Using the same notations as in this picture :

enter image description here

The element of volume is: $r^2 \sin(\theta) \, dr \, d\theta \, d\phi$

If I try to create the volume visually, I begin with integrating $r$ between $0$ and $R$ to get the radius. Now I can either:

  • integrate $\theta$ between $0$ and $\pi$ to have the area of half a circle, and then integrate $\phi$ between $0$ and $2\pi$ to have the full sphere volume

  • integrate $\theta$ between $0$ and $2\pi$ to have the area of a circle, and then integrate $\phi$ between $0$ and $\pi$ to have the full sphere volume

Of course the second method won't work because $$\int_0^{2\pi} \sin(\theta)d\theta = 0.$$

But I don't understand why it works visually but it fails when I write it down. I'm not sure there is something to understand but it still disturbs me...

  • 1
    $\begingroup$ For your second case you consider $\phi$ the polar angle and therefore the volume element changes to $$r \sin (\phi) \, dr \, d \theta \, d \phi.$$ $\endgroup$ – Mark Fantini Oct 4 '14 at 11:06
  • $\begingroup$ Your $\theta$ is measuring the angle from the positive z-axis, so the values of $\theta$ that make sense are $0\le\theta\le\pi$. $\endgroup$ – user84413 Oct 4 '14 at 17:15

To your question about choosing the bounds of integration, it is customary to use $\theta \in [0,\pi]$. It is done because of the issue with integrating the $\sin (\theta)$ as you mentioned. What you are dealing with is the 3D equivalent of integrating a sine-function. If you evaluate: $$\int_0^{2\pi} \sin(\theta)d\theta$$ You obtain: $$\left[ \cos(\theta) \right ]_0^{2\pi}=[1-1]=0$$ This is because the integral evaluates the negative regions of the sinusoid as negative area. See the pink regions of this image?

Wolfram alpha plot

However, in the case of the sphere, the entire volume enclosed is positive volume. So you want to count the negative values of the sinusoid as positive, giving the total enclosed area. Thus, you are actually integrating: $$\int_0^{2\pi} |\sin(\theta)|d\theta$$ This function looks like:

Absolute integral sine function

So to your point about the sinusoidal integral, you would assume symmetry and make the bounds different ala this: $$\int_0^{2\pi} |\sin(\theta)|d\theta=2\int_0^{\pi} \sin(\theta)d\theta$$

If you apply this, you obtain the correct volume integration.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.