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

Equate (Power Rating Of Kettle) Times (t Seconds) To

(Specific Heat For Water) Times (Mass Of Water) Times (Change In Temperature).

Build the equation as 1500 (Watts Or Joules/Second) x t (Seconds) Equals

4184 (Joules Per Kilogram Per Degree Celsius) x (1 Kilogram) x [(88 - 18) (Degrees Celsius)].  

Rewrite as 1500t J = 4184 (J/Kg/°C) x 1 (Kg) x 70 (°C), which reduces to t = 195.2533333 Seconds.

195.2533333 Seconds Divided By 3.6 Minutes (or 216 Seconds) gives (195.2533333/216) or 

0.9039506171 equivalent to 90.4% efficiency for the kettle.