the Russian alphabet) . The unit designations established by the standard are given in table. 1 - 7. International and Russian designations for relative and logarithmic units are as follows: percent (%), ppm (o/oo), ppm (pp m, ppm), bel (V, B), decibel (dB, dB), octave (- , oct), decade (-, dec), background (phon, background). 5.2. Letter designations of units must be printed in roman font. In unit designations, a dot is not used as an abbreviation sign. 5.3. Unit designations should be used after numerical values ​​of quantities and placed on the line with them (without moving to the next line). Between the last digit of the number and the designation of the unit, a space should be left equal to the minimum distance between words, which is determined for each type and size of font according to GOST 2.304-81. Exceptions are designations in the form of a sign raised above the line (clause 5.1), before which a space is not left. (Changed edition, Amendment No. 3). 5.4. In the presence of decimal in the numerical value of a quantity, the unit symbol should be placed after all digits. 5.5. When indicating the values ​​of quantities with maximum deviations, you should enclose the numerical values ​​with maximum deviations in brackets and place unit designations after the brackets or put unit designations after the numerical value of the quantity and after its maximum deviation. 5.6. It is allowed to use unit designations in column headings and in row names (sidebars) of tables. Examples:

Nominal flow. m3/h	Upper limit of readings, m 3		Dividing value of the rightmost roller, m 3, no more
100, 160, 250, 400, 600 and 1000
2500, 4000, 6000 and 10000
Traction power, kW
Overall dimensions, mm:
length
width
height
Track, mm
Clearance, mm

5.7. It is allowed to use unit designations in explanations of quantity designations for formulas. Placing symbols of units on the same line with formulas expressing dependencies between quantities or between their numerical values ​​presented in letter form is not allowed. 5.8. The letter designations of the units included in the product should be separated by dots on the middle line, like multiplication signs*. * In typewritten texts, it is allowed not to raise the period. It is allowed to separate the letter designations of units included in the work with spaces, if this does not lead to misunderstanding. 5.9. In letter designations of unit ratios, only one line should be used as a division sign: oblique or horizontal. It is allowed to use unit designations in the form of a product of unit designations raised to powers (positive and negative)**. ** If for one of the units included in the relation, the designation is set in the form of a negative degree (for example, s -1, m -1, K -1; c -1, m -1, K -1), use an oblique or horizontal line not allowed. 5.10. When using a slash, the unit symbols in the numerator and denominator should be placed on a line, and the product of the unit symbols in the denominator should be enclosed in parentheses. 5.11. When indicating a derived unit consisting of two or more units, it is not allowed to combine letter designations and names of units, i.e. For some units, give designations, and for others, names. Note. It is allowed to use combinations of special characters...°,... ¢,... ¢ ¢, % and o / oo with letter designations of units, for example...°/ s, etc.

APPLICATION 1

Mandatory

RULES FOR FORMATION OF COHERENT DERIVATIVE SI UNITS

Coherent derived units (hereinafter referred to as derived units) of the International System, as a rule, are formed using the simplest equations of connections between quantities (defining equations), in which the numerical coefficients are equal to 1. To form derived units, quantities in the connection equations are taken equal to SI units. Example. The unit of speed is formed using an equation that determines the speed of a rectilinearly and uniformly moving point

v = s/t,

Where v- speed; s- length of the traveled path; t- time of movement of the point. Substitution instead s And t their SI units gives

[v] = [s]/[t] = 1 m/s.

