Build a Mini Van de Graaff Generator
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
- Electric charge
- Electrostatics
- Van de Graaff generator
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.
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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
Aluminum tart pans, 4½" diameter, 2 Clear plastic tube, 1" diameter, 2½" length DC motor Fur, rabbit Nail or thumbtack Paper discs, 8 Pens, ballpoint, 2 Polyurethane foam, 1 in3 Power source, 3-V, DC PTFE tape, ½", 1 roll PVC Tee connector, ¾" Rubber band Rubber stopper, one-hole, size 00 Ruler, metric Scissors Speaker wire, 2 feet Tape, transparent, masking or electrical Tinsel Toothpick, plastic Tube, glass, 1" long Wire strippers
Prelab Questions
- Define the term electrostatics or static electricity.
- Define the term conduction.
- 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 Graaff Construction
- 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.
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- 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.
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- 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.
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- 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.
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- 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).
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- 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.
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- 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).
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- 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.
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- 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 PTFE tape. The rubber stopper should be near but not touching the inside wall of the PVC.
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- 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.
- 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.
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- 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.
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- Insert the clear plastic tube into the PVC Tee connector as shown in Figure 13.
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- 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.
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- 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.
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- 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.
- 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.
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- 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.
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- Using scissors, carefully cut out the circle that was traced on one aluminum tart pan.
- 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.
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- 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).
- Place the second aluminum pan on top of the other as shown in Figure 19. Secure them together with tape.
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Part A. Paper Discs
- Obtain eight small paper discs from a paper punch. Spread the paper discs on the top of the mini Van de Graaff generator.
- Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply. Observe the vibrating discs.
- 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.
- Ground the mini Van de Graaff generator by touching its top.
Part B. Tinsel
- Obtain a piece of tinsel and lay it across the top of the mini Van de Graaff generator.
- Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply.
- 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.
- Ground the Mini Van de Graaff generator by touching its top.
Part C. Fur
- 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.
- Holding the mini Van de Graaff generator in one hand, attach the DC motor to a 3 V power supply.
- 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.
- Ground the mini Van de Graaff generator by touching its top.
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