Electrostatics Experiments Experiment 1: Faraday Ice Pail and Charge Production

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1 Electrostatics Experiments Experiment 1: Faraday Ice Pail and Charge Production PURPOSE: To investigate charge production and charge transfer BACKGROUND: We will be using "charge producers" that consists of aluminum rods with an aluminum disc at one end separated from the rod by insulating plastic. One of the wands will have a white material on one side of the disc and a second will have a blue material on one side. The third type of wand will have only the aluminum disk (no material attached to it). These wands will be used to produce electrical charges. See section II-A in Appendix B for more on Charge Producers. The Faraday ice pail is used frequently in electrostatic experiments to detect/measure charge. The Faraday ice pail consists of a open wire cylinder attached to a base through a supporting (insulated) cylinder. Surrounding this inner cylinder is a second. larger open wire cylinder that sits on a cardboard plate resting on the base of the inner cylinder. These two cylinders are insulated from each other by air. See section II-C in Appendix B for more on the Faraday ice pail. We will be using the charge producers to produce charges and the Faraday ice pail to detect these charges and their relative strengths. We can also examine the relation between the charge induced on the ice pail by an inserted object and the actual charge on the object. Using the ice pail, one can then investigate the nature of charging an object by contact as compared to charging by induction. APPARATUS: Connect the electrometer (essentially a sensitive voltmeter) to the Faraday ice pail as shown in the figure. When performing the experiment, adjust the sensitivity of the electrometer such that most readings are in the upper 2/3 of the scale. Notice that the analog readings are 3 V and 30 V on the bottom scale and 10 V and 100 V on the top scale. To ground the ice pail (i.e., connect the inner pail to the shield/outer pail with your fingers ), momentarily push the "ZERO" switch on the electrometer to the "CHECK" position. The charge producers will be used as the charged objects. Before beginning, the electrometer should read "zero", indicating that there is no charge on the ice pail. RED BLACK Revised 2/ LC

2 PROCEDURE : Setup the apparatus as shown in the previous figure. Turn on the electrometer and zero it (using the 3 V scale initially). After zeroing the electrometer, you may want to set the scale at 10 V (you will probably have to change the scale when taking a variety of readings). Be sure to zero the electrometer before every group of measurements. There may be a small charge retained on the exposed plastic between the aluminum disk and the aluminum rod on the charge producers. This residual charge does not transfer readily when the disk is touched to the pail. Therefore, (before starting the experiment ) breathe on the exposed plastic of the charge producer so that the moisture in your breath will tend to remove any residual charge. Activity One - Charging by Induction 1. Gently rub the two charge producers together (the white one and the blue one). Insert the white charged object/wand into the inner pail of the Faraday ice pail, but DO NOT let the charged wand touch the pail. It is suggested that you insert the wand almost to the bottom of the inner pail. Note: You need to repeat these steps three or more times to get an "average" effect, but complete the cycle before you start taking the second set and the third set. What is the electrometer reading? V V V. Remove the white wand and again note the electrometer reading. V V V. Now, insert the blue wand into the inner pail of the Faraday ice pail, but DO NOT let the charged wand tough the pail. What is the electrometer reading? V V V. Remove the blue wand and again note the electrometer reading. V V V. For this activity, a. did the white and blue wands have the same approximate charge value? b. did the white and blue wands have the same sign for their respective charge? c. what is occurring when you rub the white and blue materials together? Revised 2/ LC

3 The rubbing of the white and blue material together is called charging by contact. The charging of the Faraday ice pail by the charged wands is called charging by induction. 2. Now rub the white and blue materials together more vigorously and take measurements as you did before. White wand in the pail: V V V. After removing the white wand: V V V. Blue wand in the pail: V V V. After removing the blue wand: V V V. For this activity, a. does it make much difference if you rub the two object vigorously or gently? why or why not? b. Would it make much difference if you insert the charged wands only a little into the pail? why or why not? Try it! Charge the wands and insert them barely into the inner pail, then a little more, and a little more. c. What is happening to the charge induced on the Faraday ice pail? 3. What would happen if you took the uncharged white and blue wands and rubbed them together inside the Faraday ice pail? Revised 2/ LC

4 Try it! Ground the charge producers and rub them together inside the ice pail. What does the electrometer read? Why? Activity Two - Charging by Contact 4. Now, gently rub the white and blue wand together. Insert the white wand, touch it to the inner pail, and then remove it. What is the electrometer reading. V. Momentarily ground the ice pail and then again touch the white wand to the inner pail. What is the electrometer reading? V. Does any charge remain on the object? 5. If you rub the blue and white wands together vigorously and touch the blue wand to the inner pail, what would you expect the electrometer to read? Should this be different then step 4 above? Activity Three - Conductors 6. Ground the charge producers again. Rub the white wand with the aluminum proof plane (the wand with the aluminum disk with no material on it). Measure the magnitude and polarity of the wands. White wand: V. Aluminum wand: V. Are these results consistent with Activity one results? What is causing one to be more positive than the other? Revised 2/ LC

5 Try rubbing the blue material with the aluminum proof plane. Measure the magnitude and polarity of the wands. Blue wand: V. Aluminum wand: V. Are these results consistent with Activity one results? What is causing one to be more positive than the other? Which of these three materials - white wand, blue wand, and aluminum wand is the best conductor? How do we know? 7. Construct a list of materials such that if a material lower on the list is rubbed with a material higher on the list, the higher material is always positive. Such a list is called an electrostatic series. Revised 2/ LC