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Two long parallel wires carry currents I₁ and I₂ in the SAME direction, separated by distance d. The force per unit length between them is:
AAttractive, μ₀ I₁ I₂ / (2π d)
BRepulsive, μ₀ I₁ I₂ / (2π d)
CAttractive, μ₀ I₁ I₂ d / 2
DZero
Answer & Solution
Correct answer: A. Attractive, μ₀ I₁ I₂ / (2π d)
1. Field from wire 1 at wire 2: B = μ₀ I₁ / (2π d).
2. Force per unit length on wire 2: F/L = I₂ B = μ₀ I₁ I₂ / (2π d).
3. Same-direction currents ATTRACT; opposite currents repel.
4. This was the historic definition of the ampere.
_Source: NCERT Class 12 Physics Ch 4 §4.7 Force between Parallel Currents_
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