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 $30cm$ and its weight negligible. The surface tension of the liquid film is

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

Answer 1

Answer: (D)

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

$2TL=WorT=\frac{W}{2L}$
$T=\frac{1.5\times 10^{-2}}{2\times 30\times 10^{-2}}=0.025N{m}^{-1}$
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 $(=W)$ .