18.7. Cool cans

Author:	Norbert van Veen
Time:	10-20 minutes
Age group:	13+
Concepts:	Heat flow, insulation, evaporation, thermodynamics, Newton's cooling law

Fig. 18.16 The two cans contain hot water (same initial temperature) and a temperature sensor. The temperature measurements are displayed on a digital board. Which one cools faster?

Introduction

This very simple experiment shows students that the cooling of objects is influenced by the environment. The two cans filled with warm water cool down. Although the initial temperature is the same, the rate of cooling differs. Does the wet handkerchief serve as insulation?

Equipment

• Two cans filled with hot water at the same initial temperature

• Two temperature sensors or thermometers

• Optional: measurement software

• A screen or digital board for projecting the temperature graphs

• Hot and cold water

• A handkerchief

Preparation

Set up the apparatus as shown in Figure 18.16. Connect the two temperature sensors to your device (if possible) and show the output on the digital schoolboard.

Ensure that hot water is available, as well as a handkerchief that can be soaked in cold water. Pour the hot water into the cans and place the temperature sensors in the cans. Wrap one of the cans in a wet handkerchief. Start the measurement. Optionally: provide graph paper to the students to draw the graph by hand.

Tip

• Place the setup on a raised platform so that students can see clearly when and how the can is wrapped.

• Since there isn’t much to see during the experiment, it is important for students to be able to follow the temperature changes closely. A projection screen is a useful aid for this experiment.

• Have some exercises or questions ready for students to work on while the measurements are running.

Procedure

1. Ask the students how to keep a hot liquid at a high temperature for as long as possible. What insulation methods do they know? Which form of heat transfer does the insulation method “reduce”?

2. Explain that you are going to conduct an experiment that demonstrates the difference in insulation between two heat sources.

3. Explain how the experiment will proceed and ask the students to write down their expectations.

4. Carefully pour the hot water into the cans and wrap one of the two cans in the wet (cold) handkerchief.

5. Start the computer measurement. Optionally, have students write down the measured values after, for example, 10 seconds, 20 seconds, etc. so that they can create their own graph.

6. Ask the students to explain why the can with the handkerchief cools faster than the other can. After all, heat flows outward in both cans, and the cold water from the handkerchief cannot enter.

7. Use a Socratic dialogue to arrive at the proper answer.

Fig. 18.17 We immediately see the difference in cooling between the two cans. The can with the wet handkerchief (green) cools much faster than the can without.

Physics background

The can with the handkerchief cools down faster, especially at the beginning. The reason for this is that, due to the lower temperature of the water in the handkerchief, the temperature difference is larger and thus the heat flow (\( \frac{\Delta Q} {\Delta t} = \frac{\lambda \cdot A \cdot \Delta T}{d} \)). Once the temperature of the water in the handkerchief is roughly similar to the temperature of the water inside the can, the cooling can be attributed to the evaporation of water from the handkerchief.

Follow-up

Isolation contests are often held to keep hot water warm for as long as possible. Following this demonstration, ask students how they would cool down a hot liquid as quickly as possible. How would you design such a cooler, and what requirements would it need to meet?

A discussion can be held with the students whether this is a fair experiment. What is needed to do a better comparison between the effect of a (wet) handkerchief?