1)

A big water drop is formed by the combination of 'n' small water drops of equal radii. The ratio of the surface  energy of 'n'  drops to the surface energy of big drop is 


A) $n^{2}:1$

B) n:1

C) $\sqrt{n}:1$

D) $\sqrt[3]{n}:1$

Answer:

Option D

Explanation:

 Volume of big drop = n(Volume of small drop)

 $\Rightarrow$     $\frac{4}{3}\pi R^{3}=n.\frac{4}{3}\pi r^{3}$

 where, R= radius of big water drop

and r = radius of small water drop

$\Rightarrow$  $R^{3}  =n r^{3}$

$\Rightarrow $     $R= n^{1/3}.r$  ........(i)

surface energy of n drops,

 $E_{2}= n \times 4 \pi r^{2}\times T$

Surface energy of big drop ,

  $E_{1}=  4 \pi R^{2}T$

$\therefore$     $\frac{E_{2}}{E_{1}}=  \frac{nr^{2}}{R^{2}}= \frac{nr^{2}}{(n^{1/3}.r)^{2}}$

  $= \frac{nr^{2}}{n^{2/3}.r^{2}}= n^{1/3}$     $  [\because $ from eq.(i)]

 $E_{2}:E_{1}=\sqrt[3]{n}:1$