What's the area of this volume?

[Jin314159]

Consider a 3d coordinate system with axis x,y and z.

We are given a circle on the x-z plane with function z^2 + (x-a)^2 = a^2. We rotate this circle 90 degrees around the z-axis. What's the volume of the resulting surface?

[arildno]

Well, shouldn't that be a quarter of a torus?
I'll opt for that and say:
V=\pi{a}^{2}*\frac{\pi}{2}a=\frac{\pi^{2}}{2}a^{3}

Edit:
The area function for a given angle measured relative to the x-axis (and with the origin as the pole) is f(\theta)=\pi{a}^{2}

By Cavalieri's principle, we have:
V=\int_{0}^{\frac{\pi}{2}}\pi{a}^{2}ad\theta