First slide

Surface tension

Question

There is a horizontal film of soap solution. On it a thread is placed in the form of a loop. The film is pierced inside the loop and the thread becomes a circular loop of radius R. If the surface tension of the loop be T, then what will be the tension in the thread ?

Moderate

A

${πR}^{2} / T$
B

${πR}^{2} T$
C

$2 πRT$
D

RT

Solution

Here, the force on semicircle is to be considered' So, Surface energy of film enclosed = Tension x length of the thread in semi-circle
$\begin{matrix} T \times {πR}^{2} = F \times πR \\ or F = TR \end{matrix}$

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