Calculating Calories in Sunlight

Name______Class______Date______

Exploration LabOBSERVATION

Calculating Calories in Sunlight

All objects in the universe have energy, which is the ability to produce a change in the object itself or a change in the object’s surroundings. Like matter, energy cannot be created or destroyed. Rather, it merely changes form. The stored chemical energy in foods, for example, is converted into energy of motion that enables you to carry out many activities your body does every day. Radiant energy from the sun is converted to heat that warms Earth, making it possible for life to exist.

Heat is the transfer of energy and can be measured in a unit called calories. A calorie can be defined as the amount of heat required to raise the temperature of one gram of water one degree Celsius (given one atmosphere of pressure). The amount of solar energy that reaches Earth’s atmosphere has been determined to be 1.94 calories per square centimeter per minute (cal/cm2/min). This value ,which is called the solar constant, differs widely for the solar energy which reaches Earth’s surface, depending on various factors including location, elevation above sea level, cloud cover and other general weather conditions, and obviously,the time of year. In this laboratory activity, you will contrast how three different solar collectors absorb the sun’s energy and compare the results with the solar constant.

OBJECTIVES

Construct three different solar collectors.

Contrast the number of calories of energy from sunlight absorbed by the water in each collector.

Compare your results to the solar constant and explain any discrepancies.

MATERIALS

•baby food jars, clean, small, labels removed (3)

•calculator (optional)

•Celsius thermometer

•construction paper, black

•or black matte paint)

•glue or tape

•graduated cylinder

Calculating Calories in Sunlight continued

Procedure

Part I—Constructing the Solar Collectors

1.Put on your lab apron. Use the paper towels to immediately wipe up any spillsonto your work area or the floor.

2.Measure and cut a piece of black construction paper that is half as long as thecircumference of the baby food jars and just as wide as the jars are high.

3.Glue or tape the construction paper onto one of the jars so that it completelycovers one half of the jar as shown in the figure below.

4.Measure and cut another piece of black construction paper that will com-pletely cover the outside of one of the jars as shown in the figure below. Glueor tape this piece of paper onto the jar.

5.Use the graduated cylinder to measure and pour 50 mL of distilled water atroom temperature into each jar.

Part II—Collecting Data

6.Use the thermometer to measure and record the temperature of the water in each jar. Remember to let the thermometer reach room temperature before making all new measurements.

7.Put the jars on the window sill or in a sunny spot on your work area.

8.Use the magnifying glass to focus the sun’s rays onto the jar with no paper. Hold the lens about 2.54 cm from the jar and at an angle as shown in the figure below Looking directly at the sun or looking at the sun through the magnifying lens cancause instant and permanent eye damage including blindness.

9.After 5 minutes, measure and record the temperature in the jar.

10.While keeping sunlight focused on the jar, repeat step 9 two more times. Average your data for this collector and record it in the data table.

11.Repeat steps 8–10 for the other two jars.

Calculating Calories in Sunlight continued

CALORIES OF ENERGY RECEIVED BY THREE SOLAR COLLECTORS

Jar without paper / jar half covered / jar completely covered
Trial / 1 / 2 / 3 / Avg / 1 / 2 / 3 / Avg / 1 / 2 / 3 / Avg
Initial temperature (C)
Final temperature (C)
Temperature change
Calories absorbed
Calories/cm/minute

PART III—COMPUTING CALORIES FROM SUNLIGHT

12.Compute the area of your magnifying glass by using the formula A = pr2, where A is the area, p is 3.14, and r is the radius of the lens. Record this value here.

______

13.One milliliter of water has a mass of one gram. Use this fact to compute then umber of calories absorbed by the water in each collector by multiplying the mass of the water by the average number of degrees the temperature changed. Record your calculations in the data table.

14.Now compute the number of calories of sunlight received per square centimeter of lens surface per minute for each collector. Record your values in the data table.

Analysis

1.Examining Data In which collector were the largest number of calories of solar energy absorbed?

______

2.Examining Data In which collector were the least number of calories of solar energy absorbed?

______

Calculating Calories in Sunlight continued

3.Recognizing Patterns In general, how do your values of the number of calories absorbed per square centimeter per minute compare with the solar constant?

______

4.Analyzing Data Compute the efficiency of each of your solar collectors.

______

Conclusions

5.Evaluating Results Give at least three reasons why the number of calories absorbed per square centimeter per minute in this activity differs from the solar constant.

______

6.Interpreting Information Based on your data, which collector is the most efficient at absorbing solar energy?

______

7.Making Predictions At what time of day would you get optimal results fro m this activity? Why?

______

Holt Environmental Science1Renewable Energy

TEACHER RESOURCE PAGE

Exploration LabOBSERVATION

Calculating Calories in Sunlight

Teacher Notes

TIME REQUIRED Onei45-minute period

SKILLS ACQUIRED

Collecting data

Experimenting

Interpreting

Measuring

Organizing and analyzing data

RATING

Easy Hard

Teacher Prep–1

Student Set-Up–2

Concept Level–2

Clean Up–1

THE SCIENTIFIC METHOD

Make ObservationsProcedure, steps 6–11

Analyze the ResultsAnalysis, questions 1–4

Draw ConclusionsConclusions,questions 5–7

Communicate the Results Analysis, questions 1–4 and Conclusion questions 5–7

MATERIALS

Test tubes or other small jars can be used in place of the baby food jars. If you use jars, have each student bring to class three jars that are the same size. If you use test tubes, have students adjust the amount of water accordingly. Black matte paint can be used in place of construction paper. If paint is used, have student spaint the jars a few days before you do the lab so that the paint will be dry.

SAFETY CAUTIONS

Looking directly at the sun or looking directly at the sun through a magnifying glass can cause instant and permanent eye damage including blindness.

TIPS AND TRICKS

Review,if necessary, how to compute an average and remind students to use the average values when answering the questions.

To compute the number of calories absorbed, the number of mLof water is equal to that same number of grams. For example, 50 mL of water has a mass of50 g. Thus, 50 g of water multiplied by the average difference between the initial land final temperatures in degrees Celsius gives the energy of absorbed calories in units.If the area of the lens is 78.5 cm2, then calculating the calories per square centimeter per minute follows: 50 g 0.5ºC = 25 calories; (25 calories 78.5cm2)/5 min = 0.064 calories/cm2/min.

Name______Class______Date______

Exploration LabOBSERVATION

Calculating Calories in Sunlight

Heat is the transfer of energy and can be measured in a unit called calories. A calorie can be defined as the amount of heat required to raise the temperature of one gram of water one degree Celsius (given one atmosphere of pressure). The amount of solar energy that reaches Earth’s atmosphere has been determined to be 1.94 calories per square centimeter per minute (cal/cm2/min). This value which is called the solar constant, differs widely for the solar energy which reaches Earth’s surface, depending on various factors including location, elevation on above sea level, cloud cover and other general weather conditions, and obviously the time of year. In this laboratory activity, you will contrast how three different solar collectors absorb the sun’s energy and compare the results with the solar constant.

OBJECTIVES

Construct three different solar collectors.

Contrast the number of calories of energy from sunlight absorbed by the water in each collector.

Compare your results to the solar constant and explain any discrepancies.

MATERIALS

•baby food jars, clean, small,labels removed (3)

•calculator (optional)

•Celsius thermometer

•construction paper, black

•(or black matte paint)

•glue or tape

•graduated cylinder

Holt Environmental Science1Renewable Energy

TEACHER RESOURCE PAGE

Name______Class______Date______

Calculating Calories in Sunlight continued

Procedure

Part I—Constructing the Solar Collectors

1.Put on your lab apron. Use the paper towels to immediately wipe up any spill son to your work area or the floor.

2.Measure and cut a piece of black construction paper that is half as long as the circumference of the baby food jars and just as wide as the jars are high.

3.Glue or tape the construction paper onto one of the jars so that it completelycovers one half of the jar as shown in the figure below.

4.Measure and cut another piece of black construction paper that will complete cover the outside of one of the jars as shown in the figure below Glue or tape this piece of paper onto the jar.

5.Use the graduated cylinder to measure and pour 50 mL of distilled water at room temperature into each jar.

Part II—Collecting Data

6.Use the thermometer to measure and record the temperature of the water in each jar. Remember to let the thermometer reach room temperature before making all new measurements.

7.Put the jars on the window ill or in a sunny spot on your work area.

8.Use the magnifying glass to focus the sun’s rays onto the jar with no paper. Hold the lens about 2.54 cm from the jar and at an angle as shown in the figure below.Looking directly at the sun or looking at the sun through the magnifying lens can cause instant and permanent eye damage including blindness.

9.After 5 minutes, measure and record the temperature in the jar.

10.While keeping sunlight focused on the jar, repeat step 9 two more times .Average your data for this collector and record it in the data table.

11.Repeat steps 8–10 for the other two jars.

Name______Class______Date______

Calculating Calories in Sunlight continued

CALORIES OF ENERGY RECEIVED BY THREE SOLAR COLLECTORS

Jar without paper / jar half covered / jar completely covered
Trial / 1 / 2 / 3 / Avg / 1 / 2 / 3 / Avg / 1 / 2 / 3 / Avg
Initial temperature (C)
Final temperature (C)
Temperature change
Calories absorbed
Calories/cm/minute

PART III—COMPUTING CALORIES FROM SUNLIGHT

12.Compute the area of your magnifying glass by using the formula A = r2, where is the area,  is 3.14, and r is the radius of the lens. Record this value here.

For a lens with a 10-cm diameter, A  (5 cm)2 3.14 x 25 cm2= 78.5 cm2.

13.One milliliter of water has a mass of one gram. Use this fact to compute the number of calories absorbed by the water in each collector by multiplying the mass of the water by the average number of degrees the temperature changed. Record your calculations in the data table.

14.Now compute the number of calories of sunlight received per square centimeter of lens surface per minute for each collector. Record your values in the data table.

Analysis

1.Examining Data In which collector were the largest number of calories of

solar energy absorbed?

The water in the jar that was completely covered with the black construction___ paper absorbed the largest number of calories from the sun______.

2.Examining Data In which collector were the least number of calories of solar energy absorbed?

The water in the jar that had no paper on it absorbed the least number of______calories from the sun.______

Name______Class______Date______

Calculating Calories in Sunlight continued

3.Recognizing Patterns In general, how do your values of the number of calories absorbed per square centimeter per minute compare with the solar constant?

Due to the inefficiencies associated with the conversion of one form of energy___ into another, most of the students’ values will be less than the solar constant.___

______

4.Analyzing Data Compute the efficiency of each of your solar collectors.

Students should compute the efficiency of each of the collectors by dividing the_ average calories/cm2/minute of each by the solar constant and multiplying the__ result by 100, which will yield the percent efficiency.______

Conclusions

5.Evaluating Results Give at least three reasons why the number of calories absorbed per square centimeter per minute in this activity differs from the solar constant.

Answers may vary. The amount of energy emitted by the sun is constant. The___ amount of solar energy that reaches Earth, however, depends on the time of____ year, cloud cover, the time of day, the amount of solar energy that is either_____ reflected or absorbed by Earth’s atmosphere, and the latitude and altitude of the location, among others. Students’ results may also differ due to experimental___ error.______

6.Interpreting Information Based on your data, which collector is the most efficient at absorbing solar energy?

The jar covered with the black construction paper is the most efficient at______absorbing solar energy.______

7.Making Predictions At what time of day would you get optimal results from this activity? Why?

Assuming no cloud cover, this activity would produce optimal results at solar___noon, when the sun is at its highest point in the sky.______

______

Holt Environmental Science1Renewable Energy