Materials Included In Kit

Additional Materials Required

Prelab Preparation

Activity C. Ready, Set, Charge! 

1. Set up a support stand with a ring clamp.
2. Wrap two pith balls in a small amount of aluminum foil, just enough to completely cover each ball.
3. Hold two pith balls together and pull both pieces of string taut.
4. With the balls next to each other and the string pulled tightly, tie both pieces of string together at the ends.
5. Hang the string over the ring clamp so the knot rests on the ring far from the rod and the pith balls hang next to each other (see Figure 8 in the Procedure).
6. Clip the excess string with scissors, if necessary.
7. Repeat steps 1–6 for each pair of pith balls.

Activity D. Curving Water

1. Fill each beaker with 150 mL of tap water.

Safety Precautions

Be alert to the potential for electrostatic shocks, especially in Activity C. Students should wear safety glasses for Activity D. Clean up any water spills immediately. The static charges in these activities will be low-voltage, but electrostatic shocks can be startling. Please follow all laboratory safety guidelines.

Disposal

All materials may be stored for future use.

Lab Hints

• For best results, set up three stations for each activity throughout the lab. This will allow 12 groups of students to rotate through four activity stations in a 45- to 50-minute lab period. A double lab period (two class periods) will allow time for both an introduction to static electricity before the lab and for a collaborative class discussion after lab.
• Each activity is a self-contained unit and may be completed in any order. Students should need only 8–10 minutes per station—keep the pace fairly brisk to avoid dawdling. Post-Lab Questions may be answered during any downtime between stations.
• Prelab preparation is an essential component of lab safety, and it is also critical for success in the lab. Standing in front of the lab station is not a good time for students to be reading the activity for the first time. Having students complete the written prelab assignment for this lab will help ensure that students are prepared for and can work safely and efficiently in the lab.
• Static-electricity experiments always work better on a dry day. Lower humidity days are better than high humidity days. Air-conditioned air or heated winter air tends to be drier and more conducive for electrostatic activities.
• Transparent tape with a matte finish works better than clear cellophane tape. It may be reused several times before discarding.
• After continuous use, the friction rods and friction pads may become permanently charged. It may be necessary to ground them occasionally in order to return them to a neutral state. Rubbing them on a grounded metal table or metal table leg is a good way to remove any accumulated charge.

Teacher Tips

• Ask students to research everyday effects of static electricity, both positive and negative (no pun intended!). Some examples are given.
  • Destructive or beneficial effects of lightning
  • Photocopy machines
  • Hazards of static electricity around gas pumps
  • Electrostatic paint sprayers
  • The need for safety footwear for workers
  • Electrostatic air cleaners where a static discharge may be hazardous

Answers to Prelab Questions

1. What is static electricity?

   Static electricity is a stationary electric charge.

2. What does it mean when an object is described as electrically neutral?

   An electrically neutral object has the same number of positive and negative charges.

3. Refer to the triboelectric series chart in the Background section.
   1. Which pair of materials below would you expect to accumulate the greatest amount of charge when rubbed together?

      Wool/Silk, Silk/Polyethylene, Polyethylene/Rabbit fur
      When rubbed together, polyethylene and rabbit fur would accumulate the greatest charge.

   2. Explain your answer.

      Polyethylene and rabbit fur are farther apart on the triboelectric series compared to the other pairs of materials listed. The farther apart two materials are in the series, the greater the transfer of charges.

Sample Data

Activity A. Sticky Charges

Activity B. The Mysterious Moving Board Activity C. Ready, Set, Charge! Activity D. Curving Water

Observations
Describe what happens to the stream of water as it flows past the charged comb.

The stream of water bends and flows into the center of the beaker.

Answers to Questions

Activity A. Sticky Charges 

1. Consider the charged pieces of tape that were created when both were removed side by side from the work surface.
   1. What type of charges (like or unlike) did this first treatment produce on the pieces of tape?

      Since the pieces of tape repelled, they had like charges.

   2. Why do you think this happened?

      The two pieces of tape were treated the same way, so each accumulated the same charge.

2. Consider the charged pieces of tape created when one was on top of the other and then pulled apart.
   1. What type of charges (like or unlike) did the second treatment produce on the pieces of tape?