Therefore, the SI unit of speed is meter per second. He equal to speed a rectilinearly and uniformly moving point, at which this point moves a distance of 1 m in a time of 1 s. If the coupling equation contains a numerical coefficient different from 1, then to form a coherent derivative of an SI unit, values ​​with values ​​in SI units are substituted into the right side, giving, after multiplication by the coefficient, the total numerical value, equal to the number 1. Example. If the equation is used to form a unit of energy

Where E- kinetic energy; m - mass material point;v is the speed of motion of a point, then the coherent SI unit of energy is formed, for example, as follows:

Therefore, the SI unit of energy is the joule (equal to the newton meter). In the examples given it is equal to kinetic energy a body weighing 2 kg moving with a speed of 1 m/s, or a body weighing 1 kg moving with a speed

APPLICATION 2

Information

Correlation of some non-systemic units with SI units

Name of quantity					Note
	Name	Designation		Relation to SI unit
		international
Length	angstrom
	x-unit			1.00206 × 10 -13 m (approx.)
Square
Weight
Solid angle	square degree			3.0462... × 10 -4 sr
Strength, weight
	kilogram-force			9.80665 N (exact)
	kilopond
	gram-force			9.83665 × 10 -3 N (exact)
	ton-force			9806.65 N (exactly)
Pressure	kilogram-force per square centimeter			98066.5 Ra (exactly)
	kilopond per square centimeter
	millimeter of water column		mm water Art.	9.80665 Ra (exactly)
	millimeter of mercury		mmHg Art.
Tension (mechanical)	kilogram-force per square millimeter			9.80665 × 10 6 Ra (exact)
	kilopond per square millimeter			9.80665 × 10 6 Ra (exact)
Work, energy
Power	Horsepower
Dynamic viscosity
Kinematic viscosity
	ohm-square millimeter per meter		Ohm × mm 2 /m
Magnetic flux	Maxwell
Magnetic induction
	gplbert			(10/4 p) A = 0.795775…A
Magnetic field strength				(10 3 / p) A/ m = 79.5775…A/ m
Amount of heat, thermodynamic potential (internal energy, enthalpy, isochoric-isothermal potential), heat of phase transformation, heat of chemical reaction	calorie (int.)			4.1858 J (exactly)
	thermochemical calorie			4.1840 J (approx.)
	calorie 15 degrees			4.1855 J (approx.)
Absorbed radiation dose
Equivalent dose of radiation, equivalent dose indicator
Exposure dose of photon radiation (exposure dose of gamma and x-ray radiation)				2.58 × 10 -4 C/kg (exact)
Activity of a nuclide in a radioactive source				3,700 × 10 10 Bq (exactly)
Length
Angle of rotation				2 p rad = 6.28… rad
Magnetomotive force, magnetic potential difference	ampereturn
Brightness
Square

Amended edition, Rev. No. 3.

APPLICATION 3

Information

1. The choice of a decimal multiple or fractional unit of an SI unit is dictated primarily by the convenience of its use. From the variety of multiple and submultiple units that can be formed using prefixes, a unit is selected that leads to numerical values ​​of the quantity acceptable in practice. In principle, multiples and submultiples are chosen so that the numerical values ​​of the quantity are in the range from 0.1 to 1000. 1.1. In some cases, it is appropriate to use the same multiple or submultiple unit even if the numerical values ​​fall outside the range of 0.1 to 1000, for example, in tables of numerical values ​​for the same quantity or when comparing these values ​​in the same text. 1.2. In some areas the same multiple or submultiple unit is always used. For example, in drawings used in mechanical engineering, linear dimensions are always expressed in millimeters. 2. In table. 1 of this appendix shows the recommended multiples and submultiples of SI units for use. Presented in table. 1 multiples and submultiples of SI units for a given physical quantity should not be considered exhaustive, since they may not cover the ranges of physical quantities in developing and emerging fields of science and technology. However, the recommended multiples and sub-multiple units of SI units contribute to the uniformity of presentation of the values ​​of physical quantities related to various areas technology. The same table also contains multiples and submultiples of units that are widely used in practice and are used along with SI units. 3. For quantities not covered in table. 1, you should use multiple and submultiple units selected in accordance with paragraph 1 of this appendix. 4. To reduce the likelihood of errors in calculations, it is recommended to substitute decimal multiples and submultiples only in the final result, and during the calculation process, express all quantities in SI units, replacing prefixes with powers of 10. 5. In Table. 2 of this appendix shows the popular units of some logarithmic quantities.

