Beacon Lesson Plan Library

Demonstrating and Calculating Electrostatic Forces

Robert Rosen

Description

A presentation demonstrating electrostatic force focuses on how electrostatic forces exist between charged objects.

Objectives

The student drafts and revises writing that: is focused, purposeful, and reflects insight into the writing situation; has an organizational pattern that provides for a logical progression of ideas; has effective use of transitional devices that contribute to a sense of completeness; has support that is substantial, specific, relevant, and concrete; demonstrates a commitment to and involvement with the subject; uses creative writing strategies as appropriate to the purpose of the paper; demonstrates a mature command of language with precision of expression; has varied sentence structure; and has few, if any, convention errors in mechanics, usage, punctuation, and spelling.

The student produces final documents that have been edited for: correct spelling; correct punctuation, including commas, colons, and common use of semicolons; correct capitalization; correct sentence formation; correct instances of possessives, subject/verb agreement, instances of noun/pronoun agreement, and the intentional use of fragments for effect; and correct formatting that appeals to readers, including appropriate use of a variety of graphics, tables, charts, and illustrations in both standard and innovative forms.

The student knows that electrical forces exist between any two charged objects.

Demonstrates ability to gather information from various sources to plan a project.

Creates potential solutions to industry problems using math and/or scientific concepts and communicates solution using industry appropriate language arts and graphic skills.

Demonstrates the ability to cooperatively work in various settings, across diverse populations.

Materials

-Teacher manuals that have descriptions of demonstration
-Books that describe physics demonstrations should be available for students to use.
-Electrostatics kits that contain pith balls; electroscope; rods made of rubber, plastic, glass; cloth and fur pads; stirrups for holding charged objects.
-A Wimhurst or Van de Graaff generator could be used to demonstrate the benchmark.
-Copies of illustrations and rubrics included in the associated file
-Calculators

Preparations

1. Facilitating teachers will:
-understand the concepts of electrostatics, gravity and force
-be able to demonstrate, through calculation, the relationships between force, mass and charge
-know the relationships between gravity, distance and force
-know the relationships between electrostatic attraction, repulsion, charge and distance
-understand the principles behind static electricity
-know the fundemental concepts of electrical charge

2. Gather necessary materials.

Procedures

KNOWLEDGE AND SKILLS:
-Students understand the relationships between mass, gravity and distance
-Students compare and contrast electrostatic force and gravitational force
-Students show, through calculation, the magnitute of gravitational force between two objects
-Students understand the relationships between mass, charge and distance
-Students show, through calculation, the magnitude of electrostatic force between two objects
-Students understand the concepts of static electricity, electrical current and electrostatics
-Students demonstrate, through presentation, an understanding of electrostatic and gravitational forces

LESSON INTRODUCTION:
(Note: Certain symbols cannot be reproduced online, such as superscript and subscript notations. These notations have been spelled out in the formulas below, but are meant to be substituted by normal notation by the teacher when using the lesson. The symbol * is used to indicate multiplication.)

1. Introduce the following basic electrostatics concepts.

a) There are two types of charge: positive and negative.
b) Charge is conserved; it is neither created nor destroyed.
c) Like charges repel and opposite charges attract.

2. These principles and other fundamental ideas are illustrated by the following demonstration. Bring an uncharged rubber rod close to a pith ball electroscope (a pith ball that is suspended from an insulating stand). The pith ball will not move. Charge the rod by rubbing it with fur. When the rod is brought close to the pith ball, the ball is attracted to the rod. The ball jumps off the rod the instant it comes into contact with the rod. The ball pops off the rod like a baseball leaving a bat. Perform the demonstration several times because the action occurs so quickly that students will miss important details unless they observe the demonstration carefully. Be sure to ground the pith ball by touching it after each trial so that the ball is neutral at the start of each trial.

3. Ask students how the rod became charged if charge can not be created. Be sure students understand that all atoms contain both positively charged particles (protons) and negatively charged particles (electrons). Atoms normally have the same number of protons as electrons and so are neutral as a whole. Electrons that are loosely bound to the fur are transferred to the rubber when the rubber rod is rubbed with the fur. This occurs because molecules making up the rod have a greater affinity for electrons than those that make up the fur. As a result of the rubbing, the rod acquires a net negative charge and the fur is left with a net positive charge. The rod-fur system is neutral as a whole because charged particles were merely separated, not created.

4. Point out that the pith ball is neutral before it comes into contact with the rod. Ask students why the neutral ball would be attracted to a charged object. The answer involves induced charges. A charged object causes charge in a nearby neutral object to separate. The pith ball is neutral as a whole, but some electrons are repelled by the negatively charged rubber rod and migrate to the side of the ball opposite the rod. The side of the ball closest to the rubber rod has a shortage of electrons and therefore is positively charged.

