Question

An electric kettle used to prepare tea, takes 2minutes to boil 4cups of water (1cup contains 200cc of water) if the room temperature is 25°C. (a) If the cost of power consumption is Rs. 1.00per unit (1unit = 1000 watt-hour), calculate the cost of boiling 4cups of water, (b) What will be the corresponding cost if the room temperature drops to 5° C?

Answer 1

Volume of water boiled = 4 × 200cc = 800cc

T₁ = 25°C  T₂ = 100°C => T₂ – T₁ = 75°C

Mass of water boiled = 800 × 1 = 800gm = 0.8kg

Q(heat req.) = MSΔθ = 0.8 × 4200 × 75 = 252000J.

1000watt – hour = 1000 × 3600 watt-sec = 1000 × 3600J