Question

A U - shaped wire is dipped in a soap solution and removed. The thin soap film formed between the wire and light slider supports a weight of $1.5\times 10^{- 2} \, N$ . The length of the slider is $30 \, cm$ . What is the surface tension of the film?

• A. $3\times 10^{- 3}Nm^{- 1}$
• B. $2\times 10^{- 5}Nm^{- 1}$
• C. $4\times 10^{- 4} \, N \, m^{- 1}$
• D. $2.5\times 10^{- 2}Nm^{- 1}$

Answer 1

Answer: (D)

A soap film has two free surfaces, so the total length of the film to be supported,
$l=2\times 30 \, cm=0.60 \, m$
Let the surface tension of the film
If the total on the slider due to surface tension,
then $f=T\times 2l=T\times 0.6 \, N$
$W=1.5\times 10^{- 2}N$
In equilibrium position, the force f on the slider due to surface tension must be balanced by the weight $\left(w\right)$ supported by the slider
i.e., $f=w=mg$
$T\times 0.6=1.5\times 10^{- 2}$
$T=\frac{1.5 \times 10^{- 2}}{0.6}$
$T=2.5\times 10^{- 2}Nm^{- 1}$