METR 104: Our Dynamic Weather (Lecture w/Lab) Some Demonstration Experiments: Effects of Air Motion, Evaporation, and Pressure Changes on Temperature Dr. Dave Dempsey Dept. of Geosciences SFSU, Fall 2012

Objectives. During and after these demonstration experiments, students should be able to:

• observe and record results of several experiments about changes in temperature measured by a thermometer
• propose and/or evaluate possible explanations for some of the observations, reasoning from prior knowledge and evidence from the experiments

Background Knowledge:

• conduction
• how most thermometers work
• wind chill
• evaporation vs. condensation (two phase changes of water)

Materials Needed. To complete this exploration,we will need:

• for the wind chill experiment:
• several thermometers (liquid-in-glass and/or thermocouples)
• an electric fan
• some water
• for the bicycle tire experiment:
• a bicycle tire
• tire pump
• a thermometer (liquid-in-glass and/or thermocouple)

I. Introduction.

In these demonstration experiments, we'll explore how air motion (wind), evaporation, and pressure changes affect (or don't affect) temperature as measured by a thermometer.

II. Instructions. For each of the demonstration experiments described below, observe and carefully record what you see, including both qualitative (descriptive) results and quantitative results (measurements). For some things, your instructor will ask for help.

Later (out of class, via iLearn) you'll be asked to refer to some of your observations (as well as Reading #6 and Lab Exploration #6) to support or disconfirm possible explanations for what you see happening. Hence, you need to record detailed and accurate notes of what you observe, for later reference.

1. Wind chill experiment: wind and temperature

You need to know:
• how a thermometer works (via conduction between the thermometer sensor and the air in contact with it)
• how to read a thermocouple output plot or a liquid-in-glass thermometer

The experimental setup: an electric fan, with several thermometers mounted in front of it and behind it.

1. The fan is initially turned off. Read the thermometers and record their temperatures.

2. Predict what will happen to the temperatures measured by the thermometers after the fan is turned on.

3. Turn the fan on "low" and wait a minute.

4. Reread the thermometers and record their temperatures.

5. Turn the fan on "high" and wait a minute.

6. Reread the thermometers and record their temperatures.

7. Compare your prediction to what you observed and discuss what might be going on.

8. Predict what will happen to the temperatures measured by the thermometers after dipping them in water and holding them in front of the fan.

9. Dip the thermometers in some water, hold them in front of the fan, and record what happens to the temperatures.

10. Compare your predictions to what you observed and discuss what might be going on.

2. Bike tire experiment: pressure and temperature

You need to know:
• a typical atmospheric pressure (in pounds per square inch) at the altitude of the classroom (which in our case is close to sea level)
• the maximum recommended pressure (in pounds per square inch) inside an inflated bike tire (printed on the side of the tire)
• how to open the valve on a bike tube (a shrader-type valve or a presta-type valve)
• how to use a hand pump to pump air into the bike tube

The experimental setup: an inflated bike tire, several thermometers, and a bike pump

1. With a thermometer, measure and record the air temperature in the room.

2. We can't easily measure the temperature of the air inside the bike tire, but given that (a) the bike tire has been sitting around in the room for a while, (b) the air temperature in the room hasn't been changing much, and (c) knowing how conduction works, what would you say about the temperature of the air inside the tire compared to the room temperature (higher, about the same, or lower)? Why?

3. Answer the same question about the valve on the bike tube.

4. With your fingers, briefly feel the metal part of the valve on the bike tube. Does it feel warm, neutral, or cool? Why, do you think? [Note: when something feels warm or cold, what you're actually feeling is your own skin temperature, because that's where the nerves are that are responsible for detecting and signaling temperature change. Similarly, a thermometer really only measures its own temperature. If your skin is warmer than it's "normal" temperature (around 91°F), your skin will feel warm or hot. When it cooler than "normal", it will feel cool or cold. If you touch an object and "it" feels warm or cold, that means that your skin has gained or lost heat (probably by conduction of heat from or to the object respectively, which will happen if the object is warmer or cooler than your skin, respectively).

5. Given the pressure exerted by the air inside the tire and the pressure of the air in the room outside the tire, what should the air inside the tire do when the valve is opened? Why? (What physical principle that we've learned about applies here?)

6. Position the sensor of a thermometer directly in front of the tire valve, so you can measure the temperature of the air immediately after it leaves the tire. For a schrader-type valve, using the tip of a ball-point pen or other narrow, pointy object, open the valve and let air out of the tire until it stops coming out, measuring its temperature the whole time. For a presta-type valve, unscrew the valve closure and press down on the pin sticking out of the valve. Record the temperature at the point when it has changed the most.

7. Quickly and briefly feel the metal part of the valve again with your fingers. Does it feel warmer, no different, or cooler than it did before the air escaped? Why, do you think? (That is, if it changed temperature, by what mechanism did it gain or lose heat?)

8. Compare the temperature of the air after it escaped from the tire to the temperature you think the air inside the tire had before the air escaped. If the temperature differs, think about the mechanism(s) by which it might have gained or lost heat to account for the temperature change. In particular, what candidate mechanisms seem possible in this case, at least in principle? Which one(s) is (are) seem the most likely to have acted here?

9. Using the tire pump, pump air back into the tire. Feel the valve again. Has its temperature changed? If so, how do you think it happened?

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