Question

Water of mass 3kg initially at 20℃ is heated in an electric kettle rated 3.0kW. Thewater is heated until it boils at 100℃. Taking specific heat capacity of water=4200Jkg/K, heat capacity of kettle=450J/kg, specific latent heat of vapourisation of water =2.3MJ/kg, calculate;

1. The heat absorbed by the water.
2. Heat absorbed by the electric kettle.
3. The time taken for the water to boil .
4. How much longer it will take to boil away all the water? 

Answer 1

1. The heat absorbed by the water. 
   • Q = MCΔθ
     = 3kg x 4200JKg^-1k^-1 x 80 
     = 240 x 4200 = 1008000J / 1.008kJ 
2. Heat absorbed by the electric kettle. 
   • Q = M_kC_kΔθ
     = 450 x 80  = 36000J
3. The time taken for the water to boil . 
   • pt = total heat absorbed
     pt = 36000 + 10080001
     3000t = 1044000
     t = ^1044000/₃₀₀₀
     t = 348 sec
4. How much longer it will take to boil away all the water? (3 marks)
   • pt = mLv
     3000t = 3 x 2.3 x 10⁶
     t = (3 ×2.3 × 10⁶)/3000
     t = 2300 seconds