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Experiment

6

Advanced Physics with Vernier – Beyond Mechanics © Vernier Software & Technology 6 – 1

Electrostatics INTRODUCTION Charge, like mass, is a fundamental property of matter. However, unlike mass, which seems to be more or less unchanging, the effects of charge are not always apparent. In fact, we use terms like “charged” or “uncharged” to describe objects when we would not think of using parallel descriptors for mass. In this experiment, you will observe charge separation and transfer and learn how to measure the electric charge of objects.

OBJECTIVES In this experiment, you will

 Use a Charge Sensor and Faraday Pail to measure electric charge.  Observe and quantify the separation of electrical charge by friction.  Observe and quantify charging by contact.  Observe and quantify charging by induction.

MATERIALS

Vernier data-collection interface two wires to ground objects Logger Pro or LabQuest App grounding wrist strap Vernier Charge Sensor charge separators Faraday Pail and Cage cotton cloth grounding plane

You will use a Charge Sensor and a Faraday Pail to measure the sign and magnitude of the electrostatic charge in these activities. This combination of equipment is capable of measuring very small amounts of charge, which can be accidentally introduced into your experiment. It is important to know how and when to discharge the sensor and when to ground objects in order to obtain satisfactory data.

You should perform Part 1 and discuss your findings before proceeding with the rest of the experiment.

Experiment 6

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PART 1 THE STATIC ELECTRICAL CHARGE ON YOUR BODY PROCEDURE This part of the experiment demonstrates the quantity of charge that can reside on your body and how you can minimize the effects of this build-up of charge in subsequent experiments.

1. Set up the Faraday pail and charge sensor as shown in Figure 1. a. Place the plastic disk that holds the Faraday pail and cage on the grounding plane. b. Place the Faraday pail and the cage on the disc. c. Connect the black lead from the

charge sensor to the grounding plane. d. Connect the red lead from the charge

sensor to the Faraday pail. e. Connect the charge sensor to the data-

collection interface and set the range switch to the +/- 10 V position.

f. Connect a grounding wire between the cage and the grounding plane.

g. This equipment is capable of measuring very small amounts of charge. As you proceed with the experiment, your equipment may accidentally or intentionally develop a charge. It will be important to remove or ground unwanted charge either in the Faraday pail or on the sensor. This is accomplished by pressing and holding the Reset button on the sensor.

h. You are now ready to measure the charge of objects that are inserted into the center of the Faraday pail.

2. Scuff your shoes on the floor or carpet. (Note: Do not use the grounding strap yet.) Insert a

finger into the pail without touching the pail or cage. Observe the readings on the meter.

3. Remove your finger. Again, observe the readings on the meter. Record your findings.

4. Touch the metal grounding plane that holds the Faraday pail with your finger.

5. Insert your finger into the pail without touching the pail. Observe the reading on the meter. Remove your finger. Again, observe the reading on the meter. Account for your observations.

6. If possible, repeat Steps 2 and 3 using a different combination of shoe type or flooring (perhaps your lab partner has a different type of shoe).

7. Ground the pail and zero the sensor by pressing and holding the Reset button on the sensor.

8. Attach the alligator clip on the grounding strap to the grounding plane and attach the grounding strap to your wrist.

9. Repeat Steps 2 and 3. (Note: This time you will use the grounding strap.) Make a record of your observations in your lab notebook.

Figure 1

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Advanced Physics with Vernier – Beyond Mechanics 6 – 3

EVALUATION OF DATA 1. Describe the reading of the charge sensor when you placed your finger in the Faraday pail

and then removed it. How do your findings compare with others in the class?

2. Account for your findings regarding charge in terms of the transfer of charged particles between you and the floor and the movement of these particles in the pail.

3. Repeat Steps 1 and 2 for the readings you observed after you touched the grounding plane.

4. Account for your observations after you used the grounding strap.

PARTS 2–4 THE SEPARATION AND MOVEMENT OF CHARGE PROCEDURE For the remainder of this experiment you will collect charge data as a function of time. During your analysis you will identify events on the graph (e.g., when an object is inserted into or removed from the pail) using features in the software called Data Marks and Data Tags. Data Marks and Data Tags are both ways to add a note to a particular moment in data collection, but Marks are added during data collection only. During data collection, press a key or tap an icon to mark an event. The time and associated sensor value is noted; laterm, you can add notes to that event.

Mark Data in Logger Pro a. While collecting data in Logger Pro, press the “D” key on the keyboard for each point of

interest. Each time an event is marked, a Data Mark helper object will be displayed on the graph.

b. When data collection is complete, double-click each Data Mark helper object and enter a description for the event.

c. If you wish to add additional tags after you have stopped data collection, you may insert a Data Tag. Turn on the Examine tool. Click the point on the graph you wish to tag and choose Tag Data from the Experiment menu or press “D.” A Data Tag helper object will appear.

Mark Data in LabQuest App

a. Before collecting data in LabQuest App, choose Data Collection from the Sensor menu. Tap Advanced, check the Enable Data Marking box, then tap OK.

b. When data collection begins, a small button labeled MARK will appear below the graph. Tap this button to mark an event.

c. When data collection is complete, tap the box to the right of the graph that contains Data Marks. A new window will appear with your data marks listed. You can name each mark by tapping in the box and entering the desired text. Tap OK when finished.

d. If you wish to add additional tags after you have stopped data collection, you may insert a Data Tag. Tap the spot on the graph you wish to tag and choose Tag Data from the Graph menu. Label Data Tags in the same way you did Data Marks.

Experiment 6

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Part 2 Charge separation due to friction

In Part 1, you explored the charge that accumulated on your body, but you did not explore the charge that might be on the floor or carpet. If you were able to measure the charge on the floor, what would you expect to observe? In this part of the experiment, you will use small dissimilar materials to explore the charge on each of them after they are rubbed together.

1. Choose New from the File menu. Change the data-collection duration to 60 seconds and the data-collection rate to 10 Hz.

2. Attach the alligator clip on the grounding strap to the grounding plane, and then attach the grounding strap to your wrist. Ground the system and discharge the charge sensor by pressing and holding the Reset button on the sensor.

3. To remove any charge from the charge separators, dab each charge separator on a damp cotton cloth. Do not rub the charge separator and cotton cloth together, as this might result in a build-up of charge on the disc. Insert the charge separators into the Faraday pail one at a time to verify that they are electrically neutral.

4. Rub the white and gray charge separators together, then hold them separately.

5. In the next step, mark each time you insert or remove the charge separator.

6. Start data collection; be ready to mark events. Insert the white charge separator into the middle of the pail without touching the pail. Wait 4 or 5 seconds, then remove it. Wait a couple of seconds, then insert the gray charge separator into the middle of the pail and wait 4 or 5 seconds. Remove it and wait a couple of seconds. Finally, position both charge separators in the pail; be careful to keep the charge separators from touching the pail or each other. Wait 4 or 5 seconds, then remove both separators and stop data collection.

7. Label the marked events for each of the charge separators. Sketch or print the graph.

8. Store this run.

Part 3 Charging by contact

In Parts 1 and 2, you inserted and removed a charged object in the Faraday pail. What do you expect would happen if you touched the pail with the charged object?

1. Continue use of the grounding strap as you did in Part 2. Ground the system and discharge the charge sensor by pressing and holding the Reset button on the sensor.

2. To remove any charge from the charge separators, dab each charge separator on a damp cotton cloth. Insert the charge separators one at a time into the Faraday pail to verify that they are electrically neutral.

3. Rub the charge separators together, then set one aside.

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Advanced Physics with Vernier – Beyond Mechanics 6 – 5

4. Start data collection; be ready to mark events. Insert one of the charge separators into the middle of the pail without touching the pail (see Figure 2). Wait 4 or 5 seconds, then remove it. Wait a couple of seconds. Now lightly rub the charge separator across the top edge of the pail and remove it from the pail. Stop data collection.

5. Label the marked events. Sketch or print the graph.

6. Store the run.

Part 4 Charging by induction

In Part 1, you found that touching the grounding plane “removed” excess charge from your finger. Suppose the pail were grounded with a conducting wire when a charged body was brought close to it. What effect would this have on the charge of the pail? In this part, you will take a closer look at the effect of “grounding” in this situation.

1. Continue use of the grounding strap as you did in Part 2. Ground the system and discharge the charge sensor by pressing and holding the Reset button on the sensor.

2. Attach a second grounding wire to the grounding plane, leaving the other end disconnected. You’ll use this later to ground the pail.

3. As you did in Part 3, make sure that the charge separators are electrically neutral.

4. Rub the charge separators together. When you collect data, use the same separator as you did in Part 3 and set the other one aside.

5. Start data collection; be ready to mark events. Insert the charge separator into the middle of the pail (be careful to not touch the side of the pail). Wait 4 or 5 seconds, then remove it. Wait another couple of seconds. Reinsert the charge separator into the pail, then ground the pail by briefly touching it with the second grounding wire. Wait 4 or 5 seconds, then remove the charge separator. Wait a couple of seconds and move the charge separator back into the pail. Wait 4 or 5 seconds, then remove the charge separator and stop data collection. Save your data file.

6. Label the marked events. Sketch or print the graph.

EVALUATION OF DATA It would be helpful for you to view the graph of charge vs. time for only one run at a time. To do this in Logger Pro, click the vertical axis label, choose More, and select Charge for the run you wish to view. In LabQuest App, tap the field next to the Filing Cabinet and select the desired run.

Part 2

1. Make note of the sign of the charge on each charge separator. Compare the magnitude of the charge on each.

2. From your answer to Question 1, account for the shape of the graph in terms of the transfer of charged particles between the charge separators. Account for the features of the graph when each of the charge separators was in the pail separately and when they were in the pail together.

Figure 2

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Part 3

1. Account for the shape of the graph in each of these regions in terms of the transfer of charged particles between the separator and the pail.

2. Did the charge in the pail return to zero at the end of the experiment? Why or why not?

Part 4

1. Account for the shape of the graph in each of these regions in terms of the transfer of charged particles between the pail and grounding plane and the role of the charge separator.

2. Did the net charge in the pail return to zero at the end of the experiment? Why or why not?

3. Compare your findings with those who used the other charge separator.

EXTENSION Describe, in general, the movement of charge in the pail/charge sensor circuit when a charged object is placed in the pail and then removed. What must the charge sensor do?