Question

Two parallel wires each of length 10 cm are 0.5 cm apart. A film of water is formed between them. If surface tension of water is 72 dynes/cm the work done in increasing the distance between the wires by 1 mm will be -

(1) 1.44 × 10–7 J

(2) 7.2 × 10–5 J

(3) 1.44 × 10–5 J

(4) 7.2 × 10–3 J

Answer 1

Correct Answer is: (3) 1.44 × 10–5J

By increasing the distance between the wires by 1 mm the increase in area of the film. ΔA = 2 × 0.1 × 10–3 = 2 × 10–4 m2

(Film has two surfaces)

Work done = TΔA

= 72 × 10–3 × 2 × 10–4 J

= 144 × 10–7 J = 1.44 × 10–5 J