Obtain Some String, a Pair of Balloons and Attempt All of the Described Experiments

/ Lesson 9 Electrostatics
California Standard Addressed
5. Electric and magnetic phenomena are related and have many practical applications. As a basis for understanding this concept:
5. e. Students know charged particles are sources of electric fields and are subject to the forces of the electric fields from other charges.
Student Performance Outcomes
Pair electric charges with the appropriate subatomic particles
Summarize charge interaction rules
Describe charging through conduction
Describe charging through induction
Accurately predict charge distribution within a system such as a charged electroscope
Diagram lines of force relative to a point charge of positive or negative sign
Define electric field
Accurately and qualitatively predict the motion of interacting charged particles / DO NOT WRITE ON THIS HANDOUT

Engage

/ 1. On a dry day, if you rub your feet across a carpeted floor and touch a door knob, you can get shocked. Why do you think this happens? / / 2. Why is this person’s hair attracted to the balloon?
/ 3. Use the illustration of the balloon and person’s hair to explain what you think is happening with the balloon.
/ 4. The diagram on the left is an illustration of a carbon atom. What is the charge associated with each of the particles that make up the atom?

Share your responses with your lab group members. Develop a consensus around the best answers for these questions. Share your ideas with the class.
Explore

Obtain some string, a pair of balloons and attempt all of the described experiments.

Test
Inflate the balloons and tie a string to each of them. / Prediction
How will the balloons behave when they are brought together? / Observations
How did the balloons behave when they are brought together?
No treatment - bring them near to one another. /
Rub your hair or wax paper against balloon A.
Bring them near to one another. /
Rub your hair or wax paper against both balloon A and balloon B.
Bring them near to one another. /

5. Compare your predictions to the evidence. Did the evidence support your ideas, or has your thinking changed?

6. Why do you think very little happened with the “No Treatment” balloons?

7. Why do you think that when one balloon was rubbed that the two balloons were attracted to one another?

8. Why do you think that when both balloons were rubbed that they repelled one another?

You will now perform a series of experiments to refine your understanding about how charged objects interact.

Consider the following set ups, and predict the direction of the forces exerted on the sticky tape.

Set Up / Predictions / Evidence

• Press a piece of sticky tape, about 20-25 cm in length, firmly onto a smooth unpainted surface, for example, a notebook or an unpainted tabletop.
• Make "handles" by folding each end of the tape to form portions that are not sticky.
• Peel the tape off the table / / • Will/did the tape acquire an electric charge?
• Do you think that the tape and the objects have the same charge or different charges?
• Predict/Describe the behavior of the tape as you bring objects toward it (e.g., a hand, a pen).
Set Up / Prediction / Evidence

• Prepare two 20-25 cm tapes as you did before.
• Peel the tapes off of the table. / / • Will/did the tapes acquire an electric charge?
• Do you think that the tapes have the same charge or different charges?
• Predict/Describe the behavior of the tapes as you bring them together.

• Prepare two 20-25 cm tapes.
• Place one tape directly on top of the other.
• Peel the “B” tape off of the table.
• Peel the “A” tape off of the table. / / • Will/did the tapes acquire an electric charge?
• Do you think that the tapes have the same charge or different charges?
• Describe the behavior of the tapes as you bring them together.

• Prepare four 20-25 cm tapes.
• Place one tape directly on top of the other.
• Repeat the process.
• Peel the “B” tapes off of the table. / / • Will/did the tapes acquire an electric charge?
• Do you think that the tapes have the same charge or different charges?
• Describe the behavior of the tapes as you bring them together.

Explain

9. Compare your predictions to the evidence. Were your ideas supported by the evidence or has your thinking changed?

10. Do “A” tapes and “B” tapes have the same electric charge? How do you know?

11. Do “A” tapes and “A” tapes have the same electric charge? How do you know?

12. Do “B” tapes and “B” tapes have the same electric charge? How do you know?

Read the following student statement:

“Objects that have the same charge push away from each other. Objects that have different kinds of charge are attracted to one another.”

13. Is this statement supported by the evidence?

Notes:
Static charge is due to the presence of excess charge of one kind or another.
Charge is carried by particles found in an atom.
Electrons carry negative charge. Protons carry positive charge.
An object with a negative charge has more electrons than protons.
An object with a positive charge has more protons than electrons.
Electrostatic forces always move charged objects to reduce potential energy. /

16. Write rules using the words positive charge and negative charge to describe how charged objects behave when brought together.

17. An inflated balloon which has been rubbed against a person's hair is touched to a neutral wall and remains attracted to it.

Which diagram best represents the charge distribution on the balloon and wall?

