Spherical Shell

Solid enclosed between two concentric spheres is called a spherical shell. For a spherical shell if R and r are outer and inner radii respectively, then the volume of the shell is

 = \frac{4}{3}\pi \left( {{R^3} - {r^3}} \right)

Or

 = \frac{\pi }{6}\left( {{D^3} - {d^3}} \right)

Example:

A sphere of radius 5cm is dropped into a cylindrical vessel partly filled with water. The diameter of the vessel is 10cm. If the sphere is completely submerged, by how much will the surface of water be raised?

Solution:

The sphere will displace a volume of water equal to that of itself and this amount of water will go up.
\therefore   Volume of sphere  = \frac{4}{3}\pi {r^3} = \frac{4}{3}\pi {\left( {\frac{5}{2}} \right)^3} = \frac{{125}}{6}\pi cu.cm

The radius of the cylinder vessel  = \frac{{10}}{2} = 5cm

The volume height occupied by \frac{{125}}{6}\pi cu.cm. of water in a cylindrical vessel of 5cm radius  = \frac{{125}}{6}\pi + \pi {\left( 5 \right)^3}

 = \frac{{125}}{{6 \times 25}} = \frac{5}{6}cm

Example:

A solid cylinder of glass, the radius of whose base is 9cm and height 12cm is melted and turned into sphere. Find the radius of the sphere so formed.

Solution:

Volume of the cylinder  = \pi {r^2}h = \pi \times {\left( 9 \right)^2} \times 12
 = 972\pi cu.cm       --- (1)

Volume of the sphere    = \frac{4}{3}\pi {r^3}                 --- (2)

By the given condition,
(1) = (2)  \Rightarrow \frac{4}{3}\pi {r^3} = 972\pi
 \Rightarrow {r^3} = 729 \Rightarrow r = 9