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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.
Solution
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
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