Table 1

Name of quantity	Designations
	SI units		units not included in SI	multiples and submultiples of non-SI units
Part I. Space and time
Flat angle	rad ; rad (radian)	m rad ; mkrad	... ° (degree)... (minute)..." (second)
Solid angle	sr ; cp (steradian)
Length	m; m (meter)		… ° (degree) … ¢ (minute) … ² (second)
Square
Volume, capacity			l(L); l (liter)
Time	s; s (second)		d ; day (day) min; min (minute)
Speed
Acceleration	m/s2; m/s 2
Part II. Periodic and related phenomena
	Hz; Hz (hertz)
Rotation frequency			min -1 ; min -1
Part III. Mechanics
Weight	kg ; kg (kilogram)		t ; t (ton)
Linear density	kg/m; kg/m	mg/m; mg/m or g/km; g/km
Density	kg/m3; kg/m 3	Mg/m3; Mg/m 3 kg/dm 3; kg/dm 3 g/cm3; g/cm 3	t/m3; t/m 3 or kg/l; kg/l	g/ml; g/ml
Quantity of movement	kg×m/s; kg × m/s
Momentum	kg × m 2 / s; kg × m 2 /s
Moment of inertia (dynamic moment of inertia)	kg × m 2, kg × m 2
Strength, weight	N ; N (newton)
Moment of power	N×m; N×m	MN × m; MN × m kN × m; kN × m mN × m; mN × m m N × m ; µN × m
Pressure	Ra; Pa (pascal)	m Ra; µPa
Voltage
Dynamic viscosity	Ra × s; Pa × s	mPa × s; mPa × s
Kinematic viscosity	m2/s; m 2 /s	mm2/s; mm 2 /s
Surface tension		mN/m; mN/m
Energy, work	J; J (joule)		(electron-volt)	GeV; GeV MeV ; MeV keV ; keV
Power	W; W (watt)
Part IV. Heat
Temperature	TO; K (kelvin)
Temperature coefficient
Heat, amount of heat
Heat flow
Thermal conductivity
Heat transfer coefficient	W/(m 2 × K)
Heat capacity		kJ/K; kJ/K
Specific heat	J/(kg × K)	kJ /(kg × K); kJ/(kg × K)
Entropy		kJ/K; kJ/K
Specific entropy	J/(kg × K)	kJ/(kg × K); kJ/(kg × K)
Specific quantity warmth	J/kg; J/kg	MJ/kg; MJ/kg kJ / kg ; kJ/kg
Specific heat of phase transformation	J/kg; J/kg	MJ/kg; MJ/kg kJ/kg; kJ/kg
Part V. Electricity and magnetism
Electric current (electric current strength)	A; A (amps)
Electric charge (amount of electricity)	WITH; Cl (pendant)
Spatial density of electric charge	C/ m 3; C/m 3	C/mm 3; C/mm 3 MS/ m 3 ; MC/m 3 S/s m 3 ; C/cm 3 kC/m3; kC/m 3 m C/ m 3; mC/m 3 m C/ m 3; µC/m 3
Surface electric charge density	S/ m 2, C/m 2	MS/ m 2 ; MC/m 2 С/ mm 2; C/mm 2 S/s m 2 ; C/cm 2 kC/m2; kC/m 2 m C/ m 2; mC/m 2 m C/ m 2; µC/m 2
Tension electric field		MV/m; MV/m kV/m; kV/m V/mm; V/mm V/cm; V/cm mV/m; mV/m mV/m; µV/m
Electrical voltage, electrical potential, electrical potential difference, electromotive force	V, V (volts)
Electrical bias	C/ m 2; C/m 2	S/s m 2 ; C/cm 2 kC/cm2; kC/cm 2 m C/ m 2; mC/m 2 m C/ m 2, µC/m 2
Electrical displacement flux
Electrical capacity	F, Ф (farad)
Absolute dielectric constant, electrical constant		m F / m , µF/m nF/m, nF/m pF / m , pF/m
Polarization	S/ m 2, C/m 2	S/s m 2, C/cm 2 kC/m2; kC/m 2 m C/ m 2, mC/m 2 m C/ m 2; µC/m 2
Electric dipole moment	S × m, Cl × m
Electric current density	A/ m 2, A/m 2	MA/ m 2, MA/m 2 A/mm 2, A/mm 2 A/s m 2, A/cm 2 kA/m2, kA/m2,
Linear electric current density		kA/m; kA/m A/mm; A/mm A/c m ; A/cm
