Question

The only possibility of heat flow in a thermos flask is through its cork which is 75 cm² in area and 5 cm thick. Its thermal conductivity is 0.0075 cal/cm sec^0C. The outside temperature is 40^0C and latent heat of ice is 80 cal g^–1. Time taken by 500 g of ice at 0^0C in the flask to melt into water at 0^0C is

• A. 2.47 hr
• B. 4.27 hr
• C. 7.42 hr
• D. 4.72 hr.

Answer 1

Answer: (A)

2.47 hr

Answer 2

$mL = \frac{KA\Delta\theta t}{\Delta x}$ ⇒ $500 \times 80 = \frac{0.0075 \times 75 \times (40 - 0)t}{5}$

⇒ t = 8.9 × 10³ sec = 2.47 hr.