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Question:

Find force per unit length at P

10⁻⁴m

10⁻⁴N/m

3×10⁻⁴N/m

0.3 N/m

Solution:

Correct option is B. 10⁻⁴N/m
Let length of wire element at P is dl. Magnetic field at P is B.
B = μ₀i/4πr + 0 + μ₀i/4πr
B = μ₀ × 5 / 4π × 5 × 10⁻² ; × 2
B = μ₀/4π × 200
B = 2 × 10⁻⁵T
F = iBdl
F/dl = iB = 5 × 2 × 10⁻⁵ = 10⁻⁴N/m.