Q. A drop of liquid of diameter 2.8 mm breaks up into 125 identical drops. The change in energy is nearly $(S = 75\ dyne\ cm^{- 1})$
A) Zero
B) 19 erg
C) 46 erg
D) 74 erg
Explanation
(d) Here, $R = 2.8/2 = 1.4\ mm = 0.14\ cm$
$= \frac{4}{3}\pi R^{3} = 125 \times \frac{4}{3}\pi r^{3}$
Or $r = R/5 = 0.14/5 = 0.028\ $cm
Change in energy = surface tension $\times$ increase in area
$= 75 \times (125 \times 4\pi r^{2} - 4\pi R^{2})$
$= 74\ erg$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.