Materials Included In Kit

Additional Materials Required

Prelab Preparation

Electroscope Assembly

1. Use scissors to cut two identical ⅜" x ¾" pieces of aluminum foil.
2. Use a pushpin 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 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 pushpin as necessary (see Figure 4).
   {12564_Preparation_Figure_4}
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).

Safety Precautions

Please follow normal laboratory safety guidelines. Use care when handling pushpins.

Disposal

The Flask Form Electroscope should be saved and stored for future use.

Lab Hints

• Enough materials are provided in this kit for one student group. This laboratory activity can reasonably be completed in one 25-minute class period. The activity may also be performed as a teacher demonstration.
• 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 more conducive for electrostatic activities.
• Remind students to rub friction rods and friction pads rapidly for at least 15 seconds in order to obtain a good charge on the rod.
• 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.
• Typically, it is difficult to positively charge the electroscope by conduction. The electrons do not readily leave the foil leaves unless there is a large “reservoir” for the electrons to go to, such as the “ground.” They do not readily flow into a positively charged friction rod.

Teacher Tips

• See Table 1 in the Background section for a list of how to create negatively and positively charged friction rods with other materials.
• Recommended negatively charged test objects: plastic Beral-type pipets, plastic straws, rubber balloons and PVC pipes make for 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 for good positively charged rods when rubbed with wool or silk. (Thicker and longer materials will sustain a positive charge better.)
• Inflated balloons can be used instead of friction rods. However, an inflated balloon will not give up its static electric charge readily. Therefore, it may be difficult to charge by conduction with a charged balloon.
• An alternative method for permanently charging by induction is as follows: charge the Flask Form 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.

Sample Data

Charge by Induction

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.

Positively Charged Friction Rod

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

Charge by Conduction

Negatively Charged Friction Rod

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

Positively Charged Friction Rod

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

Permanently Charge by Induction

Negatively Charged Friction Rod

While touching the metal ball of the electroscope and bringing the negatively charged friction 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.

Positively Charged Friction Rod

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

Unknown Charge Polarity

Student materials and answers will vary.

Answers to Questions

1. Did the Flask Form Electroscope respond differently to the positive and negative charges?

   No, the Flask Form Electroscope responded the same for either charge.

2. What is the charge on an electroscope that was charged by conduction using a negatively charged friction rod?

   The electroscope has a negative charge.

3. What is the charge on an electroscope that was permanently charged by induction using a positively charged friction rod?

   The electroscope has a negative charge.

4. Sketch the process of charging by induction. Include in the drawing the charge on the friction rod, the corresponding charge on the foil leaves and the directions the electrons flow from the ground.
   {12564_Answers_Figure_5}
5. Describe how to determine the polarity of an unknown charge on an object.

   If the unknown 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.

Introduction

In this experiment, the existence of static electric charge will be displayed with an electroscope. Rub different materials together to create positively or negatively charged objects and then determine the polarity of the charge with the electroscope.

Concepts

• Static electricity
• Conduction
• Electron affinity
• Induction
• Charge distribution

Background

Static electricity is a stationary electric charge. Atoms are composed of electrically charged particles: positively charged protons, negatively charged electrons and neutrons which carry no charge. The positive and negative charges of 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). Generally speaking, most objects have an equal number of protons and electrons and are therefore considered electrically neutral. Since protons form the rigid inner core of materials, they are not able to move about freely within an object. Therefore, the positive charge in an object remains reasonably constant. Electrons, on the other hand, are not held in place by rigid bonds. The electrostatic attraction between electrons and protons keep the electrons moving closely around the protons, but the electrons are generally not “locked” into position. Electrons have the ability to migrate throughout a material, and therefore are referred to as being delocalized. Electrons can also be completely removed from an object leaving the object positively charged, as well as added to an object to give the object an excess negative charge. The ease at which the electrons in a material can do this depends upon the atomic makeup of the material.

An electroscope (see Figure 1) works well as a detector and storage unit of static electric charge because the electrons surrounding the metal atoms in the electroscope are highly delocalized and are easily influenced. This makes the electroscope a great conductor of electric charge. Electrons in the electroscope 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 towards one another. Therefore, if an external negative charge is brought toward the metal ball of the electroscope, the negatively charged electrons in the metal ball will be repelled and move away from the external negative charge and travel into the foil leaves of the electroscope (the furthest region in the electroscope that they can migrate away from the negatively charged source). The electrons will accumulate and distribute themselves in the both foil leaves, giving the foil leaves a negative charge. Since both foil leaves are negatively charged, they repel each other and diverge. If the external charge is positive, the negatively charged electrons in the foil leaves will be attracted to this positive external charge and they will migrate into the metal ball of the electroscope. When the electrons travel into the metal ball, the protons in the foil leaves, and thus a positive charge, are left behind. Both foil leaves become positively charged, so once again, the leaves diverge because like charges repel. 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 has caused the electroscope to become temporarily polarized. When the external charge is removed, the charges in the electroscope will once again become evenly distributed. The electric polarization will be lost and the foil leaves will collapse. Any charged object brought close to the electroscope will cause the foil leaves to diverge. The leaves will collapse when the charged object is removed. If an object that is not charged is brought close to the electroscope, the foil leaves will not diverge.

The electroscope can also gain and lose electrons to become permanently charged. This can occur when the electroscope is charged by conduction, or permanently charged by induction. When the electroscope is charged by conduction, a charged rod is brought into direct contact with the uncharged electroscope. The charge then rapidly redistributes throughout the rod and the electroscope as if they were one object. The foil leaves will diverge because they both attain the same charge and repel one another. When the originally charged rod is removed, the originally uncharged electroscope will now carry a charge of the same polarity as the charged rod. The charged rod has donated some of its charge to the uncharged electroscope, and therefore has lost some of its initial charge when it is removed from the electroscope.

When the electroscope is permanently charged by induction, the externally charged rod does not touch the to-be-charged electroscope. Instead, the externally charged rod induces the electroscope to become electrically polarized. Then a grounded object touches the electroscope to remove the charge that migrates away from the external charge. Electrically grounding an object occurs when the charged 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, if a negatively charged rod is brought close to the metal ball of the electroscope, the electrons in the metal ball will 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 it allows the electrons to travel even further away from the external negative charge. When the grounded rod is removed from the metal ball, the ball will have lost electrons, and has therefore become positively charged. If a positively charged rod is brought close to the metal ball, and the ball is then grounded, the electrons within the Earth will travel into the metal ball to balance the positive charges that were left when the electrons migrated towards the positive external charge. When the ground is removed, the electroscope will be negatively charged. When the electroscope is permanently charged by induction, the permanent charge on the electroscope will be opposite to the charge on the external source.

The manner in which a substance becomes static-electrically charged is through contact with a different type of substance. When two different substances are rubbed across each other, the frictional energy may be enough to remove a few electrons from an “electron-releasing” material and transfer them to an “electron-holding” material. When this happens, both substances become static-electrically charged. The material that loses electrons becomes positively charged, and the material that gains electrons becomes negatively charged. The ability of one substance to hold onto electrons better than another when two different substances are rubbed together is a result of those substances’ atomic makeup. Certain atoms give up electrons easily, while other substances hold onto electrons tightly. Typically, in the electrostatic sense, metals tend to hold onto their electrons tighter than nonmetals. A list of the relative electron “holding” and “releasing” abilities of different common materials is shown in Table 1. If any 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 (ebonite—a form of hard rubber) are rubbed along the carpet (wool), the rubber-soled shoes will retain and collect excess electrons from the carpet. As a result, the shoes (and you) become negatively charged and the carpet becomes positively charged. 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 positively grounded doorknob, and thus reestablish a charge balance.

Objects do not always carry away a charge when they move past each other. Static charges continuously transfer between objects. Static charge may or may not accumulate 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 the actual “feeling” of a shock. Also, electrons readily dissipate into the air at sharp points. The more curved an object is, the less likely the static charge will dissipate. This is why the electroscope has a metal ball as the terminal for static charge transfer. The air inside the flask of the electroscope is a closed system and is stagnant, so any charge that dissipates off the foil leaves inside the flask will stay in the flask and maintain the overall charge of the electroscope.

Materials

Safety Precautions

Please follow normal laboratory safety guidelines.

Procedure

1. Negatively charge up a hard rubber friction rod by rapidly rubbing it with a piece of flannel or fur.
2. Charge by induction: Bring the negatively charged friction rod near the metal ball of the electroscope, but do not touch it (see Figure 2). Record your observations in the Flask Form Electroscope Worksheet. Move the charged rod away from the electroscope and record your observations in the worksheet.
   {12564_Procedure_Figure_2}
3. Recharge the friction rod if necessary.
4. Charge by conduction: Touch the metal ball of the electroscope with the negatively charged friction rod. Record your observations in the Flask Form Electroscope Worksheet. Slide the friction rod along the metal ball three or four times and then remove the friction rod. Record your observations in the worksheet. Discharge and ground the electroscope by touching the metal ball with your free hand. (Make sure to ground yourself before touching the metal ball, if necessary.) Record your observations in the worksheet (see Figure 3).
   {12564_Procedure_Figure_3}
5. Recharge the friction rod if necessary.
6. Permanently charge by induction: Touch the metal ball of the electroscope with your free hand. Bring the negatively charged friction rod near the electroscope, but do not touch the metal ball. Record your observations in the Flask Form Electroscope Worksheet. Remove your hand from the metal ball. Record your observations in the worksheet. Then move the friction rod away from the electroscope. Record all your observations in the worksheet. Touch the metal ball to discharge the electroscope.
7. Repeat Procedure steps 1–6 using a positively charged friction rod. A positively charged friction rod can be made by rubbing a glass friction rod with fur or silk. Compare the results to those obtained using the negatively charge friction rod, and record all observations in the Flask Form Electroscope Worksheet.
8. Determine the polarity of an unknown charge: Charge up the electroscope with a known charge (positive or negative). Record the name of the unknown friction rod and the friction pad used to charge it. Then, bring a charged object near the charged electroscope. How the electroscope responds will indicate the unknown charge. Record all observations in the Flask Form Electroscope Worksheet.

Student Worksheet PDF

12564_Student1.pdf