Question

The figure shows the cross-section of two long coaxial tubes carrying equal currents $I$ in opposite directions. If $B_1$ and $B_2$ are magnetic fields at point 1 and 2, as shown in figure then

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

Answer 1

The correct option is A $B_1\ne 0;B_2=0$

At distance r from centre

$B=\frac{{\mu }_o}{2\pi }\frac{\left(i_{in}\right)}{r}$ (From ampere's circle law)

For path-1, $i_{in}\ne 0\therefore B_1\ne 0$

For path-2, $i_{in}=0\therefore B_2=0$