A 5.5-kW water heater operates at 240 V.

i. Should the heater circuit have a 20A or a 30A circuit breaker? [2 marks]

ii. Assuming 85% efficiency, how long will the heater take to heat the water in

a 0.2085-m3 tank from 20°C to 80°C? The density of water is 1,000 kg/m3

and the specific heat capacity is 4,200 J/kg/°C.

The time taken is    

Explanation:

From the question we are told

      The power the water is

      The the voltage  of the heater is  is  

       The volume of the heater  

        The specific heat of water is  

         The initial temperature is

        The final temperature is

         The density of water is  

The current of the heater is mathematically represented as

substituting values we have

So since the current produced is greater than 20 A hence the heater current rating would be 30A

      The quantity of heat required to heat the water is mathematically represented as

Where m is the mass which is mathematically evaluated as

Therefore  

Since the heater has an  efficiency then the heat generate by the heater is  

Now Power is mathematically  represented as

Where t is the time taken for heater to heat the water

=>      

Substituting values  

a and c

c is 100% but i am confused in a

measure their densities same density, same sub