The electric field $E$ produced by an infinite line charge with linear charge density $\lambda$ at a distance $r$ is given by $E = \frac{\lambda}{2\pi\varepsilon_0 r}$. Rearranging for $\lambda$: $\lambda = E(2\pi\varepsilon_0 r) = \frac{E \cdot r}{2k}$ where $k = \frac{1}{4\pi\varepsilon_0} = 9 \times 10^9$ N m$^2$ C$^{-2}$. Given: $E = 9 \times 10^4$ N/C $r = 2$ cm $= 0.02$ m

Calculation: $\lambda = \frac{(9 \times 10^4) \times 0.02}{2 \times (9 \times 10^9)}$ $\lambda = \frac{1.8 \times 10^3}{18 \times 10^9} = 0.1 \times 10^{-6}$ C/m $\lambda = 0.1 \, \mu$C/m. (See NCERT Physics Class 12, Exercise 1.23).