Demonstrations

Rate of temperature increase

We will use an electric tea kettle to observe several aspects of heating a body of water. The electric kettle is an good tool since we can measure the electrical input power.

Questions

• Based on common knowledge, can you estimate the rate of temperature change in the water in the kettle?

• Based on the heat capacity of water, the mass of water, and the input power, can you estimate the rate of temperature change?

• What happens to the rate of temperature change if you double the amount of water in the kettle?

• What happens to the rate of temperature change if you double the power to the kettle?

• What will happen to the temperature as it reaches the boiling point?

• What is the mass of steam we expect to generate and how would we measure it?

Latent Heat

We can fill our electric kettle with a mixture of ice and water.

Questions

• What do we expect the temperature rise to look like over time?

• How long will it take the ice to melt?

• What happens to the temperature of the water during melting?

• Does the rate of temperature increase change over time?

Water Mixture

Hot and cold air are often mixed in buildings to achieve a comfortable temperature. The mathematics are equivalent if we use volumes of water at different temperatures. We will combine different masses of water at different temperatures and observe the differences.

• What is the eventual temperature if we mix equal quantities of warm and cool water?

• What is it at other ratios?

• Based on your observations, can you deduce a formula?

Refrigeration

If we monitor the temperature of a volume of water in a freezer, what do we expect to see?

• Will the rate of cooling be constant or changing over time?

• What happens at the point of freezing?

Power estimation

Can we estimate the power of the tea kettle given our observation of the rate of the temperature increase and the amount of water in the kettle? How close will our comparison be to the electrical measurement?

volume = 1 liter
mass_water = 1000 gram
time = 5 * 60 sec => 300 sec
heat_capacity = 4.186 joule/gram/kelvin
temperature_change = 75 kelvin

mass_water * heat_capacity
* temperature_change / time in watt => 1,046.5 watt

Temperature Rise in the ETC

Can we estimate the increase in the air temperature during a large class of students?

What assumptions do we need to make?

We can compare our estimation to the observed temperature rise on our temperature sensor.

If these two temperature rates are very different, what plausible reasons exist for the difference?