University of Virginia
Physics Department

## Evaporation and Heat Engines

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.7

Objectives

Students will

• study the process of evaporation;
• understand how evaporation is used in a heat engine (The Drinking Bird);
• understand how a heat engine operates.

Motivation for Learning

Discrepant Event-The Drinking Happy Bird

Materials
• The Drinking Happy Bird by Arora: available from Carolina Biological (Item # BA-75-3605; call 1-800-334-5551)
• Glass or paper cup, 10 cm tall
• Cold water (enough to fill the cup to the brim)

Procedure

1. Fill the cup or glass to the brim with cold water (the height of the glass should be a little lower than the top of the legs of the bird).
2. Put the head of the bird in the water so that it is completely wet.
3. Release the bird and allow it to pivot on its legs. Keep the glass with water in the same position. The bird should continue to drink in cycles.
4. Discuss this with the class. Why does the bird keep moving? See Background Information for an explanation.

Background Information

Evaporation is a process that allows liquid to change state to a gas without boiling. As for all changes in state, evaporation requires energy to occur. However, instead of needing the addition of external energy, evaporation utilizes the thermal energy of the liquid itself to complete the process. The molecules that evaporate are those with the highest kinetic energy, so that after evaporation the energy of the remaining liquid molecules is lower, therefore decreasing the temperature of the liquid. This indicates that evaporation is a cooling process. We will concentrate henceforth on this process in water.

The evaporation of water is utilized as a cooling process in many different areas. Humans and animals use it to cool themselves; when skin perspires, the water evaporates due to energy from the skin, and the skin temperature is lower. Students will be able to experience this in the activity, using both water and alcohol. Air conditioning systems also employ evaporation to lower air temperature by evaporating water into the air.

The Drinking Bird is a complex heat engine that also uses evaporation as a cooling mechanism. It almost appears to be a perpetual motion machine, but as we shall see, it does need an energy source. The bird's head is immersed in cold water, which causes its surface to be covered with water molecules. When the head is released and the bird moves back to its initial position, water evaporates from the surface of the head and its temperature drops. The bird's face and hat cover a glass bulb, similar to the bulb at the bottom of the bird's body. As the temperature inside the bulb decreases, the air pressure also decreases (We know this from the Ideal Gas Law: PV=nRT). This drop in pressure creates a pressure gradient within the bulb, and the liquid on the bottom is drawn up through the tube into the top of the bird. As more liquid enters the top half of the tube, the bird becomes top heavy and tips down, so that the head enters the glass of water a second time. Once the bird is parallel to the ground, the liquid moves back to the lower bulb, enabling the entire process to repeat.

When discussing the Drinking Bird with the students, emphasize the use of evaporation as a cooling tool, because they will be studying this in a brief student activity. Allow the students to watch the bird closely for several cycles (no more than three students at a time). The class should also understand the other physical science concepts that make the bird move, such as the Ideal Gas Law, which explains the liquid movement within the tube, and the force due to gravity, which causes the head and bottom of the bulb to rotate.

1. The alcohol should evaporate faster.

2. The hand with the alcohol will feel slightly cooler, because it evaporates more quickly than the water, thus cooling the skin at a faster rate.

3. The heat came from the body, which is a source of heat. When the liquid used this heat to evaporate, your hands felt cooler.

4. See Background Information.

### Student Activity

Materials

• One Tablespoon water
• One Tablespoon rubbing alcohol
• Two clear plastic cups

Procedure

1. Pour the water into one cup and the rubbing alcohol into the other cup.
2. Observe the two samples, and after several minutes note which liquid seems to have evaporated faster.
3. Place a few drops of water on one hand and a few drops of rubbing alcohol on the other hand.
4. Wave your two hands slowly through the air. Note any difference in how each hand feels. Does one feel cooler than the other?

Data Sheet

 Which substance seems to have evaporated faster, water or alcohol? How can you tell? When you waved your hands in the air, which felt cooler? Why did your hands feel cool? Where did the heat come from that made the liquid evaporate from your hands? Explain in your own words how the Drinking Bird keeps drinking.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.