The polarity of charge on the electroscope

The electroscope is a simple device used to detect charge. The main part of the electroscope is a very thin (about 10−7 m) piece of gold leaf hinged to a metal plate. When there is an excess of charge (a net charge) on the system, the force of repulsion of like charges causes the leaf to diverge from the plate. The tangent of the angle of divergence is roughly proportional to the amount of excess charge, with a 45 degree angle corresponding to a charge of roