Question

A thin liquid film formed between a $ U $ shaped wire and a light slider supports a weight of $ 1.5 \times 10^{-2} \, N $ , as shown in the figure. The length of the slider is $ 30 \,cm $ and its weight negligible. The surface tension of the liquid film is

• A. $ 0.0125\, N \, m^{-1} $
• B. $ 0.1 \, N \, m^{-1} $
• C. $ 0.05 \, N \, m^{-1} $
• D. $ 0.025 \, N \, m^{-1} $

Answer 1

Answer: (D)

Here, $ W= 1.5 \times 10^{-2} \, N $ , $ l = 30 \,cm = 30 \times 10^{-2}\, m $ The force due to surface tension will the balance the weight. i.e., $ F = W $
A liquid film has two free surfaces

$ 2TL=W or T=\frac{W}{2L} $
$ T=\frac{1.5\times10^{-2}}{2\times30\times10^{-2}}=0.025\,N\, m^{-1} $
A slider will support the weight when the force of surface tension acting upwards on the slider $ (= 2Sl) $ balances the downward force due to weight $ (= W) $ .