Formula symbol

DIN 1304 part 1-8
title	Formula symbol
Regulates	Formula symbols, formula set 1: 1994 General formula symbols 2: 1989 Meteorology and geophysics 3: 2006 Generation, transport and distribution of electrical energy 5: 1989 Fluid mechanics 6: 1992 Electrical communications engineering 7: 1991 Electrical machines 8: 1994 Converters with semiconductor components
Publishing year	from 1989

Formula symbols (also: size symbols ) are symbols that are used to denote physical quantities . According to DIN 1304 ("Formula symbol") and ISO / IEC 80000 ("Sizes and units"), a formula symbol consists of a basic symbol and, if necessary, of subsidiary symbols, e.g. B. Indices . The basic characters are Latin and Greek upper and lower case letters. Subsidiary characters are letters, numbers or special characters (e.g. *, ', ~, ^). Basic characters consisting of several letters are not permitted, except for parameters ( sizes of the dimension number ) such as B. the Reynolds number . ${\ displaystyle {\ mathit {Re}}}$

The formula symbols are occasionally based on English or Latin technical terms. For example, a force is often referred to by the symbol , which is derived from the English force or the Latin fortitudo . ${\ displaystyle F}$

In accordance with DIN 1304 and DIN 1313 ("sizes"), the basic characters must be set in italics in print . DIN 1338 ("Formula notation and typesetting") recommends a font with serifs . Such a font prevents, for example, the confusion of the capital I ( ) with the small l ( ). ${\ displaystyle I}$ ${\ displaystyle l}$

Application examples

{\ displaystyle R = {\ frac {U} {I}}}

means: The electrical resistance is equal to the electrical voltage divided by the electrical current strength .

{\ displaystyle R}

{\ displaystyle U}

{\ displaystyle I}

{\ displaystyle E = m \ cdot c ^ {2}}

means: The energy is equal to the mass times the square of the speed of light .

{\ displaystyle E}

{\ displaystyle m}

{\ displaystyle c}

Formula symbols can only consist of one basic symbol - as shown in the previous examples - or they can contain additional symbols to further define the size. Examples of electrical voltage are:

{\ displaystyle U _ {\ sim}, \, U _ {-}}

: AC and DC components of a voltage

{\ displaystyle {\ hat {U}} = U _ {\ mathrm {s}} = {\ sqrt {2}} \ cdot U _ {\ mathrm {eff}}}

: Peak value and rms value for sinusoidal alternating voltage

{\ displaystyle \ mathrm {Re} \ {\ underline {U}} = {\ tfrac {1} {2}} ({\ underline {U}} + {\ underline {U}} ^ {\ ast})}

: Real part in complex notation

Frequently used special characters in connection with formula symbols

meaning	presentation	example
Unit of measurement of a size	square brackets around the symbol of the relevant size	${\ displaystyle [F]}$
Peak value	Circumflex, "roof"	${\ displaystyle {\ hat {u}}}$
amount	Amount bars	${\ displaystyle \| x \|}$
Average	Overline	${\ displaystyle {\ overline {v}}}$
Expected value	angle brackets	${\ displaystyle \ langle E \ rangle}$
Quantity transformed into another frame of reference	apostrophe	${\ displaystyle x '}$
first (second) derivative according to location	one (two) apostrophe (s)	${\ displaystyle \ varphi ', \, \ varphi' '}$
first (second) derivative with respect to time	one (two) point (s) above the symbol	${\ displaystyle {\ dot {x}}, \, {\ ddot {x}}}$
complex conjugation	superscript asterisk	${\ displaystyle \ Psi ^ {\ ast}}$
Vectors	Arrow or bold face	${\ displaystyle {\ vec {v}}, \, {\ boldsymbol {v}}}$
Matrices	Underline or bold type	${\ displaystyle {\ underline {I}}, \, \ mathbf {I}}$
Transposition	superscript "T"	${\ displaystyle A ^ {\ mathsf {T}}}$
Adjoint	superscript cross	${\ displaystyle A ^ {\ dagger}}$