Lab 3 The Nature of Charged Entities-f22

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Arizona State University, Tempe *

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Feb 20, 2024

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Unit 1 Developing models for magnetism and static electricity PHS110 Lab 3: The Nature of Charged Entities This document is read-only. Make a copy for your group to fill in their responses. When finished, save it as a pdf to upload to Canvas for Lab #3. Look for boxes outlined in blue for places where your group must insert a response. Please put full names of all collaborators on this lab Names: Earlier in this unit, you started developing a model for static electricity that can explain various phenomena in terms of how some positively (+) and negatively (–) charged entities associated with the interacting objects behave. In this lab, you will consider some different possibilities for such a model and also what the nature and properties of these charged entities might be. Blow up the balloon at your lab station using the supply of air near the back of the lab and prepare to rub one side of the balloon vigorously with rabbit fur. Create a pair of "electrified" tapes like was done during last week's lab, one labeled as a B-tape (bottom tape) and the other a T-tape (top tape). It may help to review a short movie of how this is done here: Lab 2 Movie A Making Electrified Tape P1-Q1) Vigorously rub one side of the balloon with the rabbit fur and bring it near each of the electrified tapes. Describe whether each tape is attracted to, repelled from, or has no reaction to the balloon. Use this information to explain whether the balloon is positively (+) or negatively (-) charged and how you know. 1 Response (2 pts): The bottom tape is repelled while the top tape is attracted to the balloon. The top tape is positively charged and the bottom tape is negatively charged. Since the top tape attracts the balloon, the balloon must be negative. Since the bottom tape is repelled we can say the balloon is negatively charged. 1 Recall from class discussion last week it was determined that the T-tape is positively (+) charged. © 2018 Next Gen PET 1

Lab #3 (Unit 1) Rubbing the balloon with rabbit fur is very similar to rubbing it with wool. You should have found that when a balloon is rubbed on rabbit fur (or a wool sweater), the rubbed area of the balloon becomes negatively (–) charged, and hence the rubbed area of rabbit fur (or wool) becomes positively (+) charged. Consider how electrical charges on a balloon might rearrange when it is rubbed with a wool sweater. Assume the diagram to the right represents the distribution of charge on the sweater and balloon prior to the balloon being rubbed on the sweater. Both objects have equal numbers of positive (+) and negative (–) charges. The diagrams below represent three different models of how the +/– charged entities on the sweater and balloon might be arranged after they are rubbed together. (The balloon is rubbed on the left sleeve nearest to it.) Underneath the diagrams are explanations for what happens with the +/- charges during the rubbing process. Model Explanations: I. There are both positively (+) and negatively (–) charged entities in both the sweater and balloon, and both types of charges are free to move and be transferred between objects. II. There are both positively (+) and negatively (–) charged entities in both the sweater and balloon, but only the positively (+) charged entities are free to move and be transferred between objects. The negatively (–) charged entities stay fixed in place and never move . III. There are both positively (+) and negatively (–) charged entities in both the sweater and balloon, but only the negatively (-) charged entities are free to move and be transferred between objects. The positively (+) charged entities stay fixed in place and never move . 2

Lab 3: The Nature of Charged Entities P1-Q2) Explain which diagram (A, B, or C) matches which Model Explanation (I, II, or III) and why. Then discuss the three models as a group and explain which of the three models best matches your initial theory on how the charges behave and why you think so. Response (3 pts): 1C, 2A, 3B Model A matches model explanation 1 because Model A has a mix of positives and negatives in both sweater and ballon which shows the exchange of both charges. Model B shows negative charges moving to the balloon while positive charges are present in both. This shows only negative charges are free to move while positives are fixed. Model C shows concentration of positives on the sweater but both charges have moved between. All three of these slightly different models explain static electric phenomena using small + and – charged entities associated with the objects involved. Keep the three models in mind as you work with simulations which demonstrate the scientific consensus on how charges move during electrical interactions. Go to the PhET simulation Balloons and Static Electricity and press start. STEP 1. Without charging the balloon, move it first very close to the wall, and then very close to the sweater. Observe i) whether the balloon moves and which way, ii) whether the + charges or - charges move and in which direction, iii) whether the + or - charges move from one object to the other and (if so) how? P2-Q1) What would you consider the overall charge of each object in STEP 1? (You can highlight the cell with your choice.) Response (2 pts): Sweater Positive (+) Neutral Negative (-) Balloon Positive (+) Neutral Negative (-) Wall Positive (+) Neutral Negative (-) P2-Q2) For neutral objects, what do you notice about the number of positive and negative charges inside? Why does this explain the motion (or lack of motion) of the balloon as you brought it near the sweater and the wall? 3

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Lab #3 (Unit 1) [Hint: To help answer this question, go to the white box below the sweater and change from ‘Show all charges’ to ‘Show charge differences’ and observe what happens.} Response (2 pts): Because each object has equal charges, each object is neutral, which is why there is a lack of motion. STEP 2. Go back to SHOW ALL CHARGES. RUB the balloon over the sweater--it should look similar to the image to the right when you are done. You have now charged the balloon. P2-Q3) Describe what happened with the charges as a result of rubbing the balloon on the wool sweater. Are any charges created or destroyed during the rubbing? Did either charge type move inside its original object? Did any charges move from one object to the other? Explain. Response (2 pts): All the negative charges from the sweater have moved over to the balloon, and the amount of them has remained the same. P2-Q4) What would you now consider to be the overall charge of each object after rubbing in STEP 2? (You can highlight the cell with your choice.) Response (2 pts): Sweater Positive (+) Neutral Negative (-) Balloon Positive (+) Neutral Negative (-) Wall Positive (+) Neutral Negative (-) STEP 3. Slowly bring the charged balloon near to (BUT NOT TOUCHING) the wall. Leave about as much space as the width of your pinky finger between the balloon and wall. Observe the charges inside the wall and the balloon as they approach one another. P2-Q5) Describe what happens to the charges inside the wall as you bring the balloon near. Also explain what movement occurs once the balloon is released and why this happens. 4

Lab 3: The Nature of Charged Entities Response (2 pts): The balloon causes the negative charges in the wall to be pushed away, while the positive charges stay in place. After letting go, the ballon goes back to the sweater because of how strong of a positive strength it has. Polarization : You should have noticed that the charges in the wall separated temporarily when the negatively charged balloon was brought near. This process of separating charges temporarily is called polarization. Polarization is how a charged object can be attracted to a neutral one. STEP 4. Slowly bring the charged balloon away from the wall and release it midway between the wall and the sweater. P2-Q6) Describe what happens to the motion of the balloon and why this makes sense in terms of the overall charges on the objects involved. Response (2 pts): The balloon floats back to the sweater, this is because the wall is neutral and the sweater is the only positive object that will attract the balloon. STEP 5. Reset the balloons and remove the wall from the simulation. Select the two-balloon option. Get both balloons negatively charged and move them away from the sweater. P2-Q7) Describe what happens when you try to bring the balloons close to one another and why this makes sense in terms of the charges involved. Response (2 pts): When you bring the balloons to the right of the screen, each balloon repels the other one, this is because they are both negatively charged. Go to the PhET simulation John Travoltage and press start. 5

Lab #3 (Unit 1) P3-Q1) Predict what will happen to John if he rubs his foot against the carpet and why you think this will happen based upon prior experiments with rubbing objects together. Response (2 pts): His foot will get charged when rubbed against the carpet. P3-Q2) Rub John’s foot on the carpet by clicking and dragging his foot a few times. What happens? (Explain in terms of charges.) Response (3 pts): Johns foot gets negatively charged. This indicates that the carpet also carries negative charges. P3-Q3) After rubbing John’s foot on the carpet, click and drag John’s hand such that it touches the doorknob. What happened? (be specific) How can you make the results more dramatic and why does this happen? Response (2 pts): When johns hand touched the doorknob, all the charges from his foot were transferred to the doorknob (which is connected to ground). This created a static shock in his hands. If johns foot is rubbed more it will make the static shock longer, this is because there are more negative charges that need to be transferred to the doorknob (and then to the ground). P3-Q4) How is this simulation different from the balloon and sweater or balloon and wall touching each other? Are there any similarities between the two simulations? Response (3 pts): The sweater and balloon experiment shows the transfer of charges between two insulated objects. Simulation of John shows the transfer of charges from a person to a grounded object. The only similarity is that the charges are transferred through contact. 6

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Lab 3: The Nature of Charged Entities You are no doubt aware that all material can be regarded as being made of tiny particles we call atoms. It is generally accepted that an atom consists of a very dense central nucleus that contains two types of particle that are very strongly bound together; positively (+) charged protons and uncharged neutrons . Because of the very strong forces involved in the nucleus of an atom it is extremely difficult to add protons to, or remove them from, an atom. (All atoms of a particular element have the same number of protons in their nucleus, whereas atoms of different elements have different numbers of protons.) Orbiting around the nucleus are tiny negatively charged particles known as electrons that are much more loosely bound to the atom. Because of the much weaker forces involved it is relatively easy to add electrons to, or remove them from, an atom. Note that a single electron and a single proton are taken to have electric charges that are of opposite types (+ and –) but the same strength . In a ‘normal’ atom the number of electrons in orbit around the nucleus is equal to the number of protons in its nucleus. Because of this, a 'normal' atom has no overall charge, because the negative (–) charges of the electrons cancel the positive (+) charge of protons exactly. P4-Q1) In principle, to give this uncharged atom an overall positive (+) charge, you could either (1) add protons or (2) remove electrons from the atom. Which of these two do you think would be easier to achieve in real life and why do you think so? Response (2 pts): It should be easier to just remove electrons from the atom because the electrons are much more loosely bound to the atom than protons. 7

Lab #3 (Unit 1) P4-Q2) To give this uncharged atom an overall negative (–) charge, you could either (1) add electrons or (2) remove protons from the atom. Which of these two do you think would be easier to achieve in real life and why do you think so? Response (2 pts): It should be easier to add electrons to give an overall negative charge. Since electrons are just orbiting the atom, they can be removed or added much easier than protons can. 8

Lab 3: The Nature of Charged Entities P4-Q3) Reflect back on the results of your explorations with the simulations in light of your answers to the prior two questions. During the simulation, did both positive and negative charges move within any of the objects? Did both positive and negative charges transfer from object to object? Ex;plain how these answers make sense with the model of an atom. Response (2 pts): When the balloon is rubbed on the sweater, electrons move from the sweater to the balloon, leaving the balloon negatively charged and the sweater positively charged. When John rubs his foot on the carpet, electrons are transferred from the carpet to his foot, making his foot negatively charged. When John then touches the doorknob, the excess electrons from his body are transferred to the doorknob and then to the ground. The positive charges never moved or transferred which aligns with the model of an atom. Consider again the initial models for how electrical charges on a balloon might rearrange when it is rubbed with a wool sweater. Remember the diagram to the right represents the distribution of charge on the sweater and balloon prior to the balloon being rubbed on the sweater. The diagrams below represent three possible models for the charge distributions after rubbing. P4-Q4) Which of the three models do you think correctly represents the charges in the balloon and sweater after rubbing? Use evidence from the simulations to explain your choice. Response (3 pts): 9

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Lab #3 (Unit 1) B This is because the positive charges remain in the same spot without moving. Only the negative charges are transferred to the ballon which makes the balloon negatively charged. P4-Q5) Prepare a set of top and bottom tapes and attach them to the support stand. Rub together acrylic and Styrofoam and use the tapes to test their charge after rubbing, is the clear sheet of acrylic positively or negatively charged? What about the Styrofoam? How do you know from evidence in the observations? Explain. (If you get no reaction on the acrylic, try rubbing it again and pay careful attention to the location it is being rubbed. Bring the location of the rub as close as possible to the tape.) Response (2 pts): Acrylic attracted bottom tape. The acrylic is positively charged because the bottom tape was negatiely charged. The styrofoam repelled bottom tape which makes it negatively charged. Since like charges repel and unlike charges attract. P4-Q6) The ‘Before’ diagram for this situation is shown to the right. Both the acrylic sheet and the Styrofoam plate are initially uncharged . Which one of the ‘After’ diagrams below do you think best represents what the charges will look like after the styrofoam and acrylic are rubbed together and why? Use evidence from the simulations to explain your choice. 10

Lab 3: The Nature of Charged Entities Response (3 pts): A because the positive charges dont transfer in between objects and only the negative charges have transferred. Option A also has the correct number of negative charges while Option C has extra negative charges, Create a new pair of "electrified" tapes like was done during last week's lab, one labeled as a B-tape (bottom tape) and the other a T-tape (top tape). Hang them from your support stand. P5-Q1) Vigorously rub the glass rod at your table with a piece of silk. Describe whether each tape is attracted to, repelled from, or has no reaction to the glass rod when it is brought near. Use this information to explain whether the glass rod is positively (+) or negatively (-) charged after rubbing with silk and how you know. Also explain what charge the silk must have after rubbing. 2 Response (2 pts): T repels and B attracts so the glass rod is positive, so the silk must be negative. 2 Recall from class discussion last week it was determined that the T-tape is positively (+) charged. 11

Lab #3 (Unit 1) P5-Q2) Create a diagram (similar to that shown at right) representing what happens to the glass rod and the silk when they are rubbed together. Use the model of how positive and negative charges behave developed in the prior sections of this lab to guide your diagram. (You may draw this diagram on a whiteboard and upload a snapshot for this question). Under your diagram, EXPLAIN what is occurring with the electrical charges during rubbing. Response (insert snapshot of whiteboard) (3 pts): 12

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Lab 3: The Nature of Charged Entities 13

Lab #3 (Unit 1) P5-Q3) Create a fresh pair of electrified tapes if needed. Vigorously rub the PVC pipe at your table with a piece of rabbit fur. Describe whether each tape is attracted to, repelled from, or has no reaction to the PVC pipe when it is brought near. Use this information to explain whether the PVC pipe is positively (+) or negatively (-) charged after rubbing with rabbit fur and how you know. Also explain what charge the rabbit fur must have after rubbing. Response (2 pts): T attracts and B repels, so the PVC pipe is negative and the rabbit fur is positive. This means that the T tape is positive and the B tape is negative. P5-Q4) Bring your PVC pipe and rabbit fur to the water faucet at the back of the lab room. (Alternatively, bring it to the "water bending" apparatus set up on one of the back tables in the lab room. You can also try the large acrylic rod). Turn the faucet on to a slow stream (or pour water into the funnel on the lab apparatus), vigorously rub the PVC pipe with the fur again and bring the PVC close to the stream of water. (If there is no reaction try changing how fast the water is running, rubbing the PVC again, or trying a larger piece of PVC pipe). Describe what happens to the stream of water and what might be true of the charge of the water. Is it possible that the water is negatively charged? Could the water be positively charged? Is it possible the water is neutrally charged? Explain your reasoning. Movie Backup: On humid days it is sometimes difficult for the PVC pipe to retain its negative charge'. If your group didn't see any response from your water experiment, instead watch the movie at this link and record observations: Water Bending Physics Demo - Electricity & Polar Molecules Response (2 pts): After charging the PVC pipe and putting it next to the water stream, we observed that the water bent toward the PVC pipe meaning that it was attracted to the pipe. This means that the water is positive as the pvc pipe is negative. 14

Lab 3: The Nature of Charged Entities P5-Q5) In general, the water coming from a faucet is neutrally charged. Assuming this is the case with the water from the lab faucet, create a diagram representing what is happening with the charges in the water when the PVC pipe is brought near enough to bend the water stream. (You may draw this diagram on a whiteboard and take a snapshot to insert below.) On your diagram, add positive and negative charge entities to both the water stream (before and after the PVC is brought near) and the PVC. Under your diagram, EXPLAIN what is happening when the negatively charged PVC is brought near the stream of water to cause it to bend towards the PVC. Response (insert snapshot of whiteboard) (3 pts): 15

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Lab #3 (Unit 1) 16

Lab 3: The Nature of Charged Entities 17

Lab 3 The Nature of Charged Entities-f22

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