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CATCHING A CHARGE
Grades 9 - 12

Overview

This lesson introduces the concepts of electrical induction and conduction and the effects of each on an electroscope. The students should construct and use a simple electroscope and use the device to discover how charges affect an electroscope. The lesson integrates hands-on techniques with video animation. The students should use video segments to observe how charges move within electroscopes. In the post-viewing activity the students should be able to determine the charge of an unknown material based on the response of the electroscope.
ITV Series
"ELECTRICITY #2: Charging and Discharging"
"ELECTRICITY #3: Charging by Induction"
Learning Objectives
Students will be able to:
Materials
Each pair of students will need:
Pre-Viewing Activities
Immediately upon entering the classroom, blow up a balloon, rub it against a piece of wool, and stick the balloon to a wall. Ask the students, "Why does the balloon stick to the wall?" Once an answer of static electricity is reached, ask the students how they know static electricity is holding the balloon to the wall, can they observe an electrical charge? Explain to the students that we will be learning about static electricity today and how different charges affect each other. Since actual charges are too small for us to see, we will be using video and building a device called an electroscope to help us study the properties of charges.
Focus Viewing
To give the students a specific responsibility while viewing the video, have them list the motion of electrons and protons in a metal conductor and determine their motion.

Viewing Activities
START the video, Electricity #2, Changing and Discharging at the point immediately after the man with the electric charge touches the sink in the beginning of the section.
PAUSE the video when the positive ions are vibrating. Call on a student to describe the motion of the particles. Distribute the viewing worksheet at this point and have the students answer the first two questions. Have the other students in the class copy the answer into their worksheets. Pausing the video at this point, then allows the students to ask and discuss questions, and gives an opportunity to refocus the students' attention on the next segment.
Tell the students to observe in the next video segment and list in their worksheets which particle transfers charge.

RESUME the video segment.
PAUSE the video segment after the narrator states electrons transfer charge. The students should make note of the motion of the charges between the two materials. PAUSE the video when the cotton has a -1 charge. Pausing at this point allows students to hypothesize about the charge on the other object. Ask the students, from what they already know about static electricity, what would they believe the charge on the wool to be, if any. The students should be able to hypothesize that since the wool lost electrons, it now has more protons than electrons and now would be positively charged. Ask the students what would happen if the cotton and the wool continue to rub against each other. Most of the students will answer that the charge on each increases. Instruct them to compare what happens on the next video segment to their answer.

RESUME the video.
PAUSE the segment when they reach a charge of 5, -5. Ask the students if they think there is a limit to how large a charge can be developed. Have them explain their reasoning. If a student suggests the number of electrons moved is limited by the total number of electrons in the material, ask them if there might be another phenomenon acting which may prevent the charge difference between the two materials from becoming too great.
Ask the students to compare their answers with the reason given in the next segment.

RESUME the video.
PAUSE the video at the point where the narrator asks "if you are wearing a negatively charged shirt, how can you neutralize it?" Ask the students to answer the question. You may need to have the students count the positive and negative charges on the screen.
Ask the students to check their answers against the video segment. They are also to listen for and to write a definition for grounding an object in their worksheets.

RESUME the video.
PAUSE the video when "Grounding" is on the screen with the ground symbol. Check for student comprehension of the definition.

RESUME the video for one second and pause once the man's arm is on the screen. Ask the students to determine the charge on the man's arm and what they expect the charge to become when his finger touches the pipe.

RESUME the video, instructing students to verify and correct their answer based on the material presented.

PAUSE the video when the narrator asks "what happens if you have a positive charge?" Have a student answer the question. Have the students in the class write the answer. During the next segment have students verify if the answer they gave was correct.

RESUME the video and allow the video to continue through the explanation of an electroscope.

STOP the video after the description of an electroscope where the narrator says "insulated by the container that encloses it." Explain to the students that they are going to build their own electroscopes and use them in connection with the video. (Use and distribute "Directions for building an Electroscope.")

Once the students have completed the electroscopes, distribute to each pair of students a glass rod, rubber rod, wool, and silk. Instruct the students to rub the hard rubber rod with the wool. Explain that since the rubber rod has a stronger affinity with electrons than does the wool, the rod develops a negative charge. Once the rod has a charge instruct the students to touch the rod to the knob of the electroscope and explain the reaction of the electroscope on their worksheets.

Ask the students to describe in their worksheets the motion of the electron between the rod and the electroscope as shown in the next segment on their viewing worksheet.

RESUME the video.
PAUSE the video where the narrator says "charging by contact" and have the students write "charging by conduction" in their notes, explaining it is synonymous. On the viewing activity worksheet have the students mark down how they believe the charge will be distributed. Ask the students to verify their answers against the next segment.

RESUME the video.
PAUSE the video at the point where a glass rod is first seen being brought near the knob of the electroscope. Have the students discharge their electroscopes by touching them. The students should now rub their glass rods with silk and touch them to the knob of the electroscope. Explain to students that the silk has a greater affinity with electrons than does the glass. The students should determine the motion of electrons through the electroscope and draw the charge distribution of the electroscope on their worksheets.

