# Question #e4aa5

##### 1 Answer

Jan 24, 2017

A point

As you have rightly posted the figure, infinitesimal volume element

- Infinitesimal radial element
#=dr# - Recall formula which relates the arc length
#s# of a circle of radius#r# to the central angle#theta# is given by

#s=rtheta# , where angle is in radians.

Hence, arc length subtended by angle#d theta# at a distance#r# would be#=rd theta# - Consider area element as shown in the figure below

We see that area element is located at a perpendicular distance#=r sin theta# from the#z# -axis.

Using the same formula as used in step 2. above we see that arc length subtended by angle#d phi# at a distance of#rsin theta# would be#=rsin theta dphi#

Therefore we get#dV=drcdot rd theta cdot rsinthetadphi#

#=>dV=r^2 sin theta cdot dr cdot d theta cdot dphi#

**A word of caution** : Sometimes