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The ampere is defined as the steady current which, maintained in two long parallel wires 1 m apart in vacuum, produces a force per metre of length equal to:
A2 × 10⁻⁷ N
B2 × 10⁻⁷ N m⁻¹
C1 × 10⁻⁷ N m⁻¹
D4π × 10⁻⁷ N m⁻¹
Answer & Solution
Correct answer: B. 2 × 10⁻⁷ N m⁻¹
1. Force per unit length: $f = \dfrac{\mu_0 I_a I_b}{2\pi d}$.
2. Set $I_a = I_b = 1$ A and $d = 1$ m.
3. Then $f = \dfrac{4\pi\times10^{-7}\times1\times1}{2\pi\times1} = 2\times10^{-7}$.
4. So the defining force is $2\times10^{-7}$ N per metre of length.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.156_
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