Calculating Energy and Efficiency for Heating Water in a Kettle

[chawki]

Homework Statement

The density of water is 1000 kg/m³ and the specific heat capacity of water is 4.2Kj/(Kg.C)

Homework Equations

a) How much energy is required to heat 1.5 L of water at a temperature of 20 °C to the boiling point 100 ºC?
b) There is 1.5 L of water in a kettle on a cooking plate of 2.0 kW. It takes 7.0 minutes to
heat the water from a temperature of 20 ºC to the boiling point. How much electrical
energy is required? What is the efficiency of the heating process?

The Attempt at a Solution

a)
Q=m*Cp*T
Q=1.5*4.2*(100-20)
Q=504 Kj.

b)
how can we find the electrical energy?

 

[gneill]

how can we find the electrical energy?

...a cooking plate of 2.0 kW. It takes 7.0 minutes...

Do the obvious!

 

[chawki]

wow..is it like in cinematic..Power=work/time ?
so here we would have Power=energy/time?
and then 2000=E/(7*60)
E=840000 J = 840 KJ ?

 

[gneill]

Yes. Aren't general concepts wonderful?

 

[chawki]

Yes, amazing
but what about part c, what efficiency they are talking about ?
i know it will probably be Q/E or E/Q...i'm not sure

 

[gneill]

Yes, amazing
but what about part c, what efficiency they are talking about ?
i know it will probably be Q/E or E/Q...i'm not sure

Efficiency is the ratio of the (results/effort)x100. , or (output/input)x100, however you wish to look at it.

The water takes a certain amount of energy to boil as dictated by Nature. But in order to accomplish that result, the heater produced a much larger amount of energy. Obviously there was wasted energy and only part of the "efforts" of the heater went into accomplishing the desired result.

 

[chawki]

can you explain it more simply
i just got that it's the enrgy output/input
but in our case, they said ''the heating process'' are they talking about the question a) ?
and where is the energy generated here which is supposed to be the Output ..i guess!

 

[gneill]

can you explain it more simply
i just got that it's the enrgy output/input
but in our case, they said ''the heating process'' are they talking about the question a) ?
and where is the energy generated here which is supposed to be the Output ..i guess!

The output is hot water. How much energy went into the hot water?
The input is the energy provided by the heater. How much energy did it produce?

 

[chawki]

The output is hot water. How much energy went into the hot water?
The input is the energy provided by the heater. How much energy did it produce?

The energy that went into hot water is 504kj
but the question about how much energy did it produce..i'm not sure about it..probably that 840kj...but i don't see that is an energy produced
i think it's still an energy as the one of 504kj