Question

A partially filled (plastic) kettle is used to boil water.

The kettle contains tap (temperature) water before it is switched on.

If the thermostat to switch off the kettle when it boils is faulty:

• Q2d: Explain the solution strategy which would allow you to calculate the time needed to bring water to the boil.
• Q2a: Detail all the assumptions needed to arrive at the solution, whether they relate to equations used (i.e., the theoretical basis) or any values which must be assumed for variables (or properties) in order for you to complete the solution.
• Q2b: Perform (a sample set) of calculations to demonstrate the solution strategy, ensuring you show all steps, substitution of values, calculations, final answer, and units (if applicable).

Q2c: Insert (clips) of figures, charts, tables for any data used in the solution. Resize these to fit in the space(s) provided.

Answer 1

Given, oartially filled plastic kettle is used to boil water
it contains tap water at room temperature before it is weitched on

assuming that the thermostat for the kettle is faulty
d. to calculate time taken by the water to boil we follow the following strategy
1. find out volume / mass of water in kettle
2. find power rating of the kettle
3. from power rating P, time taken to heat water should be mC(Tf - Ti)/P
where C is heat capacity of water, Tf is final temperature of water = 100 C
C = 4186 J/kg
Ti = 20 C ( initial temp)
hence
t = m*334880/P
where m is mass of water in kettle in kg and P is power supplied by kettle to the water
  

a. now, in this case we have assumed many things like :
1. power input of the kettle is equal to the power supplied by the kettle to het the water.
this is not true as a fraction of energy used by kettle will be used in resistive losses and dissipated to ehe environment and hence net energy reching the water will be less than what we provide in
the form of electricity
2. We use conservation of erengy to say that heat energy input to water is the energy is uses to raise the temperature
and again we assume here there are no losses
3. We assume that water has reacehd 100C temperature but hasnt begun to boil
4. we use known values of heat capacity of water and room temperature and boiling temperature of water at 1 atmosphere pressure

b. calculations :
from power rating P, time taken to heat water should be mC(Tf - Ti)/P
where C is heat capacity of water, Tf is final temperature of water = 100 C
C = 4186 J/kg
Ti = 20 C ( initial temp)
hence
t = m*334880/P
where m is mass of water in kettle in kg and P is power supplied by kettle to the water