Infinite number of straight wires each carrying current I are equally placed as shown in the figure. Adjacent wires have current in opposite direction. Net magnetic field at point P is-

Question

G1 Physics · Accepted Answer

Each wire contributes μ₀I/(2πrₙ) with alternating sign. From the 30° geometry rₙ=n√3a, so B∝1-1/2+1/3-1/4+⋯=ln 2 for each side; combining the symmetric sets gives ln 4. Thus B⃗=μ₀I ln 4/(4π√3a)k̂, option B.