Magnetic field strength		kA/m; kA/m A/mm; A/mm A/cm; A/cm
Magnetomotive force, magnetic potential difference
Magnetic induction, magnetic flux density	T; Tl (tesla)
Magnetic flux	Wb, Wb (weber)
Magnetic vector potential	T × m; T × m	kT×m; kT × m
Inductance, mutual inductance	N; Gn (Henry)
Absolute magnetic permeability, magnetic constant		m N/ m; µH/m nH/m; nH/m
Magnetic moment	A × m 2; A m 2
Magnetization		kA/m; kA/m A/mm; A/mm
Magnetic polarization
Electrical resistance
Electrical conductivity	S ; CM (Siemens)
Electrical resistivity	W×m; Ohm × m	GW×m; GΩ × m M W × m; MΩ × m kW×m; kOhm × m W×cm; Ohm × cm mW×m; mOhm × m mW×m; µOhm × m nW×m; nOhm × m
Electrical conductivity		MS/m; MSm/m kS/m; kS/m
Reluctance
Magnetic conductivity
Impedance
Impedance module
Reactance
Active resistance
Admittance
Conductivity module
Reactive conductivity
Conductance
Active power
Reactive power
Full power			V × A, V × A
Part VI. Light and related electromagnetic radiation
Wavelength
Wave number
Radiation energy
Radiation flux, radiation power
Energy luminous intensity (radiant intensity)	W/sr; Tue/Wed
Energy brightness (radiance)	W /(sr × m 2); W/(avg × m2)
Energy illumination (irradiance)	W/m2; W/m2
Energetic luminosity (radiance)	W/m2; W/m2
The power of light
Light flow	lm ; lm (lumen)
Light energy	lm×s; lm × s		lm × h; lm × h
Brightness	cd/m2; cd/m2
Luminosity	lm/m2; lm/m 2
Illumination	l x; lux (lux)
Light exposure	lx×s; lx × s
Light equivalent of radiation flux	lm/W; lm/W
Part VII. Acoustics
Period
Batch frequency
Wavelength
Sound pressure		m Ra; µPa
Particle oscillation speed		mm/s; mm/s
Volume velocity	m3/s; m 3 /s
Sound speed
Sound energy flow, sound power
Sound intensity	W/m2; W/m2	mW/m2; mW/m2 mW/m2; µW/m 2 pW/m2; pW/m2
Specific acoustic impedance	Pa×s/m; Pa × s/m
Acoustic impedance	Pa×s/m3; Pa × s/m 3
Mechanical resistance	N×s/m; N × s/m
Equivalent absorption area of ​​a surface or object
Reverberation time
Part VIII Physical chemistry and molecular physics
Quantity of substance	mol ; mole (mol)	kmol; kmol mmol; mmol m mol ; µmol
Molar mass	kg/mol; kg/mol	g/mol; g/mol
Molar volume	m3/moi; m 3 /mol	dm 3/mol; dm 3 /mol cm 3 / mol; cm 3 /mol	l/mol; l/mol
Molar internal energy	J/mol; J/mol	kJ/mol; kJ/mol
Molar enthalpy	J/mol; J/mol	kJ/mol; kJ/mol
Chemical Potential	J/mol; J/mol	kJ/mol; kJ/mol
Chemical affinity	J/mol; J/mol	kJ/mol; kJ/mol
Molar heat capacity	J/(mol × K); J/(mol × K)
Molar entropy	J/(mol × K); J/(mol × K)
Molar concentration	mol/m3; mol/m 3	kmol/m3; kmol/m 3 mol/dm 3; mol/dm 3	mol/1; mol/l
Specific adsorption	mol/kg; mol/kg	mmol/kg; mmol/kg
Thermal diffusivity	M2/s; m 2 /s
Part IX. Ionizing radiation
Absorbed dose of radiation, kerma, absorbed dose indicator (absorbed dose of ionizing radiation)	Gy; Gr (gray)	m G y; µGy
Activity of a nuclide in a radioactive source (radionuclide activity)	Bq ; Bq (becquerel)

(Changed edition, Amendment No. 3).

table 2

Name of logarithmic quantity	Unit designation	Initial value of the quantity
Sound pressure level
Sound power level
Sound intensity level
Power Level Difference
Strengthening, weakening
Attenuation coefficient

APPLICATION 4

Information

INFORMATION DATA ABOUT COMPLIANCE WITH GOST 8.417-81 ST SEV 1052-78

1. Sections 1 - 3 (clauses 3.1 and 3.2); 4, 5 and the mandatory Appendix 1 to GOST 8.417-81 correspond to sections 1 - 5 and the appendix to ST SEV 1052-78. 2. Reference appendix 3 to GOST 8.417-81 corresponds to the information appendix to ST SEV 1052-78.

Constructing drawings is not an easy task, but without it modern world no way. After all, in order to make even the most ordinary item (a tiny bolt or nut, a shelf for books, the design of a new dress, etc.), you first need to carry out the appropriate calculations and draw a drawing of the future product. However, often one person draws it up, and another person produces something according to this scheme.

To avoid confusion in understanding the depicted object and its parameters, it is accepted all over the world symbols length, width, height and other quantities used in design. What are they? Let's find out.

Quantities

Area, height and other designations of a similar nature are not only physical, but also mathematical quantities.

Their single letter designation (used by all countries) was established in the mid-twentieth century by the International System of Units (SI) and is still used to this day. It is for this reason that all such parameters are indicated in Latin, and not in Cyrillic letters or Arabic script. In order not to create certain difficulties, when developing design documentation standards in most modern countries, it was decided to use almost the same conventions that are used in physics or geometry.

Any school graduate remembers that depending on whether a two-dimensional or three-dimensional figure (product) is depicted in the drawing, it has a set of basic parameters. If there are two dimensions, these are width and length, if there are three, height is also added.

So, first, let's find out how to correctly indicate length, width, height in the drawings.

Width

As mentioned above, in mathematics the quantity in question is one of the three spatial dimensions of any object, provided that its measurements are made in the transverse direction. So what is width famous for? It is designated by the letter “B”. This is known all over the world. Moreover, according to GOST, it is permissible to use both capital and lowercase Latin letters. The question often arises as to why this particular letter was chosen. After all, the reduction is usually made according to the first Greek or English name of the quantity. In this case, the width in English will look like “width”.

Probably the point here is that this parameter was initially most widely used in geometry. In this science, when describing figures, length, width, height are often denoted by the letters “a”, “b”, “c”. According to this tradition, when choosing, the letter "B" (or "b") was borrowed from the SI system (although symbols other than geometric ones began to be used for the other two dimensions).

Most believe that this was done so as not to confuse width (designated with the letter "B"/"b") with weight. The fact is that the latter is sometimes referred to as “W” (short for the English name weight), although the use of other letters (“G” and “P”) is also acceptable. According to international standards of the SI system, width is measured in meters or multiples (multiples) of their units. It is worth noting that in geometry it is sometimes also acceptable to use “w” to denote width, but in physics and other exact sciences such a designation is usually not used.

Length

As already indicated, in mathematics, length, height, width are three spatial dimensions. Moreover, if width is a linear dimension in the transverse direction, then length is in the longitudinal direction. Considering it as a quantity of physics, one can understand that this word means a numerical characteristic of the length of lines.

IN English language this term is called length. It is because of this that this value is denoted by the capital or lowercase initial letter of the word - “L”. Like width, length is measured in meters or their multiples (multiples).

