Two infinitely long straight parallel conductors carrying currents $I_1$ and $I_2$ are held at a distance d apart in vacuum. The force F on a length L of one of the conductors due to the other is _________.

Fill in the blank with the correct answer from the options given below.

proportional to $I_1 \times I_2 \times L$

Force per unit length is $ \frac{F}{L} = \frac{\mu_0 I_1I_2}{4\pi d}$

$\Rightarrow F = \frac{\mu_0 I_1I_2 L}{4\pi d}$

The correct answer is Option (3) → proportional to $I_1 \times I_2 \times L$