      Since the pieces of tape attracted, they had unlike charges.

   2. Why do you think this happened?

      The two pieces of tape were treated differently, where one was removed from the work surface and the other was removed from the first piece of tape. Therefore they accumulated opposite charges.
      

Activity B. The Mysterious Moving Board

3. Refer to the triboelectric series chart from the Background section.
   1. What type of charge did the Lucite^® rod have after it was rubbed with the silk cloth?

      The Lucite rod was negatively charged after being rubbed with the silk cloth.

   2. What type of charge did the silk cloth have?

      The silk cloth was positively charged.

4. Consider the wood board in this activity.
   1. During this activity, was the overall charge on the wood board positive, negative or neutral?

      The overall charge on the wood board was neutral.

   2. Explain your answer.

      Nothing was done to the board to cause it to become either positively or negatively charged. It was neutral because it had the same number of positive and negative charges.

5. Propose an explanation for the motion of the wood board during all parts of this activity.

   When the negatively charged rod was brought near the board, it repelled the negative charges at the surface of the board. As a result, the positive charges near the edge of the board were more strongly attracted to the negatively charged rod than the more distant negative charges were repelled, creating a net force of attraction between the board and the rod. When the positively charged piece of silk was brought near the board, it attracted the negative charges at the surface of the board. As a result, the negative charges near the edge of the board were more strongly attracted to the positively charged cloth than the more distant positive charges were repelled, again creating a net force of attraction between the board and the silk.

Activity C. Ready, Set, Charge!

6. The drinking straw is made of a type of polypropylene plastic. Refer to the triboelectric series in the Background section.
   1. Determine the type of charge the straw accumulated after it was rubbed with rabbit fur.

      The straw became negatively charged.

   2. After the piths balls were charged by induction, what charge (positive or negative or one of each) was on the sides of the pith balls facing each other?

      The pith balls were repelling, so both pith balls had an induced positive charge on the sides facing each other. The negative charges in the pith balls moved away from the negatively charged straw.

   3. After the piths balls were charged by conduction, what charge did the pith balls carry?

      Charging the electroscope by conduction permanently charged the pith balls with the same negative charge that was on the charged straw. Some of the negative charge was transferred from the straw to each pith ball when they touched. The pith balls repelled each other.

7. Explain why the pith balls immediately “fly away” from the charged straw after making contact.

   When the pith balls made contact with the charged straw, they immediately attained the same negative charge as the straw. Like charges repel, so the negatively charged pith ball was forced away from the negatively charged straw.

Activity D. Curving Water

8. Refer to Figure 4 in the Background section.
   1. Which atoms in the water molecules tend to be more negative?

      The oxygen atom in a water molecule has a partial negative charge.

   2. Which tend to be more positive?

      The hydrogen atoms in a water molecule have a partial positive charge.

9. Why is the water stream affected by an external electric charge?

   The polarization of water molecules make them very easily influenced by an external electric charge. If the external charge is negative, the molecules align themselves so the positive hydrogen molecules are attracted, and if the external charge is positive, the negative oxygen atoms will be attracted.

Going Further

10. Often when clothes are taken out of a dryer, certain items (e.g., a nylon sock and a polyester shirt) cling together. Explain why this happens.

    When two different fabrics tumble against each other in a dryer, one picks up electrons from the other. The two fabrics then have opposite charges that attract and the items cling together.

11. A student stood under a plastic playground slide as other students slid down the slide. Over time, the hair on the student under the slide began to “stand up” toward the underside of the slide. Explain why this occurred.

    The friction between the students that were sliding and the surface of the slide caused static charges to accumulate on the slide. When the charge was strong enough, the like charges in the hair of the person under the slide were repelled and opposite charges were attracted toward the underside of the slide. As this continued, the strands of hair became charged by induction and began to stand up.

12. A balloon is charged by rubbing it with a piece of rabbit fur. The balloon is then placed against a wall and it sticks. Explain why the balloon stuck to the wall, even though the wall was not charged.

