Two long straight parallel wires each carry a current of 10 A in the same direction. They attract each other with a force of \(2\times10^{-4}\ \mathrm{N}\) per metre of length. The distance between the wires is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 10 cm
Each wire lies in the magnetic field of the other, so each feels a force. The force per unit length between two long parallel wires is \(f=\mu_0I_aI_b/2\pi d\).
\( \begin{aligned} d &= \frac{\mu_0 I_aI_b}{2\pi f} \\ &= \frac{4\pi\times10^{-7}\times10\times10}{2\pi\times2\times10^{-4}} \\ &= 0.1\ \mathrm{m} \\ &= 10\ \mathrm{cm} \end{aligned} \)
NCERT (2026-27):
Force Between Two Parallel Currents, the Ampere – “Let fba represent the magnitude of the force Fba per unit length. Then, …”
\( f_{ba}=\frac{\mu_0 I_aI_b}{2\pi d} \)\( \qquad (4.19) \)