See illustration in associated file.

5. The separation of the pith ball’s charges lasts only while the rubber rod is close to the pith ball. If the rod is withdrawn,the electrons on the ball redistribute themselves uniformly so that all parts of the pith ball are neutral.

6. The ball is attracted to the rod because the positive side of the ball (which is attracted to the rod) is closer to the rod than the negative side of the ball (which is repelled from the rod). The force between charged objects depends on both the amount of charge and the distance separating the charges. Coulomb’s Law,
F = k(q subscript 1q subscript 2)÷r squared , gives the relationship between electrostatic force F, the charges q subscript 1 and q subscript 2 that are interacting and the distance r between the charges. The constant of proportionality k has the value k = (8.99 x 10 superscript 9) * [(N*m squared) ÷ C squared].

7. Ask students why the pith ball jumps off the rubber rod as soon as the ball touches the rod. The answer involves charging by conduction. The excess electrons on the rubber rod repel each other. When the ball comes in contact with the rod, electrons are repelled off the rod and are attracted by the induced positive charge on the pith ball. The rod thus loses electrons and the ball gains them. The ball thus acquires a net negative charge. The rod remains negatively charged, but less negatively charged than before. In general, if a charged object touches a neutral object, the neutral object acquires some of the charge from the charged object. Transfer of charge that occurs when an object touches a charged object is called charging by conduction. Charging by conduction gives the ball a charge that lasts even if the rod is moved away. The pith ball will jump off the rod as soon as the ball acquires negative charge from the rod because like charges repel.

See illustration in associated file.

8. Finally, charge a glass rod by rubbing it with silk. The pith ball will be attracted to the glass rod. Ask students to state the significance of this observation. Answer: There must be two types of charge. If there were only one type of charge then the charged ball would always react in the same way to a charged rod. The charged ball would not be attracted to some charged rods and repelled from otters.

9. Summarize the demonstration for students. Remind them that the demonstration illustrates all three basic principles of electrostatics. In addition, students were introduced to induced charges, charging by conduction, Coulomb’s Law and separation of charges by friction. Thus, this one demonstration gives students much of the background they will need to start researching an electrostatics demonstration they can present.

COMPUTATIONS:
Compute the force between charged objects using Coulomb’s Law.

Problem
Find the force between a -1.0 x 10 superscript -9 C charge and a 2.0 x 10 superscript -7 C charge. (The unit of charge is the Coulomb, abbreviated C. The Coulomb is a very large unit of charge. Most charges in electrostatics demonstrations are 10 superscript -6 C or less.) The charges are 35 cm apart.

Solution
Use F = k(q subscript 1q subscript 2)÷r squared.

Here q subscript 1 = -1.0 x 10 superscript -9 C
q subscript 2 = 2.0 x 10 superscript -7 C
r= .35 m

Thus, F = [(8.99 x 10 superscript 9)( -1.0 x 10 superscript -9 C)(2.0 x 10 superscript -7 C)] ÷ (.35) squared= -1.5 x 10 superscript -5 N

Ask students to interpret the minus sign of the result. (Since we have unlike charges, the 1.5 x 10 superscript -5 N force is a force of attraction. Positive values of F represent repulsive forces while negative values of F represent attractive forces.)


PROCEDURES:
1. Divide students into small groups of two or three.

2. Explain that each group of students will be responsible for presenting an electrostatics demonstration. The demonstration should deal with charging by conduction, induced charges, forces between charged objects or some aspect of the three basic principles of electrostatics. The demonstration presented by the group should be entertaining and informative. The group should clearly explain the principle of electrostatics illustrated by the demonstration.

3. Remind students to follow standard lab safety guidelines to protect themselves and their audience.

Safety Concerns:
Students should not be allowed to use generators or other equipment that presents the danger of shock without very close supervision. Common sense needs to prevail. Students need to be instructed on appropriate safety precautions before performing any demonstration. If a demonstration does present a hazard then students should not perform that demonstration. For example, students should not short large capacitors. A student might short a small capacitor. The instructor could then build upon the students’ demonstration by shorting a larger capacitor.

4. Have each group submit a written description of their demonstration. The written description should include a title, the name of the source of the demonstration (if the students’ presentation is based largely on some reference, then the students should give credit to that source), a materials list, a description of the demonstration procedure, and a section that explains the theory behind the demonstration.

5. After the assignment has been explained, give the student groups class time to research the principles and to prepare a presentation. Set aside one or two days for the presentations and their evaluations.

6. Students present their productions and are assessed on their products.

Assessments

Class presentations may be assessed by a rubric. See associated file.