    The balloon became negatively charged when rubbed with the rabbit fur. When the charged balloon was brought into contact with the neutral wall, the electron cloud around each atom in the surface of the wall was repelled. Therefore the atoms near the surface became slightly polarized with the center of positive charge from the nuclei closer to the charged balloon as the negative electron clouds became asymmetrical. Since electric forces get weaker as the distance between them increases, the positive charges near the surface of the wall were more strongly attracted to the negatively charged balloon than the more distant negative charges were repelled, creating a net force of attraction between the wall and the balloon.

References

Robertson, W. C. Stop Faking It! Electricity and Magnetism. NSTA Press: Arlington, VA. 2005.

Introduction

Have you ever reached for a doorknob and been zapped by a “lightning-like” spark? Learn the principles of static electricity.

Concepts

• Static electricity
• Polarity
• Negative charge/positive charge
• Electron transfer

Background

Static Electricity

Static electricity is a stationary electric charge. It occurs all around us every day, with effects ranging from lightning, to photocopy machines, to the shocks received on a dry winter day. It all occurs because of the structure of the atom. The atoms of a solid object are held tightly in place. Their positively charged nuclei are not free to move about within the solid. The negatively charged electrons, which surround the nucleus of the atom, are the key to understanding static electricity and charges on objects.

Generally speaking, most objects have an equal number of protons and electrons and are therefore considered electrically neutral. With the addition or removal of energy, the outermost electrons can be transferred from one atom to another. An atom missing electrons will have an overall positive charge—any matter made of these electron-deficient atoms will be positively charged. Conversely, if an atom has excess electrons, it will have a negative charge and materials made of these atoms will be negatively charged. Different atoms vary in their ability to give up or take on electrons. The atomic makeup or composition of a substance gives it a unique electron-holding or electron-releasing ability, and also allows a substance to become charged. When you rub your shoes along a carpet, energy is provided to transfer electrons from the carpet to your body. You become negatively charged with the excess electrons over the surface of your body. When you then reach for a doorknob, the excess electrons are suddenly released. This electron transfer from your body to the doorknob causes a spark to jump between the two and you experience a mild shock.

The static electricity caused by materials rubbing together is known as triboelectricity (tribo- from the Greek word meaning to rub). Some materials hold onto electrons more tightly than others and thus, depending on the materials, positively or negatively charged material may be produced. Table 1 shows some common substances and their electrostatic position relative to each other.

An atom holds onto its negative electrons by the force of electrical attraction of its positive nucleus. Some atoms exert stronger forces of attraction than others on their electrons. When a latex balloon and fur are rubbed together, some of the electrons from the atoms in the fur are “captured” by the atoms of the balloon, which exert stronger forces of attraction on those electrons than do the atoms of the fur. Thus, after the rubbing, the balloon has an excess of electrons and therefore a negative charge and the fur has positive charge from a deficit of electrons (see Figure 1). The same explanation will apply to many other pairs of substances that are rubbed together. The triboelectric series shown in Table 1 includes many common substances that can be used to create predictable static charges. If any two substances are rubbed together, the substance that is higher on the table becomes positively charged, while the lower substance on the chart becomes negatively charged. The farther apart the two substances are in the series, the greater the transfer of charges. Two substances near each other in the series may become either positively or negatively charged, or even remain neutral, depending on the materials. For example, if a piece of Lucite is rubbed with a latex balloon, the Lucite may remain neutral or may give up electrons and become positively charged. But if the Lucite were rubbed with rabbit fur, the fur would more likely become positively charged and the Lucite would gain electrons and have a negative charge.

Charging by Induction Versus Conduction

An electroscope (shown in Figure 2) works well as a detector and storage unit of static electric charge because the electrons surrounding the central sphere are easily influenced. This makes the electroscope a great conductor of electric charge. Electrons in the sphere will readily migrate to different regions in response to an external static electric charge. The fundamental principle of electric charge is that like charges repel and unlike charges attract. Two positive or two negative charges will move away from one another, whereas a positive charge and a negative charge will move toward one another. If an external negative charge is brought near the electroscope without touching it, the negatively charged electrons in the sphere will be repelled and migrate away from the external negative charge. The side of the electroscope near the external negative charge will thus have an induced positive charge. The above process of charging the electroscope is called charging by induction. The positive and negative charges remain in the electroscope so it maintains a net charge of zero, and remains neutral. No electrons are actually transferred into or out of the electroscope. However, the unbalanced charge distribution causes the electroscope to experience a temporary polarity—separation of charges. When the external charge is removed, the charges in the electroscope will eventually become evenly redistributed (see Figure 2).

