Measuring the specific heat capacity of water by a domestic electric boiler

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Key Learning Points

1. Energy and power
2. Specific heat capacity
3. Heat loss

	A domestic electric boiler for boiling water.

Have you used a domestic electric boiler? It is a very convenient appliance for boiling water. It can also keep the boiled water hot, so that you can prepare a hot drink anytime.

You may have measured the specific heat capacity of water in school laboratory by standard equipment like an electric heater and a joule meter. But have you ever think of using a domestic electric boiler to make the same measurement? Sound interesting? Now you will have the chance to do so. In fact, all the apparatus used in this activity can be found in an ordinary family. You will also be asked to comment on the accuracy of the result, and suggest methods for improvement.

3 L domestic electric boiler, thermometer, a measuring cup that can hold at least 1.5 L of water, stopwatch or digital watch for measuring time.

	The power rating of the electric boiler is 700 W.

1. Note the power rating of the electric boiler.
2. Boil a large amount of water with the electric boiler.
3. When the water is heating up, prepare 0.5 L of cold water with the measuring cup and use the thermometer to measure its temperature.
4. Pour out all the water after boiling. Pour in the 0.5 L cold water and turn on the power supply of the heater. Start timing with the stopwatch.
5. Stop timing as soon as the boiler switches automatically from the boiling mode to the warming mode.
6. Repeat step 3 to 5 using different volumes of water.
7. Calculate the electric energy supplied to the boiler in each case.
8. Plot a graph of electric energy supplied versus mass of water used. Find the slope of the graph. Fill in the worksheet and find the specific heat capacity of water.

1. How does the graph of energy supplied versus mass of water look like? What does it imply?
2. How do you find the specific heat capacity of water from the slope of the graph?
3. How is your experimental value of the specific heat capacity compared with the standard value?
4. Do you think the heat lost by vaporization of water to be an important source of error? Do you expect it to be large? How can you reduce the vaporization?
5. Do you think the heat lost by conduction from the boiler to the surroundings to be an important source of error? Why?
6. Why do we have to use the boiler to boil water once before taking data?
7. Does the graph of energy supplied versus mass of water pass through the origin? Explain this in terms of heat exchange with the surrounding. What then is the advantage of using the slope of the graph instead of a single trial to estimate the specific heat capacity?
8. If you are doing this experiment in a school laboratory, suggest some ways to improve the accuracy of the measurement.