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A long coaxial cable carries current $I$ (along its surface ). The current flows down the surface of inner cylinder of radius $a$ and back along the outer cylinder of radius $b$

• A
  Energy stored in magnetic field per unit length is $\frac{{{\mu _0}{I^2}}}{{8\pi }}\ln \frac{b}{a}$
• B
  Energy stored in magnetic field per unit length is $\frac{{{\mu _0}{I^2}}}{{2\pi }}\ln \frac{b}{a}$
• ✓
  Self inductance of given arrangement per unit length is $\frac{{{\mu _0}}}{{2\pi }}\ln \left( {\frac{b}{a}} \right)$
• D
  Self inductance of given arrangement per unit length is $\frac{{{\mu _0}}}{{4\pi }}\ln \left( {\frac{b}{a}} \right)$