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Q. The current in flowing along the path ABCD 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.
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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 $DAEFGCD,$ then the magnitude of magnetic field at the ecnntre will be

KVPYKVPY 2018

Solution:

Given magnetic field due to a square loop $DABCA$ at centre of cube is $B.$
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Now, loop $DAEFGCD$ can be viewed as super position of three square loops as shown below.
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So, net field at centre = resultant of fields of these three loops
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$\therefore B_{ net }=B \hat{i}+B\hat {j}-B \hat{k}$
Magnitude of resultant field is
$|B_{net}| =\sqrt{B^{2}+B^{2}+B^{2}=}\sqrt{3}.B$