Supplement 2.6: Differentials and derivatives (1/4)
Functions of one variable
In analysis in schools, functions as
are dealt with, which depend on only one variable namely . Examples:
The derivatives of are symbolically labelled with , with or with .
In accordance with the third option, the derivative is the differential quotient of two quantities and . These quantities are called differentials, because they become infinitely small differences from finitely large differences and through passing a limit
They become what is named infinitesimal differences.
Differentials do not necessarily have to be parts of quotients but may stand alone. Example:
Function of several variables
Natural processes in most cases depend on several environmental variables and this is what occurs in mathematical modelling of such processes as well. One example: the temperature at a particular location takes on different values at other locations and therefore is a function of the room coordinates , and . Moreover, it takes on different values for different times . The temperature thus is a function of four variables: .
This example of changes in temperature in space and time will be discussed in the following in order to concretely show the importance of differentials and derivatives. But first, the use of these terms shall be developed strictly formally, for what the named quantities are not of descriptive means.
A function shall be dependent on two variables and :
How can the derivative of be formed? How is it possible to show that is derived with respect to or or even both at the same time? It is clear, that the styles or do not help since they leave the question unanswered. What serves simplification is writing it as a differential quotient.
Looking at the derivative with respect to first:
Through the -symbol at the place of , it is shown that the function is derived with respect to only; -components are considered to be constant. Therefore, this is called partial derivative of with respect to . The partial derivative with respect to is written in the same way:
Sometimes, the variables taken as constant are written as an index in brackets on the bottom right, i.e.:
This convention is frequently used in thermodynamics to show, what variables shall be kept as constants; in most cases - and in this special one - the -symbol is sufficient.
Through multiplication of the partial derivatives with the differentials of the respective variables and summation of both terms, it is possible to write the differential of the function :
in short:
Above we have written down the total differential of . It is obviously not possible to show the first derivative of the function with respect to both variables. The second derivative is easier to specify, it is written .
We may find an expression for the first derivative, if the variables and depend on a common parameter . If
then the derivatives of and with respect to can be calculated. Considering these derivatives in the equation of the differential , it follows:
In this equation, is derived with respect to all variables. Therefore the expression is the total derivative of the function .
The terms and do not have to be written with the -symbol since and depend on only and not on other variables; partial derivatives thus are not necessary at this point.