Build a Flask Form Electroscope

Introduction

Build your own simple electroscope and demonstrate the existence of static-electric charges. Charge your electroscope using friction rods and friction pads to produce positively or negatively charged rods. Then use the charged electroscope to test other objects and to determine their charge polarity after being rubbed with different materials.

Concepts

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

Materials

Safety Precautions

The materials in this activity are considered nonhazardous. Please follow normal laboratory safety guidelines.

Prelab Preparation

1. Unfold the 30.5 x 30.5 cm sheet of aluminum foil.
2. Use scissors to cut a 3 x 2.5 cm rectangle from one corner of this sheet. Save this piece to make the foil leaves in step 19.
3. Wrap the remainder of aluminum foil sheet around the Styrofoam ball so that it is completely covered by the foil. Smooth out the aluminum foil along the surface, and where the corners of the foil meet. Note: The foil does not have to be completely smooth.
4. Follow the instructions along with the Preparation Flow Chart (Student PDF) to properly bend the copper wire to form the support for the foil leaves.
5. Obtain the straight 30-cm long copper wire (Flow Chart Figure a).
6. Using needle-nose pliers, bend the end of the wire 90° so that the length of the bent end is as long as the pliers-tip is wide (approximately 0.5 cm) ( Flow Chart Figure b).
7. Bend the wire 90°, 2 cm from the bent elbow tip. The wire should be all in one plane (Flow Chart Figure c).
8. After the “2-cm bend,” use the tip of the needle-nose pliers to bend wire 90° so that it makes contact with the tip of the wire and completes the loop (Flow Chart Figure d).
9. Bend the remaining wire perpendicular to the loop (Flow Chart Figure e).
10. About an inch from the loop, slightly bend the extended wire inward towards the center line of the loop (Flow Chart Figure f).
11. At the center point of the loop, bend the wire back so that it is perpendicular to the loop again (Flow Chart Figure f). Make sure that the wire and loop are square to each other so when the extended wire hangs vertically, the loop is horizontal and directly below the extended wire.
12. Use the needle-nose pliers to straighten out any portions of the wire, if necessary. Make sure the wire is straight and the bends are “crisp.” The wire loop needs to be very straight—the long bars of the loop need to be straight, parallel to each other and horizontal when positioned inside the flask.
13. Insert the extended wire through the base of the 1-holed rubber stopper (Flow Chart Figure g).
14. Place the wire/stopper unit into a 250-mL Erlenmeyer flask (Flow Chart Figure h).
15. Adjust the height of the loop inside the flask to be about 3–4 cm above the bottom of the flask. The loop should be centered inside the flask (Flow Chart Figure i).
16. Once the height is correct, insert the small cork into the hole of the rubber stopper to secure the wire at the appropriate height (Flow Chart Figure j).
17. With wire cutters, cut the excess wire extending from the top of the rubber stopper so that the end is approximately 5–6 cm above the top of the rubber stopper. Save this excess copper wire for step 21.
18. Insert the aluminum foil–covered Styrofoam ball onto the extending wire above the rubber stopper. Make sure the aluminum is in contact with the copper wire, and that the extending wire is vertical. Do not push the ball all the way down. Leave a 2- to 3-cm gap between the ball and the rubber stopper (Flow Chart Figure k).
19. Obtain the 3 x 2.5 cm piece of aluminum foil previously cut in step 2.
20. Use scissors to cut this piece into two 1.5 x 2.5 cm strips. Make sure the cut edges of the foil pieces are straight and square (Flow Chart Figure l).
21. Obtain one aluminum strip and a scrap piece of copper wire to be used as a form.
22. Loosely curl one of the 1.5-cm ends of the aluminum strip around the scrap copper wire piece. The curl should make about a ¾ turn (Flow Chart Figure m). Note: Make sure the foil is not curled tightly around the wire. It needs to be loose enough to swing freely on the wire “support-rod.”
23. Make sure the remaining area of the foil strip is smooth and flat.
24. Repeat steps 22 and 23 for the other 1.5 x 2.5 cm strip.
25. Remove the copper support/stopper assembly from the flask. Carefully place the curled ends of the foil strips onto the long bars of the copper loop so that the strips hang through the middle of the loop and are vertical. The strips should swing freely (Flow Chart Figure n).
26. Complete the electroscope by placing the loop with hanging aluminum strips inside the Erlenmeyer flask. Make sure the loop is horizontal and the foil leaves are free to swing (Flow Chart Figure o).

