Question

A co-axial cable which carrying equal current $I$ in opposite direction have cross section as shown in diagram. If $B_1$ and $B_2$ are the magnetic field at inner and outer point of cable, then which of the following relation is correct

• A. $B_1=0,B_2=0$
• B. $B_1\ne 0,B_2\ne 0$
• C. $B_1=0,B_2\ne 0$
• D. $B_1\ne 0,B_2=0$

Answer 1

The correct option is D $B_1\ne 0,B_2=0$

Using Ampere's circuital law,

$$\oint B\cdot dl={\mu }_0{I}_{enc}$$

Since both the coil carries cureent $I$ in oppsite direction.

$\therefore$ For any point outside the conductor, $I_{enc}=0$ but for any inside point, $I_{enc}\ne 0$

Hence, $B_1\ne 0\text{and}{B}_2=0$

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(D\right)$ is the correct answer.