In-class assessment may include the following problem:

Given:
Gravitational constant G = (6.67 x 10 superscript -11) ÷ [(N * m squared) ÷ kg squared]
Coulomb force constant k= (8.99 x 10 superscript 9) * [(N * m squared) ÷ C squared].
Proton mass m subscript p = 1.67 x 10 superscript -27 kg
Electron mass m subscript e = 9.11 x 10 superscript -31 kg
Proton charge= electron charge= e = 1.60 x 10 superscript -19 C
Radius of hydrogen atom= r = 5.29 x 10 superscript -1 m


a. Calculate the gravitational force between the proton and electron

b. Calculate the electrostatic force between the proton and electron

c. Determine which force is stronger- the gravitational attraction between a proton and an electron in a hydrogen atom or the electrical attraction between the proton and the electron? Specifically, what is the ratio of the stronger force to the weaker force?

Problem may be assessed by a rubric: See associated file.

Ability to work well together as a team and participate appropriately can be assessed informally by teacher and student observation.

Selection of appropriate prewriting strategies can be assessed by having the student submit evidence of planning that appropriately guides the writing process.

Test questions may include:

1. Which force is stronger between a proton and an electron?
a. The magnetic force is stronger.
b. The electrostatic force is stronger.
c. The gravitational force is stronger.
d. The frictional force is stronger.

(answer b. The electrostatic force between a proton and electron is much greater than the gravity between them.)

2. Based on class calculations, which factor affects the gravitational force the most?
a. The mass of the object affects the gravitational force the most.
b. The gravitational constant affects the gravitational force the most.
c. The distance between the objects affects the gravitational force the most.
d. The composition of the objects affects the gravitational force the most.

(answer c. The force is related to the square of the distance.)

3. Based on class calculations, which factor affects the electrostatic force the most?
a. the mass of the object affects the electrostatic force the most.
b. the electrostatic constant affects the electrostatic force the most.
c. the distance between the objects affects the electrostatic force the most.
d. the composition of the objects affects the electrostatic force the most.


(answer c. The force is related to the square of the distance)

4. A rubbed balloon or glass rod exibits attractive and repulsive behaviors. What statement best explains this observation?
a. The gravity of the objects is changed though friction.
b. The electrostatics are changed through friction.
c. The exchange of protons occurs durring rubbing.
d. The exchange of electrons occurs durring rubbing.

(answer d. Electrons are exchanged when two objects are moved against each other.)

Electrostatics Demonstration
Ideas for electrostatics demonstrations are listed below as a resource for the instructor.

1. Attract a rolling soda can using a charged rod.
2. Charge a leaf electroscope by conduction and by induction.
3. Deflect a stream of water using a charged rod.
4. Attract a meter stick balanced on a watch glass using a charged rod.
5. Display an electric field using felt particles mixed in oil.
6. Ribbons placed on a Van de Graaff generator stand on end A stack of pie pans placed on a Van de Graaff generator will fly off the generator one at a time after it is turned on.
7. Short a charged one farad capacitor produces a dramatic spark. (Such large capacitors are available from scientific supply catalogs. Use caution when shorting the capacitor!)
The electric field inside a conductor is zero. So a box wrapped in aluminum foil shields a transistor radio from radio waves outside the box.Electric fields are strongest at a sharp tip of a conductor that is located in an electric field. A spark will jump between a sharp point of a conductor and an electrostatic generator or tesla coil (mount the conductor on an insulator to avoid shock).



Multi-step Electrostatics Problem:

Gravitational forces between the proton and electron.

F= (Gm subscript 1 m subscript 2) ÷ r squared

F subscript grav = (Gm subscript p m subscript e) ÷ r squared = [(6.67 x 10 superscript -11)(1.67 x 10 superscript -27 0(9.11 x 10 superscript -31)] ÷ (5.29 x 10 superscript -11) squared = 3.63 x 10 superscript -47 N

Electrical force between the proton and the electron.

F = (kq subscript 1q subscript 2) ÷ r squared

F subscript elec = (ke squared) ÷ r squared = [(8.99 x 10 superscript 9)(1.60 x 10 superscript -19) squared] ÷ (5.29 x 10 superscript -11) squared = 8.22 x 10 superscript -8 N

Ratio of the electrical force to the gravitational force

ratio = F subscript elec ÷F subscript grav = (8.22 x 10 superscript -8 ) ÷ (3.63 x 10 superscript -47) = 2.26 x 10 superscript 39

This calculation shows that the electrical force is much stronger that the gravitational force inside an atom. In fact, the gravitational force is so small compared to the electrical force and nuclear forces that it can be ignored inside an atom.

Extensions

Enhancement:
The instructor may provide reference materials that describe physics demonstrations. In addition to teacher manuals and other books, students can use a search engine to find web sites devoted to physics demonstrations. The American Association of Physics Teachers as well as some universities maintain such web sites.
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