Unit 1 Cycle 2: Interactions and Energy



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?

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

[pic] 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 |[pic] |

|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. |[pic] [pic] |

| |Run Select |

|To rub these two objects together and examine the results, perform the following steps: |[pic] |

|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. | |

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

[pic] 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.

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

[pic] 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 |[pic] |

|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.

[pic] [pic] [pic]

Samantha Victor Kristen

[pic] 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 |[pic][pic] |

|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. | |

| | |

|[pic] 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|[pic] [pic] |

|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

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

[pic] 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.

[pic] 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.

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

[pic] 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?

[pic] 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?

[pic] 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.

[pic] 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.

[pic] What materials seem to interact with the charged tapes?

[pic] 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 |[pic] |

|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:

[pic] 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!

[pic] 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 , 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

[pic][pic][pic]

[pic] 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.

[pic][pic]

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

[pic][pic]

[pic][pic] What happens to the negative charges in the wall?

[pic] What happens to the positive charges?

[pic] Why does that happen to the negative and positive charges?

[pic] 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.

STEP 5. Touch the balloon is then touched to the wall, and it sticks.

|[pic] Are the excess negative charges in the balloon closer to the positive charges |[pic][pic] |

|in the wall or closer to the negative charges in the wall? | |

| | |

| | |

| | |

|[pic] Why does the balloon stick to the wall? (As a hint, remember that the strength| |

|of the electrostatic force depends on how close the charges are.) | |

Now think about when the person in the movie you brought an uncharged object close to the charged tapes. Your 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?

Simulator Exploration

STEP 1: Return to the simulator window and delete the negatively charged tapeOpen Cycle 3 Activity 3 Homework Setup 3. In this set-up, together with a hanging sphere and a barrier, you will see a single object on the right side of the window. When you click on the ‘Run’ button, the hanging sphere should turn red, showing that it is already positively charged. However, the object is uncharged. The window should look like that shown below. You can imagine that the hanging sphere is like the hanging charged tape in the movie, and the uncharged objects is like one of the objects the person brought near the hanging tape. .

| |

|[pic][pic] |

[pic] The object is shown to be uncharged. Does that mean there is equal amounts of positive and negative charge in the object, that there is more positive charge than negative charge, or that there is more negative charge than positive charge?

STEP 2: Now, using the selection tool, click once on the uncharged object. Four small black squares should appear around the object to show that it is selected, as shown above. and drag it so that is about halfway to the barrier.

You can now gradually move the uncharged object toward the charged tape by tapping on the right-arrow key on the computer keyboard. As soon as you see something happen to the electric charge on the object, stop moving it for now.

[pic] In terms of electric charge, what happeneds to the two sides of the object when it got closer toas it approaches the positively charged tapesphere?

[pic] Remember that we are assuming that only negative charges can move within a material, not the positive charges (which stay in place). Are negative charges within the object attracted to, or repelled from, the positively charged sphere?

[pic] Why is the side of the object furthest from the positively charged sphere blue-charged (negatively charged)?

[pic] Why is the side of the object closest to the positively charged sphere red-charged (positively charged)?

Use our model of electric charges in materials to explain how the side of the object closest to the positively red-charged tape sphere becomes negatively blue-charged, while the other side becomes positively red-charged. (Remember, our model assumes the negative charges in the object are free to move, but the positive charges are not.)

[pic] Did the positively charged sphere actually touch the object, or was it only near the object?

Since the positively charged sphere did not actually touch the object, no electric charges from the sphere actually were transferred to the object. Because the object was uncharged to begin with, it remains uncharged throughout the entire process. What happened, instead, is that the positive and negative charged got separated, with one side (the red-colored side) having more of the positive charge, and the other side (the blue-colored side) having more of the negative charge. When positive and negative charge within an uncharged object are separated in this way, we say the object is

When the negative charges in an object move closer to one side of the object, such so that this sideit becomes negatively charged, while the other side is left positively charged, the object is said to be electrically polarized. This idea will be revisited in later cycles of the PSET course.

[pic] When an uncharged object is electrically polarized, how does the amount of positive charge on one side compare to the amount of negative charge on the other side? Why is that?

STEP 3: Now suppose the object were moved even closer to the positively red-charged tape.

[pic] Do you think the hanging tape will be attracted, repelled, or not move? Why do you think so?

TTYou will now test your idea with the simulator. Very gGradually move the object closer to the tape, ending with the object next to the barrier.. Observe what happens to the sphere and to the charged sides of the object.Again, be careful not to let them touch each other. If they do touch you will have to reload the simulator and start again!

[pic] What happens to the charged tapeph? Is it attracted, repelled, or does it not move? Is this what you predicted above? If not try to explain why this happens.

[pic] Is the red-charged (positively charged) sphere closest to the red-charged side of the polarized object or closest to the blue-charged side?

[pic]You had seen earlier that the strength of the electric charge interaction depends on how close the interacting objects are to each other; the closer they are, the stronger the effect. Think of the colored two sides of the polarized object as if they were two charged objects at different distances from the charged sphere. Did the sides of the object lose their charge, remain charged to the same degree, or become more charged? How do you know? How could our model of electric charges in materials be extended to try to explain why this happens? (Hint: Think about what happened to the hanging red-charged sphere as the blue-charged object got closer to it.)

[pic] Draw a force diagram for the positively charged sphere. Draw arrows to represent the force that the negatively charged side of the object exerts on the sphere and the force that the positively charged side of the object exerts on the sphere. Pay attention to the relative lengths of the two force arrows.

[pic] Why is the positively charged sphere attracted to the uncharged, but polarized object?

[pic] STEP 4: Once an uncharged object (like the wall) is electrically polarized, do you think it stays polarized? What evidence fromUse evidence from the simulator to supports your answer. Why do you think this happens??

[pic] Now suppose the polarized object were moved away from the charged tapesphere. Do you think the object would remain polarized or not? Explain your reasoning.

To check your thinking, move the object back to the center right side of the setup window.

[pic] Does the object remain polarized or not? How can you tell?

[pic] In terms of the electric charge in the material, why did the object become un-polarized (no longer polarized)? How can our model explain why this happened?

STEP 5: Finally, in your experiments you saw that positively and negatively a negatively charged tapes wereas both also attracted to an the same uncharged object. How can this be?

[pic] Suppose the tape in the simulator were negatively blue-charged. What would happen to the negative charges in the object when it was brought close to the tape now?

[pic] Draw a sketch of the tape and object when they are close together. Use red and blue colors to show how you think the two sides of the object, and the tape, are charged. Why would the negatively blue-charged tape be attracted to the object in this case?

|To check your thinking, return to the simulator and set the charge on the sphere to be|[pic] |

|negative. To do this, double -click on the sphere to open its properties box and | |

|insert a negative sign in front of the ‘100.0’. Click on ‘OK’ and the sphere should | |

|turn blue. Then, move the object slowly toward the sphere. | |

To check your thinking return to the simulator and set the charge on the tape to be negative, in the same way as you did before. Then, move the object slowly toward the tape.

[pic] Do the two sides of the object become charged in the way you predicted? If not, sketch the simulator arrangement below. and use our model to try to explain how the object became polarized in this way.

Summarizing Questions

Answer these questions as part of the homework assignment. Be prepared to add any different ideas that may emerge during the whole class discussion.

S1: In the last observation you made above, the object became electrically polarized when the blue-charged (negatively charged) sphere was brought near to the uncharged object. In terms of the model for electric charge in materials, explain what happened in the object to make it become polarized?Suppose you brought a positively charged object near a wall.

(a) Would the wall become electrically polarized? If so, draw a picture and explain how it happens. I, and indicate whether the surface of the wall would be more positively charged, or more negatively charged. have an excess of positive charge or an excess of negative charge.

(b) If you touch the object to the wall, it will stick. Explain whyhow that happens.

S1: If you rub an inflated rubber balloon on your sweater, or on your hair, the rubbed area of the balloon becomes negatively charged. How could our model of electric charges in materials explain this?

S2: You learned in Activity 3 that if you rub an inflated rubber balloon on your hair (or on your sweater), the rubbed area of the balloon becomes negatively charged. If you now take the rubbed balloon from S1, carefully place the rubbed part of the balloon next to a wall and let go, the balloon will ‘stick’ to the wall[2]. Assume the The wall itself is uncharged and that when you touch the charged balloon to the wall, no charge is transferred from the balloon to the wall. Below, is a sketch of the balloonIf you rub an inflated balloon against your hair and then bring the rubbed part of the balloon near a pile of tiny pieces of paper, several of the pieces will fly up to the balloon. sticking against the wall (side view through the thickness of the wall). Draw some pictures and explain how this happens. (As a hint, consider that each piece of paper becomes electrically polarized when the charged balloon is brought near.)

|(a) Use red and blue colors to show charged areas of both the balloon and the wall. |[pic] |

| | |

|(b) Explain why the balloon sticks to the wall. | |

was not charged so why does the balloon stick to it? Draw a picture of the balloon sticking to the wall and use red and blue colors to show charged areas of both the balloon and the wall. (Hint: what would our model suggest happens to the negative charges in the wall as the balloon is brought close?)

S3: The magnetic interaction and the electric charge interaction have different names; therefore there must be some things different about them (otherwise there would be no reason to call them by different names). Describe some differences between the magnetic interaction and the electric charge interaction.

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[1] Actually the degree to which the negative charges are free to move is very different in different materials, but the idea that they can move somewhat in all materials is all we need for our model.

[2] This will not work very well in a humid climate, as the humidity in the air tends to remove the extra negative charges from the balloon very quickly.

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I think that when the tapes were pulled apart, the ‘sticky’ side of the T tape took some positive charges from the B tape. So, the T tape now has extra positive charges and the B tape has some missing.

I disagree. We know only negative charges can move, so the ‘sticky’ side of the T tape must have pulled some negative charges off of the B tape.

I think that when the tapes were pulled apart, the ‘sticky’ side of the T tape left some of its negative charges on the B tape. So, the B tape now has more negative charges than it has positive.

Uncharged object

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