Procedure

27. Negatively charge the PVC rod by rapidly rubbing it with the piece of wool.
28. Charge by induction: Bring the negatively charged PVC rod near the aluminum foil–covered Styrofoam ball, but do not touch it. Notice that the foil leaves inside the flask repel each other and diverge. Move the charged rod away from the electroscope and the leaves collapse and hang vertically (see Figure 1).
    {13855_Procedure_Figure_1}
29. Recharge the PVC rod by following step 27, if necessary.
30. Charge by conduction: Touch the aluminum-covered ball with the negatively charged PVC rod. Watch as the foil leaves diverge. Remove the friction rod from the ball and notice that the foil leaves stay diverged. The electroscope has become negatively charged. Discharge (ground) the electroscope by touching the ball with your free hand. (Make sure to ground yourself by touching a metal table leg or door before touching the ball, if necessary.) The foil leaves collapse (see Figure 2).
    {13855_Procedure_Figure_2}
31. Recharge the PVC rod, if necessary.
32. Permanently charge by induction: Touch the aluminum-covered ball with a free hand. Bring the negatively charged PVC rod near the ball, but do not touch it. Notice that the foil leaves do not diverge. Remove your hand from the ball and notice that the foil leaves still do not diverge. Then, move the PVC rod away from the electroscope. The foil leaves diverge (see Figure 3).
    {13855_Procedure_Figures_3and4}
33. After the electroscope has been charged by induction, bring the negatively charged PVC rod near the ball of the electroscope, but do not touch it. Notice that the foil leaves collapse. (Charging the electroscope by induction leaves it with a charge polarity opposite to that of the charged rod.)
34. Repeat Procedure steps 27–33 using a positively charged rod. Rub the test tube rapidly with the piece of wool to give it a positive charge. (Notice that the electroscope’s foil leaves behave exactly that same as they did with the negatively charged PVC rod.)
35. Determine the polarity of an unknown charge: Charge up the electroscope with a known charge (positive or negative). Then, bring a charged object near the charged electroscope. If the unknown charge from 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 (see Figure 4). (See Table 1 in the Discussion section, along with the Tips section, for a list on how to create positively or negatively charged friction rods with different materials.)

Student Worksheet PDF

13855_Student1.pdf

Teacher Tips

• 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 demonstrations.
• Be sure to rub the friction rods with the wool piece rapidly for at least 15 seconds in order to obtain a good charge on the rod.
• After continuous use, the wool piece and friction rods may become permanently charged. It may be necessary to ground the wool piece or 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.
• To charge by conduction, better results are achieved by sliding the friction rod along the surface of the foil-covered 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, 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 to charge by induction: Bring the charged rod near the aluminum-covered ball and the foil leaves diverge. Then, touch the ball with a free hand to ground the electroscope. The foil leaves collapse when the electroscope is grounded. The leaves diverge again when the hand is removed from the ball and the charged rod is moved away from the electroscope.
• 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 fair positively charged rods when rubbed with wool or silk. (Thicker and longer materials will sustain a positive charge better.)

Discussion

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 dense inner core of atoms, 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 removed from an object leaving the object positively charged, or added to an object to give the object an excess negative charge. The ease with which the electrons in a material can do this depends on the atomic composition of the material.

An electroscope 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 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 migrate away from the external negative charge and into the foil leaves of the electroscope (the furthest region away in the electroscope). The electrons will accumulate in the foil leaves, giving them a negative charge. Since both foil leaves become negatively charged, they will repel each other and diverge. If the external charge is positive, the negatively charged electrons in the foil leaves will be attracted to the positive charge and migrate into the metal ball of the electroscope. While electrons travel into the metal ball, the protons in the foil leaves 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 causes the electroscope to be 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 or 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 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 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 when it is removed from the electroscope.

When the electroscope is permanently charged by induction, the externally charged rod does not actually touch the 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 towards the positive charges. When the ground is removed, the electroscope will be negatively charged. When the electroscope is charged by induction, the permanent charge on the electroscope will be opposite to the charge on the external source.

A substance may acquire static-electric charge through contact with a different type of substance. When two different substances are rubbed across each other, 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 objects are rubbed together reflects differences in the atomic composition of different materials. 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 more tightly 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 large 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.