The current in flowing along the path $A B C D$ of a cube (shown in the left figure) produces a magnetic field at the centre of cube of magnitude $B$. Dashed line depicts the non-conducting part of the cube. Consider a cubical shape shown to the right which is identical in size and shape to the left. If the same current now flows in along the path $D A E F G C D$, then the magnitude of magnetic field at the centre will be
KVPY 2018, Advanced
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$(c)$ Given magnetic field due to a square loop $D A B C A$ at centre of cube is $B$.

Now, loop $D A E F G C D$ can be viewed as super position of three square loops as shown below.

So, net field at centre $=$ resultant of fields of these three loops

$\therefore \quad B _{\text {net }}=B \hat{ i }+B \hat{ j }-B \hat{ k }$

Magnitude of resultant field is

$\left| B _{\text {net }}\right|=\sqrt{B^{2}+B^{2}+B^{2}}=\sqrt{3} \cdot B$

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