Materials Included In Kit

Additional Materials Required

Safety Precautions

This lab activity is considered to be nonhazardous. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Disposal

All materials in this kit are reusable and should be saved for future use.

Lab Hints

• If the rubber band belt is not rotating, the rubber stopper may be hitting the inside wall of the PVC Tee connector. To fix this problem, gently wiggle the motor until the stopper is away from the inside wall of the PVC Tee connector.
• If the rubber stopper is not rotating well, the wire brushes may be hitting the rubber band or the rollers. Adjust the wire brushes so they are near, but not touching the rubber band or rollers.
• If the rubber belt is pulled too tightly, it may not rotate. Decrease the tension on the rubber band by pushing the clear plastic tube further into the PVC Tee connector.
• If the generator is not gaining a charge, the wire brushes may need to be adjusted. Again, they should be near, but not touching the rubber band belt.
• Small pieces of animal fur can be purchased from Flinn Scientific Inc. (Catalog No. AP9205). This fur may need to be cut into smaller strips in order to work with this mini Van de Graaff generator. Small strips of animal fur and tinsel may also be purchased at a local craft store.
• A 3 V power source such as two 1.5 V batteries will work best for the mini Van de Graaff generator. A power source over 3 V will cause the motor to spin extremely fast causing the rubber band to fall off the rubber stopper or the toothpick to break.
• If the plastic toothpick breaks, a round wooden toothpick may be used as a replacement.

Teacher Tips

• Have the students experiment with different dome shapes to see what will hold a charge better. For example, try using an empty soda can instead of the aluminum tart tins.
• Repeat the same demonstrations in the activity using a standard Van de Graaff generator. Compare and discuss similarities and differences between the mini Van de Graaff and a standard Van de Graaff.
• Van de Graaff generators and supplies may be purchased from Flinn Scientific Inc. Try the Van de Graaff Generator (Catalog No. AP4699), the electric plume (Catalog No. AP5986), and the Van de Graaff Generator Teacher Demonstration Set (Catalog No. AP7274).

Answers to Prelab Questions

1. Define the term electrostatics or static electricity.

   Electrostatics or static electricity is electricity at rest and involves the behavior of electric charge and electric forces.

2. Define the term conduction.

   The charging of an object by direct contact.

3. How does an object become a) negatively charged and b) positively charged?
   1. An object becomes negatively charged by gaining electrons.

      This means it has more electrons than protons and therefore has a net negative charge.

   2. An object becomes positively charged by losing electrons.

      This means it has more protons than electrons and therefore obtains a net positive charge.

Sample Data

Part A. Paper Discs

The paper discs vibrate on the top of the dome due to the vibrations of the motor and rotating belt. When a hand is held above the discs, the paper at first is attracted and then repelled. The paper seems to oscillate back and forth between the hand and the dome.

The edges of the tinsel start to rise up away from the dome. When a hand is held above the dome, the tinsel seems to be attracted to the hand. The tinsel may even fly off the dome toward the hand. If the tinsel is touched while it is still on the dome, it discharges and falls back down.

The fur starts to stand up when in contact with the dome of the running generator. When a hand is held over the dome, the fur is attracted to the hand and stands up even more.

Answers to Questions

Note: For all questions, assume the net charge on the dome of the mini Van de Graaff generator is negative.

Part A. Paper Discs

1. Did the paper discs have a positive or negative charge when in contact with the operating mini Van de Graaff generator? Explain.

   The paper discs had a negative charge. They gained this charge through conduction or direct contact with the negatively charged dome.

2. What is the charge on the hand when held over the paper discs? Explain your answer.

   The hand held over the paper discs is neutral because it is not in contact with the charged object.

3. Were the paper discs attracted to or repelled from the hand? Why do you think this occurred?

   The paper discs seem to go back and forth between the hand and the dome. This is because initially the paper was negatively charged and therefore attracted to the neutral hand. When the paper discs touched the hand, they became neutral as well and were attracted back to the negatively charged dome. This process was repeated back and forth until the paper discs fell off the dome.

4. Did the tinsel have a positive or negative charge when in contact with the operating mini Van de Graaff generator? Explain.

   The tinsel had a negative charge. It gained this charge through conduction or direct contact with the negatively charged dome.

5. Was the tinsel attracted to or repelled from the hand? Why do you think this occurred?

   The tinsel was attracted to the hand. This happened because the hand is neutral and the tinsel had a negative charge. The charge was trying to balance between the two objects.

6. Did the fur have a positive or negative charge when in contact with the operating mini Van de Graaff generator? Explain.

   The fur had a negative charge. It gained this charge through conduction or direct contact with the negatively charged dome.

7. What happened to the fur as it remained on the dome with the generator running? Explain why this happened.

   The fur started to stand up. Each hair strand became negatively charged and the hair strands started to repel each other.

8. Was the fur attracted to or repelled from the hand? Why do you think this occurred?

   The fur was attracted to the hand because the hand is neutral, and the fur had a negative charge. The charge is trying to balance between the two objects.

Introduction

The Van de Graaff generator is a hair-raising experience for many observers! Build a mini Van de Graaff generator to better understand electrostatics.

Concepts

• Conduction
• Electric forces
• Van de Graaff generator
• Electric charge
• Electrostatics

Background

The first Van de Graaff generator was built in 1929 by Dr. Robert J. Van de Graaff (1901–1967). This high voltage electrostatic generator was once used to accelerate particles for nuclear physics experiments. The design and function of a Van de Graaff generator relied on the basic principles of electrostatics.

The term electrostatics (or static electricity) means electricity at rest, and involves the behavior of electric charge and electric force. The electric charge on an object is determined by its number of protons and electrons. Protons have a positive charge, and electrons have a negative charge. Most objects in our everyday lives are neutral, which means they have equal numbers of protons and electrons. An object becomes charged by either losing or gaining electrons. This most often occurs through conduction, the charging of an object by direct contact. If an object gains electrons its net charge will be negative, and if it loses electrons its net charge will be positive. Objects gain or lose electrons depending on their molecular makeup.

Electric force is the attraction or repulsion that occurs between charged objects—objects that have opposite charges attract, while like charges repel. Attraction will also occur between a charged object and a neutral object. This is because charge tends to balance out between objects, and any difference in charge will result in a force between objects. An electric force will not exist between two or more neutral objects because charge is already balanced between them.

The Van de Graaff generator is known as an electrostatic generator because it gains a charge. It consists of a motor, rubber belt, circular dome, upper roller, lower roller, upper wire brush, and lower wire brush (see Figure 1). The motor is used to spin the lower roller, which in turn rotates the rubber belt. As the rubber belt moves across the lower roller, friction causes electrons to be transferred from one to the other. The Van de Graaff generator can obtain either a positive or negative charge depending on the composition of the rollers. If the dome gains a negative charge, this means that electrons are being carried up the belt and deposited on the dome (see Figure 1). If the dome attains a positive charge, electrons are being removed from the top of the dome and carried down the belt. Regardless whether the net charge is positive or negative, like charges will repel one another and move as far away from each other as possible, spreading to the outside of the dome. The dome of a Van de Graaff generator is often circular because charge tends to leak off objects with sharp edges. Therefore a circular or round dome will hold a charge better than any other shape.

Experiment Overview

The purpose of this lab activity is to build a mini Van de Graaff generator and investigate the effects of static electricity.

Materials

Prelab Questions

1. Define the term electrostatics or static electricity.
2. Define the term conduction.
3. How does an object become a) negatively charged and b) positively charged?

Safety Precautions

This lab is considered to be nonhazardous. Van de Graaff generators produce a very small current and therefore an accidental shock may be startling, but the shock will not cause serious harm. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Prelab—Mini Van de Graaf Construction

1. Remove the plastic tube containing the ink from the inside of a standard ballpoint pen. Using scissors cut a 1" piece from the plastic tube. Make sure the 1" piece does not contain ink. Place the 1" piece onto the post of the DC motor (see Figure 2). Discard the remaining components of the pen.
   {12740_Preparation_Figure_2}
2. Cut a 20-cm piece of PTFE tape. Fold the tape in half lengthwise (see Figure 3) and wrap it around the base of a size 00 rubber stopper (see Figures 3 and 4). Press the tape down firmly onto the rubber stopper.
   {12740_Preparation_Figure_3}
3. Cut a second 20-cm piece of PTFE tape. Fold the tape in half lengthwise and wrap it around the top section of the size 00 rubber stopper (see Figure 4). The middle section of the rubber stopper should not contain PTFE tape, only the top and bottom. Press the tape down firmly onto the rubber stopper. This stopper functions as the bottom roller.
   {12740_Preparation_Figure_4}
4. Fit the rubber stopper onto the plastic tube on the DC motor. The rubber stopper should fit very snugly onto the plastic tube (see Figure 5). If the rubber stopper does not fit tight onto the plastic, remove the stopper and wrap transparent, masking or electrical tape around the plastic tube to increase its diameter. Again, place the rubber stopper onto the plastic.
   {12740_Preparation_Figure_5}
5. Using scissors, cut the piece of polyurethane foam so it is about 2.5 cm long, 2.5 cm wide and 0.5 cm thick (see Figure 6).
   {12740_Preparation_Figure_6}
6. Wrap the foam around the body of the DC motor and secure it with a 10-cm piece of tape (see Figure 7). Note: The foam will not fit around the entire body of the motor.
   {12740_Preparation_Figure_7}
7. Using wire strippers, cut a 30-cm piece of speaker wire. Strip both ends to expose about 1 cm of frayed wiring at each end (see Figure 8).
   {12740_Preparation_Figure_8}
8. Place the speaker wire through the PVC Tee connector as shown in Figure 9. Allow 2 cm of wire to hang out of the bottom of the PVC connector. Secure the wire to the inside of the Tee connector with tape.
   {12740_Preparation_Figure_9}
9. Support a rubber band on an index finger and insert the rubber band into the PVC Tee connector as shown in Figure 10. Using the other hand, slip the DC motor into the side opening of the PVC Tee connector so that the rubber band fits around the rubber stopper between the two pieces of Teflon tape. The rubber stopper should be near but not touching the inside wall of the PVC.
   {12740_Preparation_Figure_10}
10. Bend the 2-cm end of the speaker wire up inside the Tee connector and fan out the wire so the frayed ends are near but not touching the rubber band and rubber stopper. This will be the lower wire brush.
11. Using the clear plastic tube as a template, trace a circle on the bottom of one of the aluminum tart pans as shown in Figure 11. Set this tart pan aside.
    {12740_Preparation_Figure_11}
12. Using a small nail or thumbtack, poke a hole through the side wall of the clear plastic tube, 1 cm from the top (see Figure 12). The hole should go through both sides of the plastic tube.
    {12740_Preparation_Figure_12}
13. Insert the clear plastic tube into the PVC Tee connector as shown in Figure 13.
    {12740_Preparation_Figure_13}
14. Pull the rubber band up and out the top of the clear tube, then slip the 1"-long glass tube under the rubber band (see Figure 14). Make sure the rubber band is not twisted.
    {12740_Preparation_Figure_14}
15. Lower the glass into the plastic tube and line the glass tube up with the holes in the plastic tube. Insert the plastic toothpick into the holes. The toothpick should go through the glass tube (see Figure 15). This will be the upper roller.
    {12740_Preparation_Figure_15}
16. Test the initial setup by attaching the motor to a 3 V power source. The rubber band should rotate around the rubber stopper and the glass tube. If the rubber band does not rotate, the stopper may be hitting the inside wall of the PVC connector. Gently wiggle the motor until the rubber stopper is not hitting the inside wall of the PVC connector. Once the rubber band is rotating, disconnect the power source.
17. Take the remaining 30 cm of speaker wire and strip both ends to expose 1 cm of frayed wire at each end. Fold the wire 4.5 cm from one end as shown in Figure 16.
    {12740_Preparation_Figure_16}
18. Secure the wire to the top of the clear plastic tube with a piece of tape as shown in Figure 17. Bend and fan out the frayed wire so that it is near but not touching the rubber band. This will be the upper wire brush.
    {12740_Preparation_Figure_17}
19. Using scissors, carefully cut out the circle that was traced on one aluminum tart pan.
20. Place the aluminum tart pan over the clear plastic tube and pull the free end of the speaker wire through the hole in the tart pan (see Figure 18). Secure the tart pan to the toothpick with tape.
    {12740_Preparation_Figure_18}
21. Using tape, secure the free end of the speaker wire to the inside bottom of the second (uncut) aluminum tart pan. Make sure the frayed wire is in direct contact with the aluminum tart pan (see Figure 18).
22. Place the second aluminum pan on top of the other as shown in Figure 19. Secure them together with tape.
    {12740_Preparation_Figure_19}

1. Obtain eight small paper discs from a paper punch. Spread the paper discs on the top of the mini Van de Graaff generator.
2. Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply. Observe the vibrating discs.
3. Hold the palm of your free hand over the paper discs and observe for 10–20 seconds. Disconnect the mini Van de Graaff from the power source and record your observations on the Build a Mini Van de Graaff Worksheet.
4. Ground the mini Van de Graaff generator by touching its top.

1. Obtain a piece of tinsel and lay it across the top of the mini Van de Graaff generator.
2. Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply.
3. Hold the palm of your free hand over the tinsel and observe for 10–20 seconds. Touch the tinsel with your finger and observe what happens. Disconnect the mini Van de Graaff generator from the power supply and record your observations on the worksheet.
4. Ground the Mini Van de Graaff generator by touching its top.

1. Obtain a small strip of rabbit fur. Smooth the fur down with your hand. Place the fur across the top of the mini Van de Graaff generator.
2. Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply.
3. Observe the fur for 10–20 seconds. Hold the palm of your free hand over the fur and observe for another 10 seconds. Touch the fur with your finger and observe what happens. Disconnect the mini Van de Graaff generator from the power supply and record your observations on the worksheet.
4. Ground the mini Van de Graaff generator by touching its top.

Student Worksheet PDF

12740_Student1.pdf