# Finding the value of the element dq

1. Sep 11, 2009

### warrior_1

1. The problem statement, all variables and given/known data
Ok the question states the following, explain why the element of charge, dq, located within an infinitesimally thin spherical shell or radius r is equal to rho(r)4$$\pi$$(r)^2dr, where dr is the thickness of the shell

2. Relevant equations

3. The attempt at a solution
ok i know that total charge is equal to charge density multiplied by volume, which is equal to rho*4/3pi*r^2. Hence if i were to find dq, i would have to differentiate with respect to dq/dr and solve for dq... thus dq/dr=rho(r)*dv/dr
where dv/dr=4*pi*r^2, therefore if i solve for dq i should get dq=rho(r)*4*pi*r^2*dr...

ok i have no idea if that was right or not... any hints and also why dont we differentiate rho(r)

Last edited: Sep 11, 2009