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A long conducing cylinder having a radius $â€™bâ€™$ is placed along the $z$ axis. The current density is $J = J_{a} \:r^{3}\:\hat{z}$ for the region $r< b$ where $r$ is the distance in the radial direction. The magnetic field intensity $\text{(H)}$ for the region inside the conductor (i.e. for $r< b$) is

- $\dfrac{J_{a}}{4}\:r^{4}$
- $\dfrac{J_{a}}{3}\:r^{3}$

- $\dfrac{J_{a}}{5}\:r^{4}$

- $J_{a}\:r^{3}$