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
3×103Nm1
B
2×105Nm1
C
4×104Nm1
D
2.5×102Nm1

Solution

## The correct option is B $$2.5\times { 10 }^{ -2 }N{ m }^{ -1 }$$A soap film has two free surfaces, so total length of the film to be supported,$$l=2\times 30cm=0.60m$$Let $$T=$$ surface tension of the film.If $$f=$$ total force on the slider due to surface tension,then   $$f=T\times 2l=T\times 0.6N$$          $$W=1.5\times { 10 }^{ -2 }N$$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 slideri.e., $$f=w=mg$$$$T\times 0.6=1.5\times { 10 }^{ -2 }$$$$T=\dfrac { 1.5\times { 10 }^{ -2 } }{ 0.6 }$$$$T=2.5\times { 10 }^{ -2 }N{ m }^{ -1 }$$Physics

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