First slide

Magnetism

Question

Force between two identical bar magnets whose centre are ‘r ’ meter apart is 6.4 N. When their axis are in the same line. If the separation is increased to ‘2r ’ meter, the force between them is reduced to

Moderate

A

3.2 N
B

1.6 N
C

0.8 N
D

0.4 N

Solution

Two bar magnets placed along same axis/line at distance ‘r ’(between their centers)
exert a force on each other
$F= \frac{μ_{o}}{4 π} \frac{{6M}_{1} M_{2}}{r^{4}}$
$∴F \propto \frac{M_{1} M_{2}}{r^{4}}$ Where $M_{1} & M_{2}$ are magnetic moments.
$\Rightarrow \frac{F_{1}}{F_{2}} = {(\frac{r_{2}}{r_{1}})}^{4} \Rightarrow F_{2} {=F}_{1} {(\frac{r_{1}}{r_{2}})}^{4}$
$\Rightarrow F_{2} =6.4 \frac{1}{16} = 0.4 N$ .

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