Question

1.0 kg of water is contained in a 1.25 kW kettle. Calculate the time taken for the temperature of water to rise from 25° C to its boiling point of 100°C. Specific heat capacity of water = 4.2 J g^-1K^-1.

Answer 1

m = 1.0 kg; Δt = 100 - 25 = 75° C

Specific heat capacity = 4.2 J g^-1 k^-1 = 4200 J kg^-1 k^-1

Power of kettle = 1.25 kw = 1250 watt

Let time be t second,

ε = P × t  and θ = m c Δt

∴ P × t = m c Δt

∴ 1250 × t = 1 × 4200 × 75

∴ t = `(4200 xx 75)/1250`

∴ t = 252 sec

∴ t = 4 min 12 sec