A 1500 W kettle warms 1.00 kg of water from...

Question

A 1500 W kettle warms 1.00 kg of water from 18oC to 88oC in a time of 3.6 min. How efficient is the kettle?

Answer: 91%

Answer 1

c = specific heat of liquid water = 4.22 kJ/(kg K) (from tables)

 

Note ΔT = same number in degrees Celsius or Kelvin

 

Ideally, the rate of heat transfer into the water would be:

 

Q/t = m c ΔT / t = (1 kg)[4220 J / (kg K)] [(88-18) K] / [(3.6)(60) s] = 1368 W

 

But the kettle needs 1500 W to achieves the same temperature rise over the same time interval.

 

Efficiency = ideal power / actual power = (1368 W) / (1500 W) = 0.912

 

So it is about 91.2% efficient.