First slide

Surface tension and surface energy

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

Moderate

A

$0 .0125 {Nm}^{- 1}$
B

$0 .1 {Nm}^{- 1}$
C

$0 .05 {Nm}^{- 1}$
D

$0 .025 {Nm}^{- 1}$

Solution

Here, $W = 1 .5 \times 10^{- 2} N, l = 30 cm = 30 \times 10^{- 2} m$
The force due to surface tension will balance the weight.
$i.e., F = W$
A liquid film has two free surfaces.
$2 SL = W or S = \frac{W}{2 L}$
$S = \frac{1 .5 \times 10^{- 2}}{2 \times 30 \times 10^{- 2}} = 0 .025 {Nm}^{- 1}$
A slider will support the weight when the force of surface tension acting upwards on the slider (= 2S/) balances the downward force due to weight (= W).

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