Power of a Microwave

For this project we are going to estimate the power of a microwave. To do this, you will need to have access to a microwave oven, a thermometer, a kitchen measuring cup, a microwaveable mug, and a time measuring device capable of measuring seconds.

A microwave oven converts electrical energy into heat energy. Power is a measure of how fast energy is converted from one form to another. To estimate the power of the microwave, you will determine the change in thermal energy of a container of water during different time intervals.

Change in thermal energy of a material that is always in the same phase (liquid in this case) depends on three parameters. The first is the amount of mass that is heated or cooled. It will take considerably more energy to heat a large container of cool water to a given temperature than it takes to heat a small container of cool water to the same temperature. The second parameter is the change in temperature of the material. For example, more thermal energy is required to heat one cup of water from 10° C to 90° C than is required to heat one cup of water from 10° C to 30° C. Finally, the material itself effects the change in thermal energy. Some materials are very easy to heat and others are more difficult. This parameter is referred to as specific heat capacity. The three parameters, mass, temperature change, and specific heat capacity are directly related:

Heat Energy = Mass x specific heat capacity x Temperature change.

Most measuring devices in American kitchens determine volume not mass. In order to determine the mass of water you may find it useful to know that 1 cup of water has a mass of about 230 grams. The specific heat capacity for water is 4.186 Joules of thermal energy for each gram of water for each one degree Celsius change in temperature.

To collect the data points, partially fill a mug with one cup of room temperature water and measure this temperature in degrees Celsius. Then microwave the mug of water for the specified time and remeasure the temperature of the water. (Do not microwave the thermometer!) For the table, you will need to compute the difference of the temperature after the water is heated and the water at room temperature.

Next determine the change in Heat energy for each elapsed time from the previous table using the formula:

Heat Energy = Mass x specific heat capacity x Temperature change.

Using a statistical package or a graphing calculator, plot this data set with the independent variable being the time and the dependent variable being the Heat Energy. Fit a line to the data set.

The slope of this line is the power of your microwave. According to this experiment, what is the power of your microwave?

What are the units for the power?

Interpret the R² value for this data set.

What other variables could have affected the data points that you collected? Explain.