For a charged spherical ball, electrostatic potential inside the ball varies with r as $\mathrm{V}=2{\mathrm{ar}}^2+\mathrm{b}$ Here, a and b are constant and r is the distance from the center .The volume charge density inside the ball is $-\mathrm{\lambda a\epsilon }$ The value of $\mathrm{\lambda }$ is ______ $(\mathrm{\epsilon }=$permittivity of the medium)