(a)(b)(c)(d)

18. Positively charged particles in the nucleus of an atom are called

A) neutrons B) protons C) photons D) electrons

19. A neutral atom must contain equal numbers of

(a) protons and neutrons, only

(b) electrons and neutrons, only

(d) protons and electrons, only

The diagram below shows the arrangement of three charged hollow metal spheres, A, B, and C. The arrows indicate the direction of the electric forces acting between the spheres. At least two of the spheres are positively charged.

20. Which sphere, if any, could be negatively charged?

(a) sphere A

(b) sphere B

(d) sphere C

An electron enters the space between two oppositely charged parallel plates as shown in the diagram.

21. What is the direction of the electric force on the electron as it passes between the plates?

A) toward the negative plate

B) out of the page

C) toward the positive plate

D) into the page

Elaborate

22. Conductive Sphere A has a charge +3q (q is a measurement of charge), conductive sphere B has a charge of +5q. Sphere A is brought into contact with sphere B and then removed. What charge will remain on each sphere?

Notes

Charging objects and electric fields

I. Charge

A. A convention used to predict the forces exerted between protons and electrons.

B. Charge is conserved

C. Charges on Particles

1. positive charge on protons.

2. negative charge on electrons.

D. Charge interaction rules

1. Like charges repel

2. Unlike charges attract

E. Methods of charging

1. Conduction

a. A charged object is brought into direct contact with a second object. The charge is transferred.

2. Induction

a. A charged object is brought near a second object. Charge is separated on the second object so that parts of the second object are charged differently.

The device below is called an electroscope. It is used to determine whether or not an object has an electric charge.
/ Neutral electroscope is touched with a negatively charged rod. Charges are transferred to the ball, then into the electroscope. The metal leaves inside the electroscope separate. The electroscope is negatively charged through conduction.
/ A negatively charged rod is brought near the ball of a neutral electroscope. Negative charges on the ball are repelled and accumulate on the leaves. The metal leaves inside the electroscope separate. The electroscope is negatively charged through induction.

II. Electric Field

A. The region around a charged particle where electric forces can be observed.

B. Field direction conventions

1. Field lines go out of (+) charges

2. Field lines go into (-) charges

D. Field line diagrams

The teacher will provide each of the lab groups with a computer and the webstart.jnlp

Electric Field Hockey Java Applet.

Test your ideas about the rules for positive and negative charge using Electric Field Hockey.

◊ Diagram how you plan to arrange the positive and negative charges to get the black positive charge into the goal.

You may find that you will have to add or change the position of the charges to get the black positive charge into the goal.

◊ Diagram your final successful arrangement of charges.

◊ Map the final electric fields by drawing lines of force on the final successful arrangement of charges.

Predictions / Observations

23. As you put charges onto the playing area, arrows appear on the puck.

What do you think the arrows on the puck are illustrating?

Evaluate

24. Which diagram best represents the charge distribution on a neutral electroscope when a negatively charged rod is held near

it?

(a) (b) (c) (d)

As shown in the diagram below, a charged rod is held near, but does not touch a neutral electroscope.

25. The charge on the knob becomes

(a) positive and the leaves become positive

(b) negative and the leaves become negative

(d) positive and the leaves become negative

26. Which diagram best illustrates a neutral electroscope being charged by conduction?

(a) (b) (c) (d)

In the diagram below, A is a point near a positively charged sphere.

27. Which vector best represents the direction of the electric field at point A?

(a)(b)(c)(d)

28. Which diagram best represents the electric field of a point charge?

(a) (b) (c) (d)

29. Which diagram best illustrates the electric field around two unlike charges?

(a) (b) (c) (d)

The diagram below shows the electric field in the vicinity of two charged conducting spheres, A and B.

30. What is the static electric charge on each of the conducting spheres?

(a)A is positive and B is negative.

(b)A is negative and B is positive.

(d) Both A and B are positive.

The diagram below shows some of the lines of electrical force around a positive point charge.

31. The strength of the electric field is

A) equal at points A, B, and C

B) greatest at point B

C) greatest at point C

D) greatest at point A

32. Draw two points on the paper separated by 1 centimeter. Make one of the points a positive charge and the other a negative charge. Diagram the electric field (lines of force) relative to each point. Assume gravity is negligible. Indicate the net force on each point.

33. Draw two points on the paper separated by 1 centimeter. Assign a positive charge to each point. Diagram the electric field (lines of force) relative to each point. Assume gravity is negligible. Indicate the net force on each point.

34. Summarize the charge interaction rules.

35. Define electric field.

36. Describe charging through induction.

37. Describe charging through conduction.