4.
A copper kettle of mass 1 kg contains 0.9 kg of water at 15°C. It takes 12 minutes to rise the
temperature to 95°C. If the electric energy is supplied at 220V, calculate the electric current,
assuming 10% of heat is wasted. Given specific heat of copper = 420 J kg-K-1 and specific heat of
water = 4200 J kg-1 K-1.
Answers
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Given the power rating of the electrical kettle
P=2.1kW
If time taken to raise the temperature of 1 kg of water from
20∘
→100∘C
is t sec, then the electrical energy required for this will be
=pt
=2.1t kJ
Again the heat energy reqired to raise the temperature of 1 kg of water from 20∘
→100∘C is1×4200×(100−20)J
=336KJ
So 2.1t=336
⇒t=3362.1
=160s
= 2 min 40 sec