For a charged spherical ball, electrostatic potential inside the ball varies with \( r \) as \( V = 2ar^2 + b \).
Here, \( a \) and \( b \) are constant; \( r \) is the distance from the centre. The volume charge density inside the ball is \( -\lambda a \varepsilon \). The value of \( \lambda \) is _____________.
\( \varepsilon \) = permittivity of the medium.