The magnitude of magnetic induction at the mid-point O due to the current arrangement shown in the figure is:
\( \frac{\mu_0 I}{2 \pi a} \)
\( \frac{\mu_0 I}{\pi a} \)
\( \frac{\mu_0 I}{4 \pi a} \)
- Magnetic field contributions due to segments \( BC \) and \( ET \) are outward at point \( O \).
Total magnetic field:
\[ B = \frac{\mu_0 I}{4 \pi r} + \frac{\mu_0 I}{4 \pi r} = \frac{\mu_0 I}{\pi a}. \]

In the first configuration (1) as shown in the figure, four identical charges \( q_0 \) are kept at the corners A, B, C and D of square of side length \( a \). In the second configuration (2), the same charges are shifted to mid points C, E, H, and F of the square. If \( K = \frac{1}{4\pi \epsilon_0} \), the difference between the potential energies of configuration (2) and (1) is given by: