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The magnetic force acting per meter length in two infinitely long wires placed parallel to each other at a distance of 1 meter in vacuum, carrying identical current of 1 mA is ______ N.

Asked in GSEB Board March 2020 (old course) · Force between parallel currents

• (1) 2×10⁻¹⁰
• (2) 2×10⁻⁷
• (3) 2×10⁻¹³
• (4) 2×10⁻⁴

Step-by-step solution

Why the other options are wrong

• (1) Converts only one of the two currents: 2×10⁻⁷×10⁻³×1. Both wires carry 1 mA, so the factor 10⁻³ must appear twice.
• (2) The force per metre between wires 1 m apart carrying 1 A each — the old definition of the ampere. These currents are a thousand times smaller and the force carries the square of that.
• (4) No conversion slip produces it: it is 10⁹ times the answer. Sanity check: (μ₀ I₁I₂)/(2π d)=2×10⁻⁷×(10⁻³)².

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