A ring is cut from a platinum tube 8.5 , cm internal and 8.7 , cm… - Vidyadip

MCQ

A ring is cut from a platinum tube $8.5\, cm$ internal and $8.7\, cm$ external diameter. It is supported horizontally from the pan of a balance, so that it comes in contact with the water in a glass vessel. If an extra $3.97\,gm$ weight is required to pull it away from water, the surface tension of water is ......... $dyne\, cm^{-1}$

✓
$72$
B
$70.80$
C
$63.35$
D
$60$

✓

Answer

Correct option: A.

$72$

a
$\left( {2\pi {r_1} + 2\pi {r_2}} \right)\sigma = mg$

$\left[ {2\pi \times \frac{{8.7}}{2} + 2\pi \times \frac{{8.5}}{2}} \right]\sigma = 3.97 \times 980$

$ \Rightarrow \sigma = 72\,dyne\,c{m^{ - 1}}$

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