The force between two magnetic poles separated by a distance 'd' in air is F. At what distance between them the force becomes doubled.

Explanation

To solve the problem, we need to understand the relationship between the force between two magnetic poles and the distance separating them. The force \( F \) between two magnetic poles \( M_1 \) and \( M_2 \) separated by a distance \( d \) can be expressed using the formula:

\[ F = \frac{\mu_0}{4\pi} \cdot \frac{M_1 \cdot M_2}{d^2} \]

where \( \mu_0 \) is the permeability of free space.

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