A) \[8\pi \,r\,\Delta rS\]
B) \[4\pi \,r\,\Delta rS\]
C) \[16\pi \,r\,\Delta rS\]
D) \[2\pi \,r\,\Delta rS\]
Correct Answer: A
Solution :
Work done in blowing a liquid drop or soap bubble \[W=S\times 4\pi [r_{2}^{2}-r_{1}^{2}]\] Here, \[W=S\times 4\pi [{{(r+\Delta r)}^{2}}-{{(r)}^{2}}]\] \[=S\times 4\pi [{{r}^{2}}+{{(\Delta r)}^{2}}+2\Delta r\times r-{{r}^{2}}]\] \[=8\pi \,\Delta rSr\] [\[{{(\Delta r)}^{2}}\] is very small]
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