Question

Force between two identical bar magnets whose Centres $ r $ meter apart is $4.8\, N$ when their axis are in the same line. If the separation is increased to $2r$ meter, the force between them is reduced to:

• A. 0.3 N
• B. 0.6 N
• C. 2.4 N
• D. 1.2 N

Answer 1

Answer: (A)

Force between two magnets is given by

$F=\frac{\mu_{0}}{4 \pi}\left(\frac{6 M_{1} M_{2}}{r^{4}}\right)$
$\Rightarrow F \propto \frac{1}{r^{4}}$
$\therefore \frac{F_{1}}{F_{2}}=\left(\frac{r_{2}}{r_{1}}\right)^{4}$
$\frac{4.8}{F_{2}}=\left(\frac{2 r}{r}\right)^{4}=16$
$\Rightarrow F_{2}=0.3\, N$