Question

A thin liquid film formed between a $U$-shaped wire and light slider supports a weight of $1.5\times {10}^{-2}N$ (see figure). If the length of the slider $30cm$ and its weight negligible, then 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)

$0.025N{m}^{-1}$

Here, $W=1.5\times {10}^{-2}N,$

$l=30cm=30\times {10}^{-2}m$

The force due to surface tension will 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 $\left(2Sl\right)$ balances the downward force due to weight $\left(W\right).$