Question

An electric kettle is rated 5 kW. How much time will it take to boil 15 kg of water initially 15°C. if it is connected to 230 V supply? Of One unit of electric energy costs ? 1.21, find the cost incurred

Answer 1

Given:

Energy = 5kW

Mass of water = 15kg

Temperature = 15°C

Voltage = 230V

To Find:

The cost incurred

Solution:

Q = mCvdT

where Cv = specific heat at a constant volume = 4.186KJ/kgK

dT= change in temperature = 100° - 15° = 850°C

whereas, kelvin scale dT= 273 + 85 = 358K

Substituting values -

5 = 15 × 4.186 × 358/dt

dt = 4495.764 sec

Power = Work /Time

5000J of work= 1 sec  

Thus,  4495.764sec joule of work = 5000 × 4495.764

Total work = 22478820 joule

One unit = 3600000 joule work

Units in 22478820 joule work =22478820/3600000

Total units = 6.24 units

Total electricity cost

= 6.24 × 1.21

= 7.5

Answer: The total cost incurred is Rs. 7.5