Question 97: Properties of Matter

Question

A water drop of radius $$1 \mathrm{~cm}$$ is broken into 729 equal droplets. If surface tension of water is 75 dyne/ $$\mathrm{cm}$$, then the gain in surface energy upto first decimal place will be :
(Given $$\pi=3.14$$ )

Accepted Answer

$$729 	imes {4 \over 3}\pi {r^3} = {4 \over 3}\pi {R^3}$$
$$ \Rightarrow R = 9r$$ ........ (1)
$$\Delta U = S 	imes \Delta A$$ ..... (2)
$$ \Rightarrow \Delta U = S 	imes \{  - 4\pi {R^2} + 729 	imes 4\pi {r^2}\} $$
$$ = S 	imes 4\pi \{ 729{r^2} - 81{r^2}\} $$
$$ = 7.5 	imes {10^{ - 4}}\,J$$

Answer

$$8.5 	imes 10^{-4} \mathrm{~J}$$

Answer

$$8.2 	imes 10^{-4} \mathrm{~J}$$

Answer

$$7.5 	imes 10^{-4} \mathrm{~J}$$

Answer

$$5.3 	imes 10^{-4} \mathrm{~J}$$

Properties of Matter

A water drop of radius $1 \mathrm{~cm} is broken into 729 equal droplets. If surface tension of water is 75 dyne/ \mathrm{cm}, then the gain in surface energy upto first decimal place will be : (Given \pi=3.14$ )

Choose the correct answer: