Electrostatics

Introduction

Electrostatics, or static electricity, can be witnessed almost every day in events or applications ranging from lightning and copy machines to the shocks received on a dry winter day. Demonstrate the fundamental properties of static electricity with this collection of activities. The following five demonstrations can be performed with this kit:

• Charging a Flask Form Electroscope
• Electrophorus and Condensing Electroscope
• Charge Distribution and Discharge
• Faraday Cage and “Ice Pail”
• Curving Water into a Beaker

Concepts

• Static electricity
• Electrophorus
• Induction
• Positive and negative charges
• Faraday cage
• Faraday “Ice Pail”
• Conduction
• Electroscopes
• Repulsive forces
• Polar molecule
• Charge distribution
• Lightning rod
• Insulators
• Conductors
• Static charge

Background

Static Electricity
Atoms are composed of three basic particles—positively charged protons, negatively charged electrons and neutrons, which carry no charge. The positive and negative charges on protons and electrons, respectively, are equal in magnitude, so the combination of one proton and one electron results in an electrically neutral atom (a hydrogen atom). In general, most objects have an equal number of protons and electrons and are therefore considered electrically neutral. Objects with more electrons than protons will be negatively charged.

Protons form the dense inner core of atoms and do not move about freely within an object. Electrons, on the other hand, are not held in place by rigid bonds. 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. This electrostatic attractive force between electrons and protons keeps the electrons moving closely around the protons, but they are not “locked” into position like the proton. Some (outershell) electrons have the ability to migrate throughout a material, and therefore are referred to as being delocalized. Because of the electrons’ mobility, they are also capable of being removed from an object. Static, or stationary, electricity is produced by physically separating and moving electric charges away from each other. The ease with which the electrons in a material can be removed depends on the atomic composition of the material. Benjamin Franklin (1706–1790) standardized the positive and negative of static electricity by showing that a glass rod rubbed with silk attained a positive charge. A list of the relative electron “holding” and “releasing” abilities of different common materials is shown in Table 1.

When two substances in Table 1 are rubbed together, the substance that is higher in the table will become negatively charged while the material lower in the table will become positively charged. As an example, when rubber-soled shoes are rubbed along a wool carpet, the rubber-soled shoes will collect and retain excess electrons from the carpet. As a result, the shoes (and you) become negatively charged and the carpet becomes positively charged. The physical separation of positive and negative charges causes a temporary, uneven distribution of electrons over an object. Once the charge separation occurs, the tendency is for an object to return to its neutral state. This may occur rapidly (as in a spark or large lightning bolt) or by a slow leaking of electrons from the object to the air or to other nearby objects. The electric shock you then receive when you grab a doorknob is the result of the surplus of electrons that have accumulated and redistributed throughout your body that “jump” toward the more positively grounded doorknob, and thus reestablish a charge balance. Static charge may or may not accumulate on an object depending on the conditions of the materials and the surrounding environment. In the shoe-and-carpet example above, the charge that transfers between the shoes and carpet can easily dissipate into the surrounding air, especially humid air, without causing a shock.

The Electroscope
Metals tend to be good conductors of electricity because electrons surrounding metal atoms are mobile. The mobile electrons in the ball and foil leaves of the electroscope readily migrate in response to an external static electric charge. This makes an electroscope an excellent detector and storage unit for static-electric charge.

When an external negative charge is brought toward the metal ball at the top of the flask form electroscope (see Figure 1), negatively charged electrons in the metal ball are repelled (recall: like charges repel) and migrate into the foil leaves of the electroscope (as far from the external negative charge as possible). Electrons accumulate equally in both foil leaves, giving them both a negative charge and the foil leaves repel and separate. For an external positive charge, the electrons in the foil leaves will be attracted to the positive charge and migrate into the metal ball of the electroscope. The positively charged protons in the foil leaves are left behind, so once again, the foil leaves have the same charge and they diverge. This process of charging the electroscope is called charging by induction. No electrons transfer into or out of the electroscope, they only redistribute causing a temporary polarization. The foil leaves fall when the external static charge is removed. A condensing electroscope is a special type of electroscope that uses a large plate or disk instead of a metal ball as the charge collector. The larger surface area allows the electroscope to collect more charge and makes it more sensitive to objects with smaller electric charges. The electroscope can become permanently charged by either charging it by conduction, or permanently charging by induction. When the electroscope is charged by conduction, a charged rod makes direct contact with the electroscope. The total electric charge redistributes throughout the rod and the electroscope as if they were one object. The foil leaves diverge, and when the charged rod is removed, the electroscope will carry a charge of the same polarity as the charged rod. The charged rod has donated some of its charge to the electroscope, and therefore has lost some of its initial charge.

To permanently charge an electroscope by induction, the external charged rod does not touch the electroscope. Instead, the charged rod temporarily polarizes the electroscope. Then, a grounded object (such as a hand) touches the ball of the electroscope to add or remove the charge that migrates towards or away from the external charge. An electrically grounded object is connected to the Earth through a conductor. The Earth acts as a large conductor and can be either a large reservoir for electrons or large supplier of electrons. For example, when a negatively charged rod is brought close to the metal ball of the electroscope, the electrons in the metal ball migrate away from the external negative charge and accumulate in the foil leaves. When a grounded rod touches the metal ball while the external negative charge is still there, the accumulated electrons in the foil leaves will then migrate into the ground because it is even more positive and the electrons can travel even further away from the external negative charge. Remove the hand from the ball, and then the charged rod and the electroscope will have lost electrons, and therefore has become positively charged. When the electroscope is charged by induction, the permanent charge on the electroscope will be opposite to the charge on the external source.

Electrons readily dissipate into the air at sharp points. The more gradually curved an object is, the less likely the static charge will dissipate. The metal ball of the electroscope helps to maintain electric charge and works well as the terminal for static charge transfer. A sharp-pointed pin quickly dissipates electric charge due to its high curvature. This is the principle behind lightning rods (another Benjamin Franklin invention). A lightning rod “bleeds” away excess static electric charge that may build up on a house or barn during an electrical storm, or on the wings of a plane as it flies through the air, into the air at its sharp tip. However, if the lightning rod does not bleed off enough static electricity, it also acts as a ground to conduct the electricity from a lightning bolt away from a structure, if lightning should strike.

The Electrophorus
The electrophorus is another device that can be used to store and transfer electric charge. Invented by little-known Johan Wilcke (1732–1796) in 1762, the electrophorus design was perfected by Alessandro Volta (1745–1827) (of battery fame). The device was quickly adopted by physical scientists to fill the need for a reliable and easy-to-use source of electric charge for experimental research in electrostatics. The device stores a large amount of electric charge due to the large surface area of the plate.

The Faraday Cage and “Ice Pail”
Another fundamental property of static electricity that was demonstrated by Benjamin Franklin showed that no external electric charge can penetrate the interior of a conducting shell. This means that if a conducting shell acquires an electric charge, all the charge resides on the outside surface of the shell. The interior remains charge-free. Conducting shells exhibit this principle because like charges repel each other to a point of the greatest separation possible and the exterior charges distribute themselves in such a way as to cancel any interior electric charge. On a conducting shell, the greatest separation occurs when the charge accumulates on the largest surface area—the outside surface. Therefore, all the charge accumulates here, and no charge remains on the inside surface or inside the conducting shell. A charged object that touches the inside surface of a conducting shell will lose all of its charge to the outside surface of the conducting shell. Michael Faraday (1791–1867) demonstrated this principle using a metal cage (now known as a Faraday cage) and an “ice pail.” When he sat inside a metal cage, it protected him from external sources of static electricity, even very high voltage sources (such as those produced by a Van de Graaff generator, invented in the early 1900s). When he added static charge to a metal ice pail, he was able to collect static charge only from the outside surface. No charges resided on the inside of the pail.

Curving Water into a Beaker
The triangular shape of a water molecule, plus its extreme polarity (due to hydrogen bonding) make water molecules easily influenced by electric charges. When an external electric charge is near water, the water molecules align themselves accordingly with opposite charges attracting each other. When a negatively charged comb is placed close to a water stream, the positive ends of the water molecules (the hydrogen) are attracted to the negative charge and the electrostatic attraction causes the water to bend toward the charge. Demonstration 1: Charging a Flask Form Electroscope In this demonstration, the existence of static electric charge will be displayed with an electroscope. Rub different materials together to create positively or negatively charged objects and show how the electroscope responds to the different charges. Demonstration 2: Electrophorus and Condensing Electroscope Demonstrate the technique used to charge an electrophorus and determine its charge using an electroscope. Demonstration 3: Charge Distribution and Discharge Demonstrate how charges distribute using two condensing electroscopes. Also, show how a lightning rod can be used to protect homes, buildings and airplanes. Demonstration 4: Faraday’s Cage and “Ice Pail” Show students that static charges distribute on the outside surface of a metal frame and not on the interior surface. Demonstration 5: Curving Water into a Beaker Demonstrate that water molecules can be easily polarized by an external electric charge.

Experiment Overview

Charging a Flask Form Electroscope
In this demonstration, the existence of static electric charge will be displayed with an electroscope. Rub different materials together to create positively or negatively charged objects and show how the electroscope responds to the different charges.

Electrophorus and Condensing Electroscope
Demonstrate the technique used to charge an electrophorus and determine its charge using an electroscope.

Demonstration 3: Charge Distribution and Discharge
Demonstrate how charges distribute using two condensing electroscopes. Also, show how a lightning rod can be used to protect homes, buildings and airplanes.

Demonstration 4: Faraday Cage and “Ice Pail”
Show students that static charges distribute on the outside surface of a metal frame and not on the interior surface.

Demonstration 5: Curving Water into a Beaker
Demonstrate that water molecules can be easily polarized by an external electric charge.

Materials

Safety Precautions

Be alert to the potential dangers associated with electrostatic shocks. The static charges in this demonstration will be low voltage, but static-electricity shocks can still be startling. Handle the pin carefully. Practice the electrostatic demonstrations prior to performing for students. Please follow all normal laboratory safety guidelines.

Disposal

The materials from each demonstration may be stored and saved in their original containers for future use.

Prelab Preparation

Demonstration 1: Charging a Flask Form Electroscope

Electroscope Assembly

1. Use scissors to cut two identical ⅜" x ¾" pieces of aluminum foil.
2. Use the pin included in the kit to poke a hole near the center of the short edge of one piece of aluminum foil. Make sure the hole is large enough to fit over the hook of the rod, hook and rubber stopper assembly.
3. Repeat step 2 for the other aluminum foil piece.
4. Place the foil pieces on the tabletop and press down to flatten and smooth them.
5. Carefully hang the aluminum foil pieces on the hook, making sure not to bend the aluminum foil. The aluminum foil pieces should hang together and be able to move freely on the hook. If the aluminum foil “sticks” to the hook, remove the foil and widen the hole with the pin as necessary (see Figure 2).
   {13530_Preparation_Figure_2}
6. Insert the hook and aluminum foil into the 250-mL Erlenmeyer flask and press the rubber stopper into the opening (see Figure 1 in the Background section.)

Demonstration 2: Electrophorus and Condensing Electroscope

Assembly

1. Screw the insulated handle into the threaded nut on the top of the electrophorus disk as shown in Figure 3.
   {13530_Preparation_Figure_3}
2. Remove the thin protective film from both sides of the acrylic sheet, if necessary. It is usually colored blue, but it may also be colorless.
3. Place one self-adhesive foot pad in each corner of the insulating acrylic sheet. Rest the insulating sheet on its foot pads on a flat surface to use as a charging base for the electroscope.
4. Obtain the friction pads and the flask form electroscope.

Procedure

Demonstration 1: Charging a Flask Form Electroscope 

1. Negatively charge a hard rubber friction rod by rapidly rubbing it with a piece of flannel.
2. Charge by induction: Bring the negatively charged friction rod near the metal ball of the flask form electroscope, but do not touch it (see Figure 4). Students should record observations on the Flask Form Electroscope Worksheet. Move the charged rod away from the electroscope and have students record their observations in the worksheet.
   {13530_Procedure_Figure_4}
3. Recharge the friction rod if necessary.
4. Charge by conduction: Touch the metal ball of the flask form electroscope with the negatively charged friction rod (see Figure 5). Students should record observations on the Flask Form Electroscope Worksheet. Slide the friction rod along the metal ball three or four times and then remove the friction rod. Have students record their observations in the worksheet.
   {13530_Procedure_Figure_5}
5. Discharge and ground the flask form electroscope by touching the metal ball with your free hand (see Figure 5). (Make sure to ground yourself before touching the metal ball, if necessary.) Have students record their observations on the worksheet.
6. Recharge the friction rod if necessary.
7. Permanently charge by induction: Touch the metal ball of the flask form electroscope with your free hand. Bring the negatively charged friction rod near the electroscope, but do not touch the metal ball. Students should record observations in the Flask Form Electroscope Worksheet. Remove your hand from the metal ball. Have students record their observations in the worksheet. Then move the friction rod away from the electroscope. Students should record all observations in the worksheet.
8. Bring the charged rod near the flask form electroscope again, and have students record their observations on the Flask Form Electroscope Worksheet.
9. Touch the metal ball to discharge the electroscope.
10. Repeat Procedure steps 1–9 using a positively charged friction rod. The positively charged friction rod may be made by rubbing a glass friction rod with silk. Have students compare the results to those obtained using the negatively charged friction rod, and record all observations on the Flask Form Electroscope Worksheet.

Demonstration 2: Electrophorus and Condensing Electroscope

Charging the Electrophorus

1. First, charge the insulating plate by rubbing the acrylic plate with the flannel or silk pad. The acrylic plate will be positively charged.
2. Touch the neutral electrophorus disk to the charged insulating plate. When the disk is in contact with the charged plate, the disk will attain an induced charge (see Figure 6).
   {13530_Procedure_Figure_6}
3. Touch the metal disk to ground the electrophorus and then remove the electrophorus from the insulating plate (see Figure 7). The electrophorus will have gained a negative charge.
   {13530_Procedure_Figure_7}
4. Show that the electrophorus has a charge by bringing it near one of the flask form electroscopes. Students should record their observations on the Electrophorus Worksheet.
5. Charge one of the flask form electroscopes by conduction using the rubber friction rod (refer to Demonstration 1). Make sure students observe the process of charging the electroscope with the friction rod.
6. Charge the other flask form electroscope by conduction using the electrophorus. Is it easier to charge an electroscope with the electrophorus or a friction rod? Does the electrophorus appear to have more or less charge than the original rubber friction rod? Students should record their observations in the Electrophorus Worksheet.

Condensing Electroscope

1. Discharge the flask form electroscopes, electrophorus, insulating plate and friction pads (if necessary).
2. Place the PVC tube and electrophorus disk (with the handle removed) on one of the flask form electroscopes as shown in Figure 8. Make sure the ball of the flask form electroscope is about 1 cm below the top rim of the PVC tube. It may be necessary to push down or pull up the rod of the flask form electroscope to move the metal ball to the proper level. Note: Do not pull on the metal ball. The electrophorus disk should rest flat on the PVC tube and the threaded post attached to the disk should be in contact with the metal ball (the hole of the threaded post should accommodate the curvature of the ball and maintain a flat, stable surface). This type of electroscope is known as a condensing electroscope.
   {13530_Procedure_Figure_8}
3. Charge an electrophorus with handle as discussed in Charging the Electrophorus Procedure steps 1 and 2.
4. Hold the charged electrophorus about 30 cm above the condensing flask form electroscope, with the disk parallel to the tabletop. Slowly lower the charged electrophorus. Approximately how far away is the electrophorus from the metal ball of the electroscope when the foil leaves begin to diverge?
5. Remove the disk and PVC tube from the electroscope and repeat steps 4 and 5 for the “normal” flask form electroscope. Does the condensing electroscope respond to the charge from the electrophorus at a greater distance compared to the “normal” flask form electroscope? Students should record all their observations in the Electrophorus Worksheet.

Demonstration 3: Charge Distribution and Discharge

1. Assemble two condensing electroscopes (refer to Demonstrations 1 and 2).
2. Charge the rubber friction rod using the flannel friction pad and charge one condensing electroscope by conduction (or permanently by induction) with the friction rod (refer to Demonstration 1). The foil leaves should diverge.
3. Holding the uncharged condensing electroscope by the glass flask, move the electroscope until the two disks touch. What happens to the foil leaves of each electroscope? Students should record their observations on the Charge Distribution and Discharge Worksheet.
4. Discharge both condensing electroscopes.
5. Obtain the pin and plastic tubing.
6. Carefully push the pin through the center of the plastic tubing as shown in Figure 9.
   {13530_Procedure_Figure_9}
7. Recharge one condensing electroscope with the charged rubber friction rod (step 2). Make sure the electroscope has a maximum charge.
8. Holding on to the plastic tubing, position the head of the pin just above the disk of the condensing electroscope.
9. Touch the pin head to the disk and observe the foil leaves. Students should record their observations on the Charge Distribution and Discharge Worksheet.
10. Discharge the condensing electroscopes completely.
11. Recharge the rubber friction rod using the flannel friction pad.
12. Holding on to the plastic tubing, position the point of the pin just above the disk of the uncharged condensing electroscope, without touching the disk.
13. Touch the charged friction rod to the head of the pin, while holding the pin just above the disk of the condensing electroscope. Observe the leaves of the electroscope. Students should record their observations on the Charge Distribution and Discharge Worksheet.
14. Move the friction rod and the pin away from the electroscope. Does the electroscope have a charge? Students should record their observations on the Charge Distribution and Discharge Worksheet.

Demonstration 4: Faraday Cage and “Ice Pail”

1. Assemble one condensing electroscope (refer to Demonstration 2).
2. Center and place the 2" diameter metal tube on the plate of the condensing electroscope (see Figure 10).
   {13530_Procedure_Figure_10}
3. Assemble a proof plane by using the nut to attach the 1" diameter washer to the threaded end of the electrophorus handle (see Figure 11).
   {13530_Procedure_Figure_11}
4. Place a flask form electroscope about 30 cm away from the condensing electroscope with the “pail.”
5. Charge the rubber friction rod using the flannel pad (refer to Demonstration 1).
6. Bring the charged rod near the ball of the flask form electroscope, but do not touch it. Students should record their observations on the Faraday’s Cage and Pail Worksheet.
7. Recharge the rubber friction rod, if necessary.
8. Wearing a rubber glove, or other insulating device, hold the metal strainer, bowl-side up, just above the metal ball of the flask form electroscope, without touching it. Make sure the bowl of the metal strainer is centered above the ball and the rim of the bowl is level (parallel) with the tabletop (see Figure 12).
   {13530_Procedure_Figure_12}
9. Bring the charged rod near the ball of the flask form electroscope that has the metal strainer over it. Students should record their observations on the Faraday’s Cage and Pail Worksheet.
10. Recharge the friction rod, if necessary, and charge the condensing electroscope with the “pail” by conduction (refer to Demonstrations 1 and 2). The foil leaves should diverge.
11. Holding the insulated handle of the proof plane, touch the washer to the flask form electroscope to see if there is any charge on the proof plane. If there is, discharge the flask form electroscope and the proof plane and try again. Repeat this until both the electroscope and the proof plane are discharged.
12. Holding onto the insulated handle of the discharged proof plane, carefully scrape the washer of the proof plane about five or six times along the outside surface of the metal “pail” to collect any electrostatic charge.
13. Transfer the charge collected, if any, on the proof plane to the other (uncharged) flask form electroscope. Students should record their observations on the Faraday’s Cage and Pail Worksheet.
14. Discharge the flask form electroscope and proof plane. Recharge the condensing electroscope with the “pail,” if necessary, using the rubber friction rod.
15. Holding onto the insulated handle of the discharged proof plane, carefully scrape the washer of the proof plane about five or six times along the inside surface of the metal “pail” to collect any electrostatic charge.
16. Transfer the charge collected, if any, on the proof plane to the other (uncharged) flask form electroscope. Students should record their observations on the Faraday’s Cage and Pail Worksheet.

Demonstration 5: Curving Water into a Beaker

1. Place a large beaker on the demonstration table.
2. Fill up the syringe with 10 mL of water.
3. Rub the comb with the flannel friction pad, or run the comb through your hair a few times.
4. Hold the syringe, pointing down (plunger pointing up) about 10 cm above the beaker, and about 3 cm away from the outside edge of the beaker (see Figure 13).
   {13530_Procedure_Figure_13}
5. Hold the charged comb over the beaker about 2–3 cm below the syringe tip.
6. Slowly, and continuously press on the syringe plunger to create a water stream while holding the items in the same relative positions. Watch as the water bends towards the comb and the water stream lands in the beaker. Note: The positions of the syringe, beaker and comb as well as the firmness with which to press on the syringe will require practice and a bit of trial and error.
7. Refill the syringe and repeat as necessary to illustrate the concept.
8. (Optional) Repeat the demonstration using mineral oil or other petroleum-based nonpolar solvent. The nonpolar solvent will not be as readily attracted to the electric charge.

Student Worksheet PDF

13530_Student1.pdf

Teacher Tips

• Before class, prepare copies of the student worksheets for each student. The Background information can also be copied and supplied to the students at the instructor’s discretion. Students should refer to their textbooks for further information regarding static electricity, if necessary.
• Students should review the questions in the worksheet before the demonstration, and then record their observations and answer the necessary questions as they follow along with each demonstration.
• Be sure to rub the friction rods with the fabric pieces rapidly for at least 30 seconds in order to obtain a good charge on the rod.
• After continuous use, the fabric pieces and friction rods may become permanently charged. It may be necessary to ground the fabric and the friction rods 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. Rubbing them with a wet towel and allowing them to air dry is another proven method.
• Static electricity experiments always work best 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 thus more conducive for electrostatic demonstrations.
• The Build a Flask Form Electroscope (Catalog No. AP6328) is a great hands-on laboratory kit for students to explore electrostatics further, and is available from Flinn Scientific.

  Demonstration 1: Charging a Flask Form Electroscope

• To charge by conduction, better results are achieved by sliding the friction rod along the surface of the electroscope ball a few times, instead of just touching it.
• Typically, it is difficult to positively charge the electroscope by conduction. The electrons do not readily leave the metal unless there is a large “reservoir” for the electrons to go to such as the “ground.” The electrons easily flow into a “grounded” hand that touches the electroscope. However, they do not readily flow into a positively charged friction rod. To positively charge the electroscope, it is best to charge it by induction using a negatively charged rod. Permanently charging by induction with a negatively charged rod will leave the electroscope with a positive charge. This positive charge can then be used as a test charge.
• An alternative method for permanently charging by induction is as follows: charge the electroscope by induction first so the foil leaves diverge. Then, while the electroscope is charged by induction, touch the electroscope with a free hand to ground it. The foil leaves will collapse when the electroscope is grounded. They will diverge again once the hand is removed and the friction rod is moved away from the electroscope.
• Determine the polarity of an unknown charge: Charge up the electroscope with a known charge (positive or negative). Students can record the polarity of the charge in a table on a separate sheet of paper. Students should then record the name of the unknown friction rod and the friction pad to be used in the table. Charge up the unknown friction rod and bring it near the charged electroscope. How the electroscope responds will indicate the unknown charge. Students should record all observations in their table.
• Recommended negatively charged test objects: plastic Beral-type pipets, plastic straws, rubber balloons and PVC pipes make excellent negatively charged rods when rubbed with wool, flannel or fur.
• Recommended positively charged test objects: Lucite^® friction rods, glass friction rods, glass stirring rods and curled-up overhead transparency sheets (acetate) make good positively charged rods when rubbed with wool or silk. (Thicker and longer materials will retain the positive charge better.)

  Demonstration 2: Electrophorus and Condensing Electroscope

• Rub the insulating plate rapidly for at least 30 seconds in order to obtain a good charge on the plate. After continuous use, items may become permanently charged. It may be necessary to ground them occasionally to return them to their neutral state. The acrylic plate and the glass flasks have a propensity for accumulating charge and are difficult to discharge through simple grounding techniques. If these items become difficult to discharge, rub a wet towel over the material and then allow the items to air dry.
• Use the electrophorus to charge the friction rods to show that the charge can be transferred through multiple objects before it charges the electroscope. A proof plane is another good charge transferring device. It takes a small amount of charge from the electrophorus and transfers it to other objects, while the electrophorus maintains its large charge.

  Demonstration 3: Charge Distribution and Discharge

• Use silk and the glass friction rod to experiment with positive charges.

  Demonstration 4: Faraday’s Cage and “Ice Pail”

• Repeat the Faraday’s Cage and “Ice Pail” Procedure using a positively charged friction rod, if desired.
• Another demonstration with the “ice pail” is to charge a proof plane and then insert it into the “ice pail” of the discharged condensing electroscope—the foil leaves will diverge. Touch the bottom of the “pail” with the proof plane—the foil leaves should still be diverged. Remove the proof plane from the “pail” and touch the proof plane to an uncharged flask form electroscope—no leaf deflection will occur. All the charge on the proof plane (inside the “ice pail”) was transferred to the “ice pail.”
• The simple “ice pail” used in this demonstration is not “perfect.” A completely enclosed spherical conducting shell is ideal. The ice pail with an open top may lead to some charge being collected from the inside of the “pail.” However, any charge collected from the inside should be less than that collected from the outside.

  Demonstration 5: Curving Water into a Beaker

• An alternative approach is to set up a thin stream of water pouring from a faucet, and bring the charged comb near the water stream and observe the water’s response.
• This demonstration can also be performed using a charged, inflated balloon.

Further Extensions

Demonstration 1: Charging a Flask Form Electroscope

(Optional)—Balloon Electroscope Assembly

1. Cut two pieces of string to approximately 30 cm.
2. Inflate one balloon to approximately ¾-size (do not over-inflate) and tie the end of the balloon closed.
3. Repeat step 2 for the second balloon, inflating it to the same size as the first balloon.
4. Tie one piece of string to the knot of one of the balloons.
5. Repeat step 4 for the second balloon.
6. Tie the loose end of string from one of the balloons onto the clothes hanger. Tie it approximately one-quarter from one end of the hanger (see Figure 14).
   {13530_Extensions_Figure_14}
7. Repeat step 6 for the other string. Tie this string on the opposite side of the hanger so the hanger stays balanced and the balloons are just touching each other (see Figure 14).
8. Hang the clothes hanger and balloons over the edge of a tabletop using a support stand and support stand clamp. If the area surrounding the balloons is highly charged, the demonstrations may not work well. Wipe down a highly charged area with a damp cloth and dry the area with a towel.

Demonstration 2: Electrophorus and Condensing Electroscope 

Parallel Plate Capacitor

1. Assemble the condensing electroscope. Make sure all the pieces are discharged.
2. Cut a piece of paper slightly larger than the electrophorus disk.
3. Tape the paper cutout to the bottom of the electrophorus disk with handle.
4. Clip one connector cord from the high voltage DC power supply onto the edge of the condensing electroscope plate. Make sure the disk remains balanced on the PVC tube. The other connector cord should remain disconnected—do not touch.
5. Center and place the electrophorus on top of the condensing electroscope disk, as shown in Figure 15. Make sure the two disks do not touch and that the connector cord clip does not touch the top plate.
   {13530_Extensions_Figure_15}
6. Turn on the power supply and adjust the voltage to approximately 1000 V DC.
7. Allow the capacitor to charge for about one minute.
8. After one minute, touch the top plate with a finger to ground it.
9. Disconnect the connector clip from the bottom disk making sure everything remains balanced. Turn off the power supply.
10. Slowly raise the electrophorus (touching only the insulated handle) and observe the foil leaves of the electroscope. Students should record their observations on a separate sheet of paper.

Sample Data

Demonstration 1: Charging a Flask Form Electroscope

Charge by Induction
Electroscope response to the negatively charged friction rod

Foil leaves repel each other and diverge when friction rod is brought near the electroscope. When the friction rod is removed, the foil leaves collapse and close back together.

Electroscope response to the positively charged friction rod

The foil leaves respond the same way as when charged with the negative friction rod.

Charge by Conduction
What happens when the negatively charged friction rod touches the metal ball of the electroscope and is then removed?

The foil leaves diverge and remain diverged after the friction rod is removed. After grounding the electroscope, the leaves collapse and close back together.

How does the electroscope respond to the positively charged friction rod?

The foil leaves respond the same way to the positive charge as when charged with the negative friction rod. It took more effort to charge the electroscope using the positively charged friction rod.

What happens when the charged electroscope is touched by a finger?

The electroscope discharges and the foil leaves fall.

Permanently Charge by Induction
Describe the process of permanently charging by induction. Use a sketch, if desired, to show both the charge on the friction rod and the resulting charge on the foil leaves.

While touching the metal ball of the electroscope and bringing the negatively charged friction rod near, the foil leaves hang down and do not diverge. When the hand is removed, the foil leaves continue to hang straight down. After removing the negatively charged friction rod from the area, the foil leaves diverge. The leaves collapse when the electroscope is grounded.

The foil leaves respond the same way to the positive charge as when charged with the negative friction rod. It appears easier to permanently charge by induction using the positively charged friction as compared to charging by conduction using the positively charged friction rod.

How does the charged electroscope respond to the charged friction rod after being permanently charged by induction?

The foil leaves fall when the friction rod is brought back near the electroscope, meaning the two are oppositely charged.

Demonstration 2: Electrophorus and Condensing Electroscope

Electrophorus
Describe the response of the electroscope. Does the electrophorus appear to have more or less charge than the original friction rod?

The leaves on the electroscope spread out very far, even when the electrophorus is far away. It appears that the electrophorus has considerably more charge on it than the friction rod.

Describe the processes of charging the electroscope with the electrophorus and with the friction rod. Which apparatus appears to charge the electroscope better—the electrophorus or the friction rod?

The process was similar to charging the electroscope with a friction rod. The leaves on the electroscope separated a great distance and the snap of a discharge was heard as the metal disk touched the ball of the electroscope. When the disk was removed the electroscope was permanently charged. The disk did not need to be rubbed along the metal ball, as required for the friction rod. Simply touching the ball with the disk was all that was needed to charge the electroscope.

Condensing Electroscope
Compare the sensitivity of the condensing electroscope to the “normal” flask form electroscope.

The condensing electroscope is much more sensitive than the “normal” flask form electroscope. The condensing electroscope was affected by the charged electrophorus at a distance of about 15 cm. The electrophorus had to be about 5 cm away from the “normal” flask form electroscope before the leaves separated.

Demonstration 3: Charge Distribution and Discharge

What happens to the foil leaves when one charged electroscope touches an uncharged electroscope?

The originally charged electroscope loses charge and the foil leaves fall slightly. The originally uncharged electroscope gains charge and the foil leaves diverge. The foil leaves on each electroscope appear to diverge by about the same amount, indicating that each electroscope carries about the same amount of charge.

When the pin makes contact with the charged electroscope, what happens to the foil leaves?

The foil leaves fall slowly. It takes about 10 seconds for the electroscope to completely discharge.

What happens to the electroscope when the pin is held near the electroscope and is then touched by the charged friction rod?

The electroscope becomes charged and the foil leaves diverge. The electroscope is permanently charged because once the pin and friction rod are removed, the foil leaves remain diverged.

Demonstration 4: Faraday Cage and “Ice Pail”

Faraday’s Cage
How does the electroscope respond when the charged friction rod is brought near it?

The foil leaves diverge.

Describe what happens when the friction rod is brought near the charged electroscope surrounded by the Faraday cage.

The foil leaves do not move when the negatively charged friction rod is brought near the electroscope inside the Faraday cage.

Faraday “Ice Pail”
Does the proof plane collect any static electric charge from the outside surface of the metal tube (pail)? How can you tell?

Yes, the proof plane collects a static electric charge because when the proof plane touches the flask form electroscope, the foil leaves diverge slightly.

Does the proof plane collect any static electric charge from the interior surface of the metal tube? How can you tell?

No, the proof plane does not collect a static electric charge from the interior surface. The proof plane does not transfer any charge to the flask form electroscope—there is no foil deflection.

Demonstration 5: Curving Water into a Beaker

What happens to the water as it flows past the charged comb?

The water bends and pours into the beaker.

Answers to Questions

Demonstration 1: Charging a Flask Form Electroscope

1. Does an electroscope indicate the type of charge (positive or negative) that it carries?

   No, the electroscope does not indicate the type of charge, only that a charge exists.

2. Does charging by induction leave an electroscope permanently charged?

   No, charging by induction does not leave the electroscope permanently charged. Once the charged friction rod is removed, the foil leaves fall.

3. What is the charge on an electroscope when it is charged by conduction using a negatively charged friction rod?

   The electroscope has a negative charge.

4. An electroscope is permanently charged by induction using a glass rod that was rubbed with silk. A rubber rod rubbed with flannel is then brought near the electroscope. How will the foil leaves respond? Explain.