Once the students have completed their diagrams, instruct them to verify the charge distribution shown in their worksheets against the next segment. Instruct the students to make any necessary corrections to their worksheets.

RESUME the video.
PAUSE the video at the point where the video shows the complete charge distribution on the electroscope, allowing the students to make corrections.

Remove Electricity #2: Charging and Discharging from the VCR. Insert Electricity #3: Charging by Induction, cued so it is at the point where the narrator says, "Have you ever noticed that an electroscope, when approached by a charged rod, seems to anticipate what is about to happen?"

Instruct the students to discharge their electroscopes, charge their glass rods with the silk and bring their rods near the electroscopes without touching the knob on the top of the electroscope. Have them to write down what they notice in their worksheets and why they believe it happens. In the next segment tell the students to write in their worksheet the reason given for the reaction of the electroscope.

START the video.
PAUSE the video when the narrator says "as a result the leaves repel themselves and diverge." Have the students copy the charge distribution of the electroscope from the screen, after which they should calculate the charge on the knob and the charge on the leaves of the device as well as the total charge on the electroscope. The students should perform this calculation by adding the number of positive charges and subtracting the number of negative charges. Ask the students to verify their charge distributions against the next video segment.

RESUME the video.
PAUSE the video when the video states "Induced separation of charge." Instruct the students to copy this down and write a definition from what they have just seen and heard. Ask the students what they believe happens to the charge when the rod is removed. Tell the students to verify their answers against the video if they were correct.

RESUME the video.
PAUSE the video once the charges have returned to their original position and the leaves have collapsed. Instruct the students to rub the glass rod. The charged rod is held near it and checked against their previous answer. Start with the silk, recharging the rod, and hold it near the knob of the electroscope without touching it. While holding the rod in this position, tell the students to touch the knob of the electroscope with their fingers and observe what happens to the device. The students should then remove their fingers and then remove the rod. Once again, the students are to write down their observations. On the worksheet, have the students write how a charge travels through the ground and what the charge is in the electroscope.

RESUME the video. Instruct the students to list the three movements of the electrons as the video reviews them. They are to label this section "charging by induction."
Post-Viewing Activities
Tell the students that they are going to perform a similar experiment as to what they have just done, except that they are going to use a hard rubber rod rather than glass. During each step of the procedure they are to describe the motion of the electrons and the charge on the knob and leaves of the electroscope. Have the students charge the hard rubber rod by rubbing it with wool. Once the rod has developed a negative charge the students should hold it near the knob of a completely discharge electroscope. Next, the students should touch the knob of the electroscope, still holding the rod near the knob. The students should remove their fingers from the device. Finally, the students should pull the rod away from the device.

The electrostatic force between charged bodies can be calculated using a relationship known as Coulomb's Law, F= kq1q2 /r2, where F is the force of attraction or repulsion between the charged bodies in newtons, N, k is the electrostatic constant, 9.0 x 109 Nm2/C2, q and q are the magnitude and sign of the charges in coulombs, C, and r is the distance between the centers of the charged bodies in meters, m. Using this relationship, have the students determine what would happen to the force between the two charged bodies if the distance between the bodies is doubled, tripled or divided in half. Have the students graph the relationship between electrostatic force and the distance between the bodies. Have the students determine what would happen to the force between two charged bodies if the magnitude of one or both of the charges is doubled, tripled, or halved.
Action Plan
Place the students into small groups to complete one of the following projects. At the completion of the project the groups would be responsible to present their findings to the class.

The students should take home the electroscopes they built. Knowing that a hard rubber comb run their hair will acquire a negative static electric charge, they are to devise a method using their electroscopes to determine if statically charged objects, such as a person rubbing their feet on the carpet, or the front of a television picture tube, have a net negative or positive charge. They must be able to explain the motion of charge within the electroscope and how they used this to determine the charge of these objects.

Computers are susceptible to damage by electrostatic charges. Research why they are so delicate and how designers have helped alleviate this problem.

We use current electricity in most of our everyday appliances. However, the production of it is by a method different from that of static electricity. Investigate this method.
Extensions
Social Studies:
Ben Franklin is considered a discoverer of electricity. Investigate what were some of the early uses Franklin devised for his discovery. George Westinghouse and Thomas Alva Edison promoted two different types of electrical generation and distribution. Investigate what were the two methods, what method was finally chosen, and why.

Science:
Most of our examples of static electricity involve the rubbing of two solids together, however, lightning is a large static electric discharge yet there are no two solids rubbing together. Investigate how lightning is produced and describe the different types of lightning. The relationship between two statically charged bodies is very similar to the relationship between gravitationally attracted bodies. Investigate similarities and differences between these two relationships.


Master Teachers:Ken Abbott and William Leacock

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