A) 36 erg
B) 288 erg
C) 144 erg
D) 72 erg
Correct Answer: C
Solution :
[c] Work done = Surface tension \[\times \] increase in area of the film \[W=S\times \Delta A\] \[Increase\text{ }in\text{ }area\text{ }=Final\text{ }area\text{ }-\text{ }initial\text{ }area\]\[=10\times (0.5+0.1)-10\times 0.5=1c{{m}^{2}}\] \[\therefore \,\,\,W=72\times 2\times 1=144erg\] [\[\therefore \] There are 2 free surfaces; \[\therefore \,\,\Delta A=2\times 1\]].
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