The electroscope can also gain or lose electrons to become charged. This can occur when the electroscope is charged by conduction. In this case, a charged rod is brought into direct contact with the uncharged sphere (see Figure 2). The charge then redistributes throughout the rod and the electroscope as if they were one object. When the charged rod is removed, the electroscope will retain a charge of the same polarity as the charged rod. The charged rod has donated some of its charge to the electroscope.

Charges and Neutral Objects

When a negatively charged object is brought near a neutral object, the electron cloud around each atom in the neutral object is repelled and the atoms near the surface become slightly polarized. That is, the atoms closer to the charged object acquire a small, temporary partial positive charge when the negative electron cloud is no longer symmetrical. On average, the negatively charged electron cloud is farther away. Electric forces get weaker as the distance between them increases. As a result, the positive charges near the edge of the neutral object are more strongly attracted to the negatively charged object than the more distant negative charges are repelled, creating a net force of attraction between the two objects (see Figure 3). If a positively charged object were brought near the same neutral object, the electron cloud would be more strongly attracted, again creating a net force of attraction. Polar Molecules

Water is a polar molecule, characterized by partial charge separation between the oxygen and hydrogen atoms in its O—H bonds. Molecules are classified as polar or nonpolar based on the nature of the electron sharing between atoms. In polar covalent bonds the bonding electrons are not equally shared between atoms. In the case of water molecules, the oxygen attracts the bonding electrons more strongly than hydrogen. Each oxygen atom is described as bearing a partial negative charge and the hydrogen atoms having a partial positive charge with the overall charge of the molecule being neutral (see Figure 4). Therefore, when either a positively or negatively charged object is brought near, water molecules will align themselves accordingly with opposite charges facing each other.

Experiment Overview

The purpose of this activity-stations lab is to investigate static electricity. Four mini-lab stations are set up around the classroom. Each activity focuses on a particular concept associated with electrostatics. The activities may be completed in any order.

1. Sticky Charges—Investigate positive and negative electric charges and observe how like and unlike charges behave.
2. The Mysterious Moving Board—Discover how an uncharged insulator can be “mysteriously” attracted by a charged object.
3. Ready, Set, Charge!—Distinguish between charging by conduction and induction.
4. Curving Water—Observe the effect of an electric charge on a polar molecule.

Materials

Prelab Questions

1. What is static electricity?
2. What does it mean when an object is described as electrically neutral?
3. Refer to the triboelectric series chart in the Background section.
   1. Which pair of materials would you expect to accumulate the greatest amount of charge when rubbed together?

      Wool/Silk, Silk/Polyethylene, Polyethylene/Rabbit fur

   2. Explain your answer.

Safety Precautions

Wear safety glasses. Clean up any water spills immediately. Be alert to the potential for electrostatic shocks. Although static charges in this activity will be low-voltage, electrostatic shocks can be startling. Please follow all laboratory safety guidelines.

Procedure

Activity A. Sticky Charges

1. Tear off a 7-cm piece of tape.
2. Make a “handle” by folding over approximately 1 cm at one end so the sticky sides are together (see Figure 5).
   {12208_Procedure_Figure_5}
3. Press the sticky side down on a desk or table top with the folded end hanging over the edge (see Figure 6).
   {12208_Procedure_Figure_6}
4. Smooth the piece of tape down so the entire length is firmly pressed onto the work surface.
5. Repeat steps 1–4 with a second piece of tape, pressing it down next to, but not touching the first piece.
6. Grasping one handle in each hand, quickly pull the pieces of tape off the work surface, but do not let them touch each other.
7. Slowly bring the non-sticky sides toward each other and observe how the pieces behave. Record your observations for Activity A on the Investigating Static Electricity Worksheet.
8. Repeat steps 3 and 4 with one of the pieces of tape.
9. Press the second piece of tape on top of the first, smoothing it down completely.
10. Grasp both handles in one hand and quickly pull both pieces of tape off the work surface at the same time.
11. Quickly pull the two pieces apart by the handles. Note: The tape may curl and stick to your hand or finger. Quickly detach the tape from your hand without letting the two pieces touch. A lab partner may help, removing the tape with a wood pencil.
12. Repeat step 7.