Height

The presence of this value indicates that we have to deal with something more complex - three-dimensional space. Unlike length and width, height numerically characterizes the size of an object in the vertical direction.

In English it is written as "height". Therefore, according to international standards, it is denoted by the Latin letter “H” / “h”. In addition to height, in drawings sometimes this letter also acts as a designation for depth. Height, width and length - all these parameters are measured in meters and their multiples and submultiples (kilometers, centimeters, millimeters, etc.).

Radius and diameter

In addition to the parameters discussed, when drawing up drawings you have to deal with others.

For example, when working with circles, it becomes necessary to determine their radius. This is the name of the segment that connects two points. The first of them is the center. The second is located directly on the circle itself. In Latin this word looks like "radius". Hence the lowercase or capital “R”/“r”.

When drawing circles, in addition to the radius, you often have to deal with a phenomenon close to it - diameter. It is also a line segment connecting two points on a circle. In this case, it necessarily passes through the center.

Numerically, the diameter is equal to two radii. In English this word is written like this: "diameter". Hence the abbreviation - large or small Latin letter “D” / “d”. Often the diameter in the drawings is indicated using a crossed out circle - “Ø”.

Although this is a common abbreviation, it is worth keeping in mind that GOST provides for the use of only the Latin “D” / “d”.

Thickness

Most of us remember school mathematics lessons. Even then, teachers told us that it is customary to use the Latin letter “s” to denote a quantity such as area. However, according to generally accepted standards, a completely different parameter is written in drawings in this way - thickness.

Why is that? It is known that in the case of height, width, length, the designation by letters could be explained by their writing or tradition. It’s just that thickness in English looks like “thickness”, and in Latin it looks like “crassities”. It is also not clear why, unlike other quantities, thickness can only be indicated in lowercase letters. The notation "s" is also used to describe the thickness of pages, walls, ribs, etc.

Perimeter and area

Unlike all the quantities listed above, the word “perimeter” does not come from Latin or English, but from Greek. It is derived from "περιμετρέο" ("measure the circumference"). And today this term has retained its meaning (the total length of the boundaries of the figure). Subsequently, the word entered the English language (“perimeter”) and was fixed in the SI system in the form of an abbreviation with the letter “P”.

Area is a quantity that shows quantitative characteristics geometric figure having two dimensions (length and width). Unlike everything listed earlier, it is measured in square meters (as well as in submultiples and multiples thereof). As for the letter designation of the area, it differs in different areas. For example, in mathematics this is the Latin letter “S”, familiar to everyone since childhood. Why this is so - no information.

Some people unknowingly think that this is due to English spelling the words "square". However, in it the mathematical area is "area", and "square" is the area in the architectural sense. By the way, it is worth remembering that “square” is the name of the geometric figure “square”. So you should be careful when studying drawings in English. Due to the translation of “area” in some disciplines, the letter “A” is used as a designation. In rare cases, "F" is also used, but in physics this letter stands for a quantity called "force" ("fortis").

Other common abbreviations

The designations for height, width, length, thickness, radius, and diameter are the most commonly used when drawing up drawings. However, there are other quantities that are also often present in them. For example, lowercase "t". In physics this means “temperature”, but according to GOST Unified system design documentation, this letter is a step (coil springs, etc.). However, it is not used when it comes to gears and threads.

The capital and lowercase letter “A”/“a” (according to the same standards) in the drawings is used to denote not the area, but the center-to-center and center-to-center distance. In addition to different sizes, in drawings it is often necessary to indicate angles of different sizes. For this purpose, it is customary to use lowercase letters of the Greek alphabet. The most commonly used are “α”, “β”, “γ” and “δ”. However, it is acceptable to use others.

What standard defines the letter designation of length, width, height, area and other quantities?

As mentioned above, so that there is no misunderstanding when reading the drawing, representatives different nations Common lettering standards have been adopted. In other words, if you are in doubt about the interpretation of a particular abbreviation, look at GOSTs. This way you will learn how to correctly indicate height, width, length, diameter, radius, and so on.

Studying physics at school lasts several years. At the same time, students are faced with the problem that the same letters represent completely different quantities. Most often this fact concerns Latin letters. How then to solve problems?

There is no need to be afraid of such a repetition. Scientists tried to introduce them into the notation so that identical letters would not appear in the same formula. Most often, students encounter the Latin n. It can be lowercase or uppercase. Therefore, the question logically arises about what n is in physics, that is, in a certain formula encountered by the student.

What does the capital letter N stand for in physics?

Most often in school courses it occurs when studying mechanics. After all, there it can be immediately in spirit meanings - the power and strength of a normal support reaction. Naturally, these concepts do not overlap, because they are used in different sections of mechanics and are measured in different units. Therefore, you always need to define exactly what n is in physics.

Power is the rate of change of energy in a system. This is a scalar quantity, that is just a number. Its unit of measurement is the watt (W).

The normal ground reaction force is the force exerted on the body by the support or suspension. In addition to the numerical value, it has a direction, that is, it is a vector quantity. Moreover, it is always perpendicular to the surface on which the external influence is made. The unit of this N is newton (N).

What is N in physics, in addition to the quantities already indicated? It could be:

Avogadro's constant;

magnification of the optical device;

substance concentration;

Debye number;

total radiation power.

What does the lowercase letter n stand for in physics?

The list of names that may be hidden behind it is quite extensive. The notation n in physics is used for the following concepts:

refractive index, and it can be absolute or relative;

neutron - neutral elementary particle with a mass slightly greater than that of a proton;

rotation frequency (used to replace the Greek letter "nu", since it is very similar to the Latin "ve") - the number of repetitions of revolutions per unit of time, measured in hertz (Hz).

What does n mean in physics, besides the quantities already indicated? It turns out that it hides the fundamental quantum number (quantum physics), concentration and Loschmidt constant (molecular physics). By the way, when calculating the concentration of a substance, you need to know the value, which is also written with the Latin “en”. It will be discussed below.

What physical quantity can be denoted by n and N?

Its name comes from the Latin word numerus, translated as “number”, “quantity”. Therefore, the answer to the question of what n means in physics is quite simple. This is the number of any objects, bodies, particles - everything that is discussed in a certain task.

Moreover, “quantity” is one of the few physical quantities that do not have a unit of measurement. It's just a number, without a name. For example, if the problem involves 10 particles, then n will simply be equal to 10. But if it turns out that the lowercase “en” is already taken, then you have to use a capital letter.

Formulas containing capital N

The first of them determines power, which is equal to the ratio of work to time:

IN molecular physics there is such a thing as chemical quantity substances. Denoted by the Greek letter "nu". To count it, you should divide the number of particles by Avogadro's number :

By the way, the last value is also denoted by the so popular letter N. Only it always has a subscript - A.

To determine electric charge, you will need the formula:

Another formula with N in physics - oscillation frequency. To count it, you need to divide their number by time:

The letter “en” appears in the formula for the circulation period:

Formulas containing lowercase n

In a school physics course, this letter is most often associated with the refractive index of a substance. Therefore, it is important to know the formulas with its application.

So, for the absolute refractive index the formula is written as follows:

Here c is the speed of light in a vacuum, v is its speed in a refractive medium.

The formula for the relative refractive index is somewhat more complicated:

n 21 = v 1: v 2 = n 2: n 1,

where n 1 and n 2 are the absolute refractive indices of the first and second medium, v 1 and v 2 are the speeds of the light wave in these substances.

How to find n in physics? A formula will help us with this, which requires knowing the angles of incidence and refraction of the beam, that is, n 21 = sin α: sin γ.

What is n equal to in physics if it is the refractive index?

Usually the tables give values ​​for absolute refractive index various substances. Do not forget that this value depends not only on the properties of the medium, but also on the wavelength. Table values ​​of the refractive index are given for the optical range.

So, it became clear what n is in physics. To avoid any questions, it is worth considering some examples.

Power task

№1. During plowing, the tractor pulls the plow evenly. At the same time, he applies a force of 10 kN. With this movement, it covers 1.2 km within 10 minutes. It is necessary to determine the power it develops.

Converting units to SI. You can start with force, 10 N equals 10,000 N. Then the distance: 1.2 × 1000 = 1200 m. Time left - 10 × 60 = 600 s.

Selection of formulas. As mentioned above, N = A: t. But the task has no meaning for the work. To calculate it, another formula is useful: A = F × S. The final form of the formula for power looks like this: N = (F × S) : t.

Solution. Let's first calculate the work and then the power. Then the first action gives 10,000 × 1,200 = 12,000,000 J. The second action gives 12,000,000: 600 = 20,000 W.

Answer. The tractor power is 20,000 W.

Refractive index problems

№2. The absolute refractive index of glass is 1.5. The speed of light propagation in glass is less than in vacuum. You need to determine how many times.

There is no need to convert data to SI.

When choosing formulas, you need to focus on this one: n = c: v.

Solution. From this formula it is clear that v = c: n. This means that the speed of light in glass is equal to the speed of light in a vacuum divided by the refractive index. That is, it decreases by one and a half times.

Answer. The speed of light propagation in glass is 1.5 times less than in vacuum.

№3. There are two transparent media available. The speed of light in the first of them is 225,000 km/s, in the second it is 25,000 km/s less. A ray of light goes from the first medium to the second. The angle of incidence α is 30º. Calculate the value of the angle of refraction.

Do I need to convert to SI? Speeds are given in non-system units. However, when substituted into formulas, they will be reduced. Therefore, there is no need to convert speeds to m/s.

Selecting the formulas necessary to solve the problem. You will need to use the law of light refraction: n 21 = sin α: sin γ. And also: n = с: v.

Solution. In the first formula, n 21 is the ratio of the two refractive indices of the substances in question, that is, n 2 and n 1. If we write down the second indicated formula for the proposed media, we get the following: n 1 = c: v 1 and n 2 = c: v 2. If we make the ratio of the last two expressions, it turns out that n 21 = v 1: v 2. Substituting it into the formula for the law of refraction, we can derive the following expression for the sine of the refraction angle: sin γ = sin α × (v 2: v 1).

We substitute the values ​​of the indicated speeds and the sine of 30º (equal to 0.5) into the formula, it turns out that the sine of the refraction angle is equal to 0.44. According to the Bradis table, it turns out that the angle γ is equal to 26º.

Answer. The refraction angle is 26º.

Tasks for the circulation period

№4. Blades windmill rotate with a period of 5 seconds. Calculate the number of revolutions of these blades in 1 hour.

You only need to convert time to SI units for 1 hour. It will be equal to 3,600 seconds.

Selection of formulas. The period of rotation and the number of revolutions are related by the formula T = t: N.

Solution. From the above formula, the number of revolutions is determined by the ratio of time to period. Thus, N = 3600: 5 = 720.

Answer. The number of revolutions of the mill blades is 720.

№5. An airplane propeller rotates at a frequency of 25 Hz. How long will it take the propeller to make 3,000 revolutions?

All data is given in SI, so there is no need to translate anything.

Required formula: frequency ν = N: t. From it you only need to derive the formula for the unknown time. It is a divisor, so it is supposed to be found by dividing N by ν.

Solution. Dividing 3,000 by 25 gives the number 120. It will be measured in seconds.

Answer. An airplane propeller makes 3000 revolutions in 120 s.

Let's sum it up

When a student encounters a formula containing n or N in a physics problem, he needs deal with two points. The first one is from which physics section equality is given. This may be clear from the title in the textbook, reference book, or the words of the teacher. Then you should decide what is hidden behind the many-sided “en”. Moreover, the name of the units of measurement helps with this, if, of course, its value is given. Another option is also allowed: look carefully at the remaining letters in the formula. Perhaps they will turn out to be familiar and will give a hint on the issue at hand.