ENERGY AND THE ENVIRONMENT
Comparing the Efficiency of Different Ways of Heating Water
Take-Home Experiment Write-Up
This purpose of this experiment was to investigate different ways of heating water at home, and to find
the most efficient way. Three of the most common methods are heating water using a microwave,
boiling it in a kettle or on a stove.
To find the efficiency, water was heated in the three different ways and the temperature was recorded
over a period of time using an oven thermometer.
Using information about:
a) the temperature change of the water over time
b) the specific heat capacity of the water and
c) the mass of the water,
the total power used to heat the water was calculated,
also measured using a power meter.
. The total power input into the device was
Finally, the efficiency was found as a ratio between the power used,
and the power input,
.
More specifically, to determine the efficiency of the microwave, a 200mL glass of water was heated over
different time intervals to measure the time dependency of the temperature. The heating time was set
to 10s, increasing by 10s up to 150s (This is because, in order to read the thermometer, the microwave
would have to be stopped and the door opened for each reading. Interrupting the heating in this way
would cause inaccuracies. Instead, a fresh sample of water was used and the heating time was increased
for each consecutive measurement) and both the initial temperature and final temperature were
recorded. To minimize errors, the initial temperature was roughly uniform for all samples.
To test the kettle, 1L of water was poured into it and heated up. The temperature was recorded for
different time intervals, starting from 30s, increasing by 30s for each consecutive measurement, up to
300s.
Finally, 2L of water was heated on a stove, and the temperature of the water was recorded every 10s up
to 300s.
In this case, "efficiency" means the power heating the water - a quantity we can calculate from
temperature measurements - divided by the power consumed by the heating device - a quantity we can
measure.
For each experiment, a power meter was used to measure the power input to the heating device.
Furthermore, before measuring the temperature of the water, the water was stirred to avoid
temperature gradients in the fluid.
The main formulas used are as follows:
Efficiency ,  
Puseul
P
 el
Pinput Pinput
The power used,
, is determined using the formula:
Pel  (c p )( m)( s )
Physics and Astronomy Outreach Program at the University of British Columbia
Where
is the specific heat capacity of the water, [J/K.g] ,
is the mass of the water that is heated [g]
is the slope of temperature vs. time graph
is measured using a power meter, a device that measures the power consumption of any
electrical device.
Physics and Astronomy Outreach Program at the University of British Columbia
MICROWAVE
The data obtained for the microwave is shown in Figure 1 below. The linear fit of the obtained data,
yields a value of the slope
.
Using:
a) Mass of the water used,
b) The literature value of
The electrical power put into the water is found to be:
The power input,
.
as measured using the kilowatt meter, was 1031 W.
The efficiency was thus found to be 40.6%
Sample Calculations:
Values
1. Electrical Power Used
Pel  (c p )( m)( s )
 (4.18)( 200)(0.5)
Pel  419W
2.
Cp (J/K.g)
4.18
m (g)
200
s
0.50
Pel (W)
419
Average
Pin(W)
1030.67
Efficiency
 
Pel
Pinput
 
419
 0.406  41%
1031
Figure 1: Heating in a Microwave
Physics and Astronomy Outreach Program at the University of British Columbia
Physics and Astronomy Outreach Program at the University of British Columbia
KETTLE
The data obtained while heating 1L of water in a kettle is shown in Figure 2. The slope of the graph was
determined to be:
.
Using:
a) Mass of the water used,
b) The literature value of
The electrical power put into the water is found to be:
The power input,
.
as measured using the kilowatt meter, was 1314 W.
The efficiency was thus found to be 86%
Sample Calculations:
3. Electrical Power Used
Pel  (c p )( m)( s )
 (4.18)(1000)(0.27)
Pel  1133 W
4.
Values
Cp (J/K.g)
4.18
m (g)
1000
s
0.27
Pel (W)
1133
Efficiency
 
Pel
Pinput
 
1133
 0.86  86%
1314
Average
Pin(W)
Figure 2: Heating in a Kettle
Physics and Astronomy Outreach Program at the University of British Columbia
1314
STOVE
The data obtained while heating 2L of water on a stove top is shown in Figure 3. The slope of the graph
was determined over the linear portion of the graph to be
.. Notice how the graph slowly
tapers off after t=2500 s to eventually become flat. This is where the water starts to boil and a phase
change occurs; the heat being supplied to the water is used for evaporation and not to raise the
temperature of the water.
Using:
c) Mass of the water used,
d) The literature value of
The electrical power put into the water is found to be:
The power input,
.
as measured using the kilowatt meter, was 1240 W.
The efficiency was thus found to be 20%
Sample Calculations:
5. Electrical Power Used
Pel  (c p )( m)( s )
Values
Cp (J/K.g)
m (g)
s
4.18
2000.0
0.03
Pel (W)
251
Average
Pin(W)
1240.0
 (4.18)( 2000)(0.03)
Pel  251W
6.
Efficiency
 
Pel
Pinput
 
251
 0.20  20%
1240
Figure 3: Heating on a Stove
Physics and Astronomy Outreach Program at the University of British Columbia
Conclusion
There is a noticeably large difference between the three methods of heating water. A kettle (efficiency =
86%) was found to be more than twice as efficient as a microwave (41% efficient) and 8 times as
efficient as using a stove to heat water (even though the pot on the stove was covered to minimize heat
loss to the surroundings).
Anoushka Rajan 2010/12/19
Physics and Astronomy Outreach Program at the University of British Columbia