Question

Force between two identical short bar magnets whose centres are metre apart is 4.8 when their axes are in

Answer 1

The required "option (d) 0.3 N" is correct.

Step-by-step explanation:

The complete question:

Force between two identical bar magnets whose centres are r metre apart is 4.8 N when their axes are in the same line. If separation is increased to 2r, the force between them is reduced to

(a) 2.4 N           (b) 0.6 N      (c) 1.2 N     (d) 0.3 N

The two magnets placed along same axis/line at distance r(between their centres) exert a force on each other.

We know that,

Force, $F=\dfrac{6m_{1}m_{1}}{r^4}$

Where, $m_{1}$ and $m_{2}$ are magnetic moments

∴$\dfrac{F_{2}}{F_{1}}= (\dfrac{r_{1}}{r_{2}})^4$

⇒ $\dfrac{F_{2}}{F_{1}}= (\dfrac{1}{2})^4=\dfrac{1}{16}$

⇒ $F_{2}=\dfrac{F_{1}}{16}$

$=\dfrac{4.8}{16}=0.3N$

Hence, the required "option (d) 0.3 N" is correct.