Two long thin parallel wires carry equal current I and are suspended by insulating threads of length L, each thread making angle θ with the vertical as shown. If the mass per unit length of each wire is λ, find I.

Question

G1 Physics · Accepted Answer

The wire separation is d=2L sinθ. Magnetic force per unit length is μ₀I²/(2πd). Equilibrium gives this equal to λg tanθ. Solving yields I=2sinθ√[πλgL/(μ₀cosθ)], option D.