Activity 23: Homework

Name:______Date:______Group:______

Purpose

In Activity 2 you explored what happens when two charged objects are brought near each other. In this homework assignment you will explore what happens when a charged object and an uncharged object are brought near each other.

use a model of electric charges in materials to try and account for some phenomena associated with electric charge interactions. At various points you will use a simulator to help you think about this, but first the model itself will be described.

A Model for Charges in Materials

You already know that there are two types of electric charge, which we call positive and negative. You also know that like charges repel and unlike charges attract. Scientists also find it useful to think about all material as containing a very large number of very small charges, both positive and negative. In our model we will assume that these charges cannot be created or destroyed.

It is also useful to imagine that the positive charges in a material cannot move, but that the negative charges are free (at least somewhat) to move around[1]. Further, some materials have the property that it is relatively easy to remove some of their negative charges, while other materials have the property that it is relatively easy to add extra negative charges.

Uncharged and charged objects

Our model assumes that all materials contain large numbers of both positive and negative charges, yet most objects around us seem to be uncharged. However, when objects do become charged, you have observed that some become negatively charged, while others become positively charged. How can the model account for these observations?

Using the model, how could you account for the observation that most objects are uncharged overall?

How could the model be used to explain why a certain object is negatively charged? What about a positively charged object?

Charging by rubbing

You are probably aware that oOne way of charging objects is to rub them together. How could the model explain what happens when two uncharged objects are rubbed together, leaving them both charged?

Simulator Exploration
STEP 1: To start you will use a simulator to investigate the charging of objects that are rubbed together. Open Cycle 3 Activity 3 Homework Setup 1. The setup shows two objects, made of different materials.
To rub these two objects together and examine the results, perform the following steps:
·  Click on the ‘Run’ button
·  Make sure the selection tool is active.
·  Now place the cursor on one of the two objects and drag it sideways so that it is touching the second object.
·  Use the cursor to and click-and-drag one of the objects up and down several times while it is touching the second object
·  Move the objects so they are not touching any more.
STEP 2: Now answer the following questions. /

Run Select

How does the simulator represent that the two objects become charged as a result of being rubbed together?

Does the simulator suggest that the two objects became charged in the same way (that is, both positive or both negative) or are they oppositely chargesd (one positive and the other negative)? How is this represented?

The simulator represents areas of overall positive charge using red, and areas of overall negative charge using blue. From this you can see that, when the objects were rubbed together, the rubbed surface of one became positively charged, while the rubbed surface of the other became negatively charged.

How could our model for electric charges in materials explain how this happened? (Remember, only negative charges are free to move.)

Why do you think the simulator represents neutral objects in a magnenta (reddisch-blue) color?

Charging the Tapes

In the previous activity you created pairs of charged tapes by ripping them apart very quickly. It can be shown that the tapes you labeled B (bottom) became negatively charged in this process, while the tapes you labeled T (top) became positively charged. /

Three students were discussing how they thought the model of charges in materials could account for how the two tapes became charged during this process.

Samantha Victor Kristen

Do you agree with Samantha, Victor, Kristen, or none of them? Explain your reasoning.

Interactions between Charged TapesObjects

In the previous activity, after charging the tapes you brought them close together to examine the electric charge interactions between them. Now you will use the simulator to look at the these interactions between charged objects again, and try to determine some characteristics of these interactions.

Simulator Exploration

STEP 1: Open Cycle 3 Activity 3 Homework Setup 2. In this set-up you will see two a small ballsspheres (, suspended from a threads), on the left hand side of the window, one at each end of the window. These balls spheres will represent the tapes.two objects on the right hand side, and a ‘barrier’ in the middle. When you cClick on the ‘Run’ button, and the balls spheres should turn red, showing that they are bothit is already positively charged. However, the two objects are not yet charged Thus, in this setup the two positively charged spheres represents two positively charged tapes (two “top” tapes) that are currently a long distance apart.
Suppose you were to bring these two charged tapes spheres closer together. What do you think would happen to them, and why? /
Charge the two objects, by rubbing them together, just as you did in the previous setup. (You should keep rubbing them until they become fully charged.)
Next, select the blue-charged object and turn it around, by clicking on the black dot on its stand. /

Initial Ideas

Suppose you were to bring the blue-charged object close to the hanging red-chargesd sphere. What do you think would happen, and why?

What if you brought the red-charged object close to the hanging red-charged sphere? Would anything different happen and, if so, why?

STEP 2: You should now test your thinking with the simulator. Using the selection tool, click on the tape sphere on the left of the windowblue-charged objects (actually on the ball), and drag it across the window, toward the other tapespherehanging red-charged sphere. Do this in several steps, halving the remaining distance between the tapes spheres each time, and making sure you release your mouse button after each step. End with the blue-charged object at the barrier.

Describe what happens to the two red-charged sphere tapes spheres as they the blue-charged object isare brought closer together. Is this what you predicted would happen?

Now return the blue-charged object to the right side of the window, select the red-charged object, and drag it across the window.

Describe what happens to the red-charged sphere as the red-charged object is brought closer. Is this what you predicted would happen?

Does the strength of the interaction between the two tapes spherescharged objects depend on how close together they arethe distance between them? What evidence from the simulator supports your idea?

In Activity 1 of this chapter2 you found that only certain types of metals were attracted to magnets. Suppose you conducted a similar experiment, but this time brought different materials (metals and non-metals) near an electrically charged object instead of a magnet. Do you think all types of materials would interact with the charged object, or only certain types of materials? If only certain types, what types do you think would interact?

For materials that you think will interact with the electrically charged object, do you think the interaction would always be attraction, always repulsion, or do you think some materials will attract and some will repel? Why do you think so?

Exploration #1: What kinds of materials interact with a charged tape?

Open the movie, C3A23_tapes.mpg that should be on your Student Resources CD.

STEP 1. Run Part 1 of the movie, Interactions of charged tapes, from the beginning to 3 min 28 seconds. This part of the movie provides a review of what you had done with the tapes in Experiment #1 in Activity 23.

STEP 2: Now run Part 2, Interactions of other materials with charged tapes, from 3 min 28 seconds to the end. The person in the movie will bring various uncharged objects near charged B and T tapes.

Stop the movie after each object is brought near the charged tapes, and record your observations (attract, repel, or no effect) in the following table.

Table: Observations with Charged Tape and Materials (from movie)

Material / Reaction with B-Tape / Reaction with T-Tape
Finger
Glass rod
Paper clip
Paper
Iron nail
Polystyrene
Wooden splint
Aluminum foil
Foam padding

STEP 3. Look over the data in the Table.

What materials seem to interact with the charged tapes?

Does the reaction depend on which type of charge is on the tape or not?

Now think about when the person in the movie brought an uncharged object close to the charged tapes. The experiments showed that the tapes were attracted to the uncharged object, so it seems that there can be electric charge interactions between charged and uncharged objects. How can our model of charges in materials help to explain this phenomenon?

STEP 2: You will now examine the interaction between tapes spheres that are oppositely charged. First, return the tape sphere you moved to the left end of the window. Next, double-click on this tape (ball)sphere to bring up its Properties Box. The charge on the tape sphere should currently be set to ’20.0’. To change this to a negatively charged tape simply insert a negative sign in front the ’20.0, as shown to the right. Finally, click on the ‘OK” button - the tape (ball)sphere should now change color to blue. /

Simulator Exploration #2:

Suppose you were to bring these two charged tapes spheres closer together. What do you think would happen to them, and why?

Now gradually bring the negatively charged tape sphere closer to the positively charged one, using the same procedure as you did above. However, be careful not to let them touch each other. If they do touch you will have to reload the simulator and start again!

Do these tapes spheres behave as you predicted? In this case, does the strength of the interaction between the two tapes spheres depend on how close together they are?

Interactions between Charged Tapes and Uncharged Objects

STEP 1: If you have access to a balloon, blow it up. Then rub the balloon against a sweater (or your hair) and place the rubbed part of the balloon against a wall. You should observe that the balloon sticks to the wall. (If it is too humid in the room, the balloon may not stick, but you should still observe some attraction between the balloon and the wall.)

In the rest of this exploration, you willcan either run watch a movie of the “Balloon and Static Electricity” simulator at to help you understand why the balloon sticks to the wall. You can also open the simulator yourself by going to http://phet.colorado.edu/web-pages/simulations-base.html, or you can watch a movie of the simulator. The movie is on your Student Resources CD and is called “Balloon_Wall.mov.” . Thise simulation was developed by , one of a suite of simulations from the Physics Education Technology Project (PhET) Project , was developed by a team of researchers and designers at the University of Colorado-Boulder.

STEP 2: The Either play the movie Balloon_Wall.mov or open the simulation on the web. It shows a sweater, balloon (with positive and negative charges not shown since the balloon is neutral) and part of a wall. If you are running the simulator, the screen should look like the following. Make sure the following setup options are selected:

·  The ‘Show all charges’ option should be selected

·  The ‘Ignore Initial Balloon Charge’ option should NOT be checked

·  The ‘Wall’ option should be checked

Is the wall charged positively, negatively, or is it neutral? How do you know?

STEP 3. TRub the balloon is rubbed against the sweater to charge it negatively.

STEP 4. TBring the charged balloon is then brought near the wall, but do not touch the wall yet.

What happens to the negative charges in the wall?

What happens to the positive charges?

Why does that happen to the negative and positive charges?

Since the balloon has not yet touched the wall, do any negative charges actually move from the balloon to the wall during this process?

Remember that according to our model, only negative charges can move within a material; the positive charges stay fixed. Whenever the negative charges in a material are displaced relative to the positive charges, we say the material is electrically polarized. In this case, the wall has become electrically polarized in the area closest to the balloon. There are still the same number of positive and negative charges in the wall, so it is still uncharged overall. What has happened is that the distribution of negative charge has changed, so the front surface of the wall closest to the balloon is more positively charged, while the region to the right of the surface and the other side of the wall is more negatively charged.