   The foil leaves will spread out further when the rubber rod is brought close to the electroscope. This occurs because the electroscope is originally negatively charged—charging by induction with a positive friction rod causes the electroscope to be negatively charged. A rubber rod rubbed with flannel produces a negative charge on the rubber rod, so the rubber rod and the electroscope will have the same charge polarity.

5. Describe how an electroscope may be used to determine the polarity of an unknown charge on an object.

   Charge the electroscope with a known charge (positive or negative) using one of the standard materials. Bring the object with unknown charge near the electroscope. If the charge on the object causes the foil leaves to diverge further, then the unknown charge has the same polarity as the electroscope. If the unknown charge causes the foil leaves to collapse, then the unknown charge has the opposite polarity. 

Demonstration 2: Electrophorus and Condensing Electroscope

1. Describe how to charge the electrophorus.

   See Procedure steps. Student responses should follow closely to the Procedure.

2. Why does the condensing electroscope respond to the same charged friction rod with foil leaves that diverge further compared to the “normal” electroscope?

   The larger surface area plate “collects” more charge and therefore the foil leaves experience a larger repulsive force between the like charges.

Demonstration 3: Charge Distribution and Discharge

1. Why does the pin discharge the electroscope?

   The pin discharged the electroscope because of the high amount of curvature at the pin point. This causes the static charge to “bleed” into the atmosphere easier than from the lower curvature metal ball and circular disk.

2. Explain how the pin charged the electroscope, even when no contact was made.

   The pin quickly discharged the friction rod and the static charge “bled” into the atmosphere at the pin point. Some of the dissipating charge was collected on the plate of the condensing electroscope and it gained a charge.

3. Do you believe the purpose of a lightning rod is to attract lightning, or to prevent lightning strikes? Explain.

   The purpose of a lightning rod is to “bleed” away static charge into the atmosphere at the point, and into the ground at the base. Its main use is to prevent lightning strikes instead of actually causing lightning strikes to occur on the lightning rod. However, if not enough charge is dissipated from the object, the lightning rod does make a good ground that can divert the electricity in case of a lightning strike.

Demonstration 4: Faraday Cage and “Ice Pail”

1. Do static electric charges travel through the Faraday cage? Explain.

   No, the static charges do not travel inside the Faraday cage. When the charged friction rods are brought near the electroscopes inside the cage, the foil leaves do not move.
   

2. What is the purpose of the proof plane?

   The proof plane is a device that can collect a small amount of static electric charge from one object and then transfer the charge to other objects.

3. Why do you believe charges do not accumulate on the inside of a metal container?

   The static charges repel each other and want to move as far away from each other as possible. This only occurs on the outside of the container. The charges would be too close together on the interior surfaces.

4. Do you believe it is safer to sit inside a car or lie underneath a car during a lightning storm? Explain.

   It is safer to be inside a car during a lightning storm because if the car is struck by lightning the static charge will remain on the outside surface of the car. Lying underneath the car may keep a person low to the ground and protected from the elements, but if the car is struck by lightning, the electric charge will be on the outside surface of the car, and the individual is near the outside surface and could easily be electrocuted.

Demonstration 5: Curving Water into a Beaker

1. Draw of picture of the shape of a water molecule. Which atoms in the molecule tend to be more negative and which tend to be more positive?
   {13530_Answers_Figure_17}
   Hydrogen atoms tend to be more positive, while the oxygen atom tends to be more negative.
2. Why is water easily polarized by an external electric charge?

   The triangular shape and highly polarized water molecules (due to hydrogen bonding) make the water molecule very easily influenced by an external electric charge.

Discussion

Demonstration 2: Electrophorus and Condensing Electroscope

Capacitance
Capacitance is a term used to describe the electrical storage capacity of a device. Electrical charges tend to store on metal plates that are separated by air or other non-conducting material.

Capacitance of a parallel plate capacitor can be calculated using Equation 1.

C = capacitance
A = surface area of plate
ε = permittivity of the material (Greek, lower-case epsilon)
d = separation distance between plates

The potential difference between the plates (the voltage) on the capacitor is related to the capacitance by Equation 2.

V = voltage
Q = charge

Equation 2 shows that as the capacitance decreases, the voltage or potential difference, between the plates increases. In this demonstration, as the distance between the plates (d) increases, the capacitance decreases (Equation 1). Therefore, the voltage will increase. The electroscope shows this change in voltage as the plates are separated by displaying greater foil leaf separation. In this case, the electroscope is used to indicate voltage instead of charge (the charge remains constant).

This demonstration simulates how a computer keyboard works. The keys of a computer keyboard press on spongy material call a dielectric. The thicker the dielectric, the higher the capacitance. When a key is pressed, the dielectric is squeezed and its capacitance decreases. The amount voltage increase indicates the key on the keyboard that was pressed and this information is used by the computer to display the proper character.