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# Volume integral

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 Title: Volume integral Author: World Heritage Encyclopedia Language: English Subject: Collection: Multivariable Calculus Publisher: World Heritage Encyclopedia Publication Date:

### Volume integral

In mathematics—in particular, in multivariable calculus—a volume integral refers to an integral over a 3-dimensional domain, that is, it is a special case of multiple integrals. Volume integrals are especially important in physics for many applications, for example, to calculate flux densities.

## Contents

• In coordinates 1
• Example 1 2

## In coordinates

It can also mean a triple integral within a region D in R3 of a function f(x,y,z), and is usually written as:

\iiint\limits_D f(x,y,z)\,dx\,dy\,dz.

A volume integral in cylindrical coordinates is

\iiint\limits_D f(r,\theta,z)\,r\,dr\,d\theta\,dz,

and a volume integral in spherical coordinates (using the convention for angles with \theta as the azimuth and \phi measured from the polar axis (see more on conventions)) has the form

\iiint\limits_D f(r,\theta,\phi)\,r^2 \sin\phi \,dr \,d\theta\, d\phi .

## Example 1

Integrating the function f(x,y,z) = 1 over a unit cube yields the following result:

\int\limits_0^1\int\limits_0^1\int\limits_0^1 1 \,dx\, dy \,dz = \int\limits_0^1\int\limits_0^1 (1 - 0) \,dy \,dz = \int\limits_0^1 (1 - 0) dz = 1 - 0 = 1

So the volume of the unit cube is 1 as expected. This is rather trivial however, and a volume integral is far more powerful. For instance if we have a scalar function \begin{align} f\colon \mathbb{R}^3 &\to \mathbb{R} \end{align} describing the density of the cube at a given point (x,y,z) by f = x+y+z then performing the volume integral will give the total mass of the cube:

\int\limits_0^1\int\limits_0^1\int\limits_0^1 \left(x + y + z\right) \, dx \,dy \,dz = \int\limits_0^1\int\limits_0^1 \left(\frac 12 + y + z\right) \, dy \,dz = \int \limits_0^1 \left(1 + z\right) \, dz = \frac 32