Location: You will need to come to Dana 126 in order to check out the experimental equipment. You should do the experiment outside Dana, either on the loading dock on the east end, in the courtyard, or on the street side of the building. The best time to do it is between the hours of 11 am and 3 pm, so that the sun will be more overhead. You need to pick as clear a day as possible. After taking data, you should return the apparatus to Dana 126. You can use the computers in Dana 126 to analyze your data.

Purpose:

To measure the solar insolation reaching the surface of the earth at our location.

Equipment:

A black-painted aluminum block mounted in a styrofoam insulation box with a glass cover, a wooden dowel, a thermometer, and a stop watch.

Introduction:

Solar insolation is defined as the rate at which energy reaches a unit area of surface which is oriented at right angles to the direction of the sunlight. At the top of the earth's atmosphere the insolation is about 1360 W/m². This value is known as the solar constant. As your text explains, the atmosphere absorbs and reflects about one-half of the incident sunlight, so that the insolation that reaches the surface of the earth is about 680 W/m². The intensity will be different at different latitudes and during the morning or evening when the sun is lower in the sky. Cloud cover will also affect the readings.

You will determine the solar insolation in Davidson, NC on a typical day in the fall. You will measure how the temperature of a black-painted aluminum block increases when it is exposed to the sun. If the black paint is a perfect absorber (a perfect blackbody), then the amount of solar energy incident on the block is equal to the thermal energy absorbed by the block.

Definitions:           I = solar insolation

                           E = energy from the sun incident on the block

                           Q = gain of thermal energy by the block

                           A = area of block

                           T = temperature in ºC

                           c = specific heat of aluminum = 900 J/kg ºC

                           t = time

By definition:                or       

where D t is the time during which the block is exposed to the sun and D T is the temperature change of the block during that time.

Since   ,    then     .

We can solve this last equation for the insolation:

.

Thus, your task is to measure the temperature as a function of time. You can then plot temperature T versus time t. That line should be very close to a straight line with slope ΔT/Δt. We can then determine I from the above equation.

1. The aluminum block and its glass cover have already been enclosed in the Styrofoam insulation box, which has a cover. There is a digital thermometer and a stopwatch inside the enclosure. A wooden dowel sticks up from the top of the Styrofoam box.
2. Record the mass of the aluminum block from the bottom of the Styrofoam box.
3. Open the box while you are still in the laboratory and remove the wooden dowel, the thermometer and the stopwatch. Close the cover.
4. Take the Styrofoam box, the stopwatch and the digital thermometer outside to the courtyard, the loading dock or the sidewalk on the street side of Dana. Orient the Styrofoam box at a right angle to the sun by propping it against the cardboard half cylinder. Use the weight on the other side of the box to keep the box from slipping. Adjust the box so that the wooden dowel does not cast a shadow; thus, the sunlight is perpendicular to the box.
5. Gently remove the probe from its clip and unwind the cable. Slide the probe as far as it will go into the hole in the cutout underneath the glass cover. Turn on the thermometer.
6. Wait for the thermometer to stabilize and then record the initial temperature in ºC.
7. Make sure that the sunlight is still perpendicular to the box. If not, readjust the box. Now, open the cover of the Styrofoam box and expose the aluminum block to the sun. Start the stopwatch and record the temperature every 30 seconds for 10 minutes.
8. If you think your data may have been disturbed in some way, e.g., by a cloud or person passing in front of the sun, you should repeat your measurements.
9. Bring everything back into the laboratory.
10. Measure the area of the aluminum block.
11. Leave everything in the same condition as you found it.

Data Analysis:

1. Using Microsoft Excel, make a plot of the temperature T versus the time t. Use Excel to draw a trendline through the data so that you can determine the slope ΔT/Δt of the best straight-line portion of your plot.
2. Use the equation for I on the previous page to determine the insolation at Davidson, NC during that period of time.

Written Report:

Each student will submit an individual report that follows the format listed below. Your report should be neatly handwritten (or typed on the computer) using complete sentences. The total length of the report should not be longer than two or three pages. Pledge the report to indicate that the written report reflects your own understanding of the experiment. The report should include your name, the name of any partner(s), the date the experiment was performed, and the following sections:

The report does not have to be voluminous, but must be such that someone else reading it could understand the theory, repeat your work and verify your results.