Materials Included In Kit

Additional Materials Required

Prelab Preparation

Identify an appropriate area for the outdoor track. It should be fairly smooth and level. A minimum of 3 meters is sufficient for timing the cars, and a longer track may be used if desired. Mark a “Start” and “Finish” line with chalk or tape. If the track is wide enough, two or three cars may be tested at the same time.

Safety Precautions

Although the current generated by the solar panel is small and not harmful, use caution when connecting the wires. Do not touch bare wires that are part of a “live” circuit. Wear sunscreen and sunglasses when working in bright sunlight. Never look directly at the Sun. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Lab Hints

• Enough materials are provided in this kit for eight groups of students. Both parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• This activity may be extended over several days, giving students time to test more variables.
• The wheels and gear are meant to fit tightly onto the axles. If the fit is too tight, if may be necessary to gently tap in the axle with a hammer or rubber mallet.
• Ideally, testing of the solar cars should be conducted outdoors on a sunny day. If this is not possible, then indoor testing may be conducted with the proper equipment and some practice. A utility lamp that is rated for at least 150 W is needed. Use a clear, 150-W bulb (available from Flinn Scientific, AP6421) and connect the lamp to an extension cord. One person should hold the lamp just behind the solar car and walk along as the car moves, keeping the same distance and angle between the lamp and the car. Use caution as the bulb will get hot.

Teacher Tips

• The first solar-powered satellite is the Vanguard I, launched in 1958. As of this writing, it is the oldest man-made satellite orbiting Earth. The solar cells lasted nearly 7 years.
• Some knowledge of atomic structure is helpful in understanding how photovoltaic cells work, but not required to complete the activity.
• Students may determine the angle of the sun with a straight straw and a protractor. Point the straw at the sun, parallel to the sun’s rays. The correct angle is achieved when the straw does not cast a shadow, only a ring. The angle between the straw and the ground may be measured with a protractor.
• For another use of solar energy, try Cooking with Solar Energy—Flinn STEM Design Challenge™ (Catalog No. AP8048). Students construct a solar oven and then modify the design to increase the efficiency of the oven.

Answers to Prelab Questions

1. What are some advantages of using solar energy to generate electricity?

   Advantages of solar energy are it is a renewable source, does not pollute, and there is an abundant supply.

2. What are some drawbacks to using solar energy?

   Using solar energy to generate electricity is currently expensive and not very efficient, only available during the day and dependent upon weather conditions.

Sample Data

Sample Data Table A

*Trials were run with a 150-W incandescent lamp.

Sample Data Table B

*Trials were run with a 150-W incandescent lamp. Solar panel set at a 15-degree angle from the horizontal.

Answers to Questions

Design Challenge

1. Make a list of variables that might affect the car’s performance. Variables include angle of solar panel, angle of Sun’s rays (time of day), intensity of light, weight of car, size of gear, frontwheel drive vs. rear-wheel drive, aerodynamics of car, stability of car, alignment of wheels, amount of friction in moving parts, surface that car travels over and how level the track is.
2. Which of the variables listed may be tested by modifying the solar car?

   Modifications to the car may include changing the angle of the solar panel, the weight of the car, its aerodynamics, size of gear and using front-wheel or rear-wheel drive.

3. Which of the variables listed cannot be controlled?

   The intensity of the sunlight cannot be controlled, and if testing is limited to the class period, then the effect of the angle of the Sun cannot be tested. The track is also predetermined.

4. Why is it important to change one variable at a time?

   Changing only one variable at a time while all others remain the same enables the experimenter to determine what effect, if any, the change has on the performance of the car.

Post-Lab

1. Describe the final design of your group’s solar car and give a reason for each modification. Student answers will very.

   Sample data are based on changing the angle of the solar panel to receive more direct light rays.

2. Calculate and record the speed of the car for each of the five trials and determine the average speed.

   See Sample Data Table B.

3. Was your final design from Part B faster than the prototype built in Part A? If so, by how much?

   Yes, the average speed increased from 0.58 m/s to 0.64 m/s (a 10% increase).

4. If allowed to make other changes, and more materials were available, what else might be done to improve the car’s performance?

   Accept all reasonable answers, which may include adding more solar cells in series or in parallel, use a higher voltage motor, and change the gear ratio.

5. Describe the types of energy involved and how energy is transferred in making a solar car run.

   Energy from the sun (electromagnetic energy) causes electrons to flow in the solar panel (electrical energy). The flow of electrons produced electricity, which causes the motor to turn (mechanical energy). The mechanical energy of the motor is transferred to the gears and from the gears to the wheels. The turning wheels propel the car forward, giving the car kinetic energy.

References

Junior Solar Sprint. “So…You Want to Build a Model Solar Car.” http://www.nrel.gov/docs/gen/fy01/30826.pdf (accessed January 2016).

Introduction

Not only is the sun a source of heat and light, it’s a source of electricity too! Solar cells are used to convert sunlight to electricity. Solar cells can provide electricity for all kinds of equipment, from calculators and watches to roadside emergency phones and even vehicles. Use solar technology to make your own solar-powered model car!

Concepts

• Solar energy
• Energy conversion
• Photovoltaic cells

Background

Solar energy, the conversion of sunlight to electricity, has enormous potential as a clean source of renewable energy to replace fossil fuels. Although solar energy has powered satellites and spacecraft for more than 50 years, it accounts for less than 1% of electricity generated in the United States today.

A solar cell, also called a photovoltaic cell (PV cell), is a light-sensitive semiconductor device that uses the photoelectric effect to convert sunlight into electricity (see Figure 1). Conventional solar cells contain a silicon diode as the semiconductor. The diode is created by layering n-type silicon (silicon doped with an impurity that has one more valence electron than silicon) to p-type silicon (silicon doped with an impurity that has one fewer valence electron than silicon). The different properties of the materials at the p-n junction give rise to a potential difference at the interface. Photons of light striking the silicon surface excite electrons to different energy states and create “electron-hole pairs.” Electrons move toward the positive side of the junction, “electron holes” toward the negative side, and the resulting flow of electrons generates an electric current. The amount of current produced by a PV cell is proportional to the amount of light striking the cell. Wires attached to the “p” and “n” silicon layers allow the electricity to power calculators, watches, recharge batteries, electric motors and many more electrical devices. Panels of solar cells connected together generate enough electricity to power satellites.

An important factor limiting the use of solar energy is the trade-off between cost and efficiency. Photovoltaic cells convert only 15–20% of the Sun’s radiant energy to electric energy, much less than the chemical energy of fossil fuels (about 35% is converted to electric energy). Scientists and engineers are continually researching ways to improve solar cell efficiency and bring down the cost. Each year teams of scientists, engineers, and even students participate in solar car challenges where they design and build solar-powered vehicles capable of carrying one or more passengers on a cross-country trip. Finding ways to economically harness the Sun’s energy is an important goal toward more clean energy alternatives.

Experiment Overview

The purpose of this activity is to design a car that runs on solar power. The lab begins with an introductory activity to build and test a solar car prototype. Then different variables are tested to determine their effects on the car’s performance. Finally, the prototype is modified to increase the efficiency of the car, measured by its speed.

Materials

Prelab Questions

1. What are some advantages of using solar energy to generate electricity?
2. What are some drawbacks to using solar energy?

Safety Precautions

Although the current generated by the solar panel is small and not harmful, use caution when connecting the wires. Do not touch bare wires that are part of a “live” circuit. Wear sunscreen and sunglasses when working in bright sunlight. Never look directly at the sun. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part A. Introductory Activity

A1. Chassis Assembly

1. With a pencil, draw a line across the cardboard base 2 cm from one end.
2. Repeat step 1 at the other end (see Figure 2).
   {14056_Procedure_Figure_2}
3. Use scissors to cut the straw into four pieces, each 4 cm long.
4. Tape one piece of straw along one line on the base, with the end of the straw even with the edge of the base.
5. Repeat step 4 with a second straw on the opposite end of the line.
6. Repeat steps 4 and 5 with the other two pieces of straw on the other line (see Figure 3).
   {14056_Procedure_Figure_3}
7. Obtain the accessory bag of wheels, axles and gears.
8. Insert the end of one axle into one wheel hole.
9. Insert the other end of the axle through the straws on one end of the base.
10. Press a second wheel onto the free end of the axle. Note: If the fit is too tight, set the axle vertically with one wheel resting on the work surface. Press down on the top wheel, being careful to not bend the axle with too much pressure. It may be necessary to gently tap the wheel with a hammer or rubber mallet.
11. Select the largest gear from the accessory bag.
12. Place the flat side of the gear on the work surface.
13. Insert a second axle into the gear (see Figure 4).
    {14056_Procedure_Figure_4}
14. Lift the axle and gear and push the gear about 1 cm onto the axle.
15. Insert the end of the axle with the gear into a wheel hole.
16. Repeat steps 9–10 with the other end of the base.
17. Attach the tires to the wheels.
18. Obtain the motor and the smallest gear from the accessory bag.
19. Insert the motor shaft through the hole in the gear with the flat side of the gear facing the motor (see Figure 5).
    {14056_Procedure_Figure_5}
20. Mount the motor on top of the base with tape so the small outer gear attached to the motor meshes with the large gear on the axle. Note: It may be necessary to raise the motor up slightly by placing a small piece of folded paper under the motor. Test for proper alignment by setting the car on the work surface and moving it back and forth. When the motor is properly placed, the gear on the motor should mesh with the gear on the axle and turn easily without slipping.

A2. Attaching the Solar Panel

1. Obtain a solar panel.
2. Note the ends of the wires. If less than 1 cm of bare wire is visible, use wire strippers to remove more of the insulation.
3. Repeat step 2 for the motor wires.
4. Make a loop of masking tape with the sticky side out and place it in the center of the upper side of the chassis.
5. Gently press the solar panel on top of the tape to secure it near the center of the chassis (see Figure 6).
   {14056_Procedure_Figure_6}
6. Bend the black wires of the solar panel and the motor so they are under the chassis.
7. Turn the chassis over so the solar panel is covered.
8. Twist the bare ends together securely and tape the wires to the underside of the chassis so they won’t touch the work surface.
9. Repeat steps 6–8 with the red wires of the solar panel and motor. Make sure the wires will not interfere with the motion of the car. Note: This step should be done with the solar panel covered.

A3. Testing the Car

1. Take the car outside to an area designated by the instructor, keeping the solar panel covered with your hand or a piece of paper.
2. Set the car on the ground and uncover the solar panel.
3. The car should start moving across the ground. If it does not, check the following.
   1. The motor does not turn. Check the wire connections.
   2. The motor spins, but the car does not move. Check the gear alignment. Make any necessary adjustments.
4. Note which direction the car moves. If the motor is in front, the car has “front-wheel drive.” If the motor is in the back as the car moves, the car has “rear-wheel drive.” (Note: Switching the wire connecting will reverse the current, and the motor will spin in the opposite direction.)
5. If the car veers to the right or left, check the axle alignment and adjust as needed.
6. Once the car is functioning well, go to the start of the prepared race track.
7. Set the car down at the start, and time how long it takes to travel 3 meters. Record the time and any observations in Data Table A on the Solar-Powered Cars Worksheet.
8. Repeat step 7 for a total of 5 trials.
9. Calculate the car’s speed for each trial and the average speed. Record the values in the data table.

Part B. Design Challenge

The challenge is to modify the solar car from the Introductory Activity in order to achieve a faster average speed than in Part A on the same 3-m track. The solar panel, motor, motor gear, wheels and axles must remain the same. The cardboard base must be used for the chassis, but it may be modified.

Form a working group with other students and consider the following.

1. Make a list of variables that might affect the car’s performance.
2. Which of the variables listed may be tested by modifying the solar car?
3. Which of the variables listed cannot be controlled?
4. Why is it important to change one variable at a time?

Once the final design of solar car is ready, conduct five trials on the track and record the time for each trial in Data Table B on the worksheet.

Student Worksheet PDF

14056_Student1.pdf