Activity B. The Mysterious Moving Board

1. Place the cap in the center of the work area. Clear any objects away from the board.
2. Balance the board on the cap (see Figure 7). Note: The board does not need to be perfectly balanced. Just make sure neither end touches the work surface.
   {12208_Procedure_Figure_7}
3. Make sure the Lucite friction rod is not charged by holding it with both hands for several seconds.
4. Bring the uncharged end of the Lucite rod very close to, but not touching, one corner of the balanced board.
5. Observe what happens. Record observations for Activity B on the Investigating Static Electricity worksheet.
6. Charge the Lucite friction rod by rapidly rubbing it with a piece of silk cloth.
7. Bring the charged end of the Lucite rod near one corner of the balanced board.
8. Again, observe what happens and record all observations on the worksheet.
9. Repeat steps 6–8, bringing the charged Lucite rod near a corner at the opposite end of the board.
10. Fold the silk cloth into quarters.
11. Charge the silk cloth by rapidly rubbing it with the Lucite rod.
12. Bring the silk cloth near one corner of the balanced board.
13. Observe and record what happens.

Activity C. Ready, Set, Charge!

Part 1. Charge by Induction

1. Check to make sure the pith balls are hanging side by side from the ring clamp and away from the support rod. Adjust the position of the pith balls if necessary (see Figure 8).
   {12208_Procedure_Figure_8}
2. If the pith balls are not touching each other, discharge them by holding each one in a hand for a few seconds.
3. Charge the drinking straw by rapidly rubbing it with the fur friction pad.
4. Bring the end of the straw between the strings near the top without touching the metal ring.
5. Slowly lower the straw between the strings until the end of the straw is between the two pith balls and observe what happens.
6. Record your observations for Activity C on the Investigating Static Electricity worksheet.
7. Hold the straw between the two pith balls for several seconds.
8. Carefully remove the straw from between the two pith balls and observe what happens. Record your observations on the worksheet.

Part 2. Charge by Conduction

9. Discharge the pith balls by holding each in a hand for several seconds. The pith balls should be hanging side by side and touching.
10. One partner should grasp each string about halfway between the ring and the pith ball and hold the pith balls away from each other (see Figure 9).
    {12208_Procedure_Figure_9}
11. A second partner should charge the drinking straw by rapidly rubbing it with the fur friction pad.
12. Bring the charged straw near one pith ball and let the straw just touch the ball.
13. Repeat steps 11–12 with the second pith ball.
14. Slowly bring the pith balls closer to each other and let go of the strings.
15. Record your observations on the worksheet.
16. Discharge the pith balls as in step 9.
17. While one partner is holding the uncharged pith ball by the string, the other partner should touch the pith ball with the charged straw.
18. Record your observations on the worksheet.
19. Let both pith balls hang freely and record your observations.
20. Repeat step 9.

Activity D. Curving Water

1. Draw up 10–12 mL of water from the beaker into the syringe.
2. Charge the comb by rapidly rubbing it with the wool friction pad.
3. Hold the syringe, pointing down (plunger pointing up) about 10 cm above the beaker, and near the inner edge of the beaker (see Figure 10).
   {12208_Procedure_Figure_10}
4. Hold the charged comb over the center of the beaker, about 2–3 cm below the syringe tip.
5. While holding the syringe and the comb in the same relative positions, slowly and continuously press on the syringe plunger to create a water stream. Note: The positions of the syringe and comb, as well as the firmness with which to press on the syringe may require practice.
6. Repeat steps 1–5 as needed to observe the effect of the charged comb on the stream of water.
7. Record your observations for Activity D on the Investigating Static Electricity worksheet.

Student Worksheet PDF

12208_Student1.pdf