Question

A frame made of a metallic wire enclosing O surface area A is covered with a soap film. If the area of the frame metallic wire is reduced by 50% the energy of the soap film will be changed by 

(A) 100% 

(B) 75% 

(C) -50% 

(D) 25%

Answer 1

Correct option: (C) -50%

Explanation:

Surface energy = surface Tension × effective area

i.e. E = T × 2A

E = 2AT

(E₁ / E₂) = (A₁ / A₂) × (T₁ / T₂)

A₂ = (A₁ / 2) is same

(E₁ / E₂) = {A₁ / (A₁ / 2)} = 2

i.e. (E₂ / E₁) = (1/2)

∴ {(E₂ –E₁) / E₁} = {(1 – 2) / 2}

 i.e. decrease in energy = [(– 1) / 2]

 ∴ % decrease in energy = [(– 1) / 2] × 100 = – 50%