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					ENERGY TRANSFER: CONVECTION—LAB ACTIVITY


Energy Transfer: Convection
(Activity #13)



Purpose             Determine how energy is transferred by convection in water.


Background          Convection
                    Convection is the transfer of thermal energy by the movement of a
                    substance from one place to another. Convection takes place only in
                    liquids and gases.
                    For example, the lower
                    layers of the atmosphere
                    absorb infrared energy
                    emitted by the Earth’s
                    surface. The warmed
                    gas expands, rises, and
                    transfers thermal energy
                    to higher layers of the
                    atmosphere. Convection
                    in the atmosphere is
                    responsible for the
                    redistribution of thermal
                    energy from the
                    equatorial regions to the
                    polar regions as well as from the surface upward.
                    Convection is generally used to describe vertical motion. Advection is
                    sometimes used to describe horizontal motion, such as winds.
                    Convection also occurs in Earth’s oceans as
                    warmed water at the surface expands away and
                    cooler water from below wells up to the surface. In
                    water warmed at the bottom, the motion of the
                    warmed water is from the bottom toward the top.
                    As the surface water becomes cooler, it sinks
                    toward the bottom.




                        EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES                 177
                                          ENERGY TRANSFER: CONVECTION—LAB ACTIVITY


Materials and
Equipment             • Xplorer GLX                            • ice cubes (6)
Required              • PASPORT Temperature Sensors (2)        • string
                      • Large Tripod Base & Rod                • plastic bag, approx. 1-L size
                      • Buret Clamp (2)                        • tape
                      • large straight-sided clear container   • water
                      • heat lamp (or 150-W lamp)              • white paper
                      • weight                                 • lab coat (optional)
                      • food coloring




Safety Notes        Remember, follow the directions for using the equipment.
                    Wear safety glasses and follow standard laboratory safety procedures.
                    Be careful when handling the heat lamp.

                    Keep water away from electrical outlets.



Pre-Lab             How quickly will water temperatures change as a result of heating and
Question            cooling due to a heat lamp shining on one part of the water and ice
                    added to another part of the water?




Procedure           GLX Setup
                      1. Plug the PASPORT Temperature Sensors into the Xplorer
                          GLX.


                       2. Use the arrow keys (           ) to highlight the Flash folder,
                          select the Flash folder (press       ), use the arrow keys to
                          highlight the file, and then open (press F1) the GLX
                          configuration file entitled

                                                   13 Convection CF

                       3. Open the Graph display (press    , F1).
                          Note: The sensors have been configured to collect data once
                          every 2 seconds.




178   EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES
ENERGY TRANSFER: CONVECTION—LAB ACTIVITY


Procedure,             Equipment Setup
continued                 1. Set up the tank of water. Fill the tank with room temperature
                             water to approximately 10 cm from the top.

                          2. Tape white
                             paper to one
                             side of the tank
                             to form a
                             backdrop so
                             you can more
                             easily view the
                             convection
                             currents in the
                             water.

                          3. Position the
                             heat lamp near one end of the tank.

                          4. Prepare the ice cubes by putting them in a plastic bag.
                             Squeeze as much air out of the bag as possible, and then
                             tie a piece of string around one end of the bag. Tie the other
                             end of the string to a weight.

                          5. Use the clamps and support rod to mount the two
                             Temperature Sensors at the middle of the tank so the tip of
                             one sensor is almost at the bottom of the water and the tip
                             of the other sensor is about 4 cm below the surface of the
                             water.

                       Record Data
                       The process is to turn on the heat lamp so it warms the water at
                       one end of the tank and then put the bag of ice cubes into the
                       water at the other end of the tank. The weight attached to the bag
                       of ice cubes will be an “anchor” to keep the bag in place. Use the
                       Temperature Sensors to measure the change in temperature of the
                       water near the bottom and near the top.

                          1. Press the Start/Stop (      ) key to begin recording data.
                             Turn on the heat lamp.

                          2. Carefully place the weighted bag of ice into the water at the
                             other end of the tank.

                                Note: The bag of ice will displace some of the water, so
                                watch the level of the water in the tank. Make sure that
                                none of the water spills out. The level of the water needs
                                to be within 5 cm of the top of the tank after you add the
                                ice, so put more room-temperature water into the tank if
                                needed.


                        EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES                 179
                                       ENERGY TRANSFER: CONVECTION—LAB ACTIVITY


Procedure,             3. Wait 10 minutes for the temperature change to take effect, then
continued                 add 1 or 2 drops of food coloring into the water near the heat
                          lamp, and 1 or 2 drops of food coloring into the water near the
                          ice cubes. Note the time you added the food coloring.

                       4. Observe the motion of the food coloring to look for evidence of
                          convection. Record your observations. Be sure to include the
                          time relative to when you added the food coloring to the water of
                          each recorded observation.

                       5. After 20 minutes, press the Start/Stop key to stop recording
                          data.

                        6. Carefully remove the bag of ice cubes and turn off the heat
                           lamp. Continue to observe the motion of the food coloring.

                    Analyze
                       1. Examine your graph of temperature versus time to determine
                          how energy is transferred by convection.
                          (Hint: Compare the starting temperature for each sensor to the
                          final temperature. Which area of the water tank became colder?
                          Which area of the water tank became warmer?)

                       2. Make a sketch of your graphs. Be sure to label the x and y axes.
                          Add notes to your graph concerning the appearance of the food
                          coloring in the water at various points in time.

                       3. Save your GLX file (press       , select Data Files, press F2).
                          Result: Your GLX file will be saved in the RAM folder.

                              Note: Your teacher may ask you to rename your file so you
                              can find it later. If so, use the following procedure:
                              1. Select the Files menu (press F4) and select Rename.
                              2. Key in the new name, and press the Save key (F2).


Analysis/           Analysis questions
Synthesis              1. What was the starting temperature for the water in both parts of
Questions                 the tank?

                       2. What was the final temperature of the water in the top part of the
                          tank?

                       3. What was the final temperature of the water in the bottom part of
                          the tank?




180   EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES
ENERGY TRANSFER: CONVECTION—LAB ACTIVITY


Analysis/           Synthesis questions

Synthesis               1. Examine your graph of Temperature versus Time. How was
                          energy transferred by convection throughout the water tank?
Questions,
continued              2. Do your results support your prediction?

                       3. What evidence did you observe in addition to the temperature
                          that shows convection in the water?




                        EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES             181
                                    ENERGY TRANSFER: CONVECTION—LAB ACTIVITY




182   EXPLORATIONS IN EARTH & ENVIRONMENTAL SCIENCES

				
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