# 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 ${\ displaystyle A ^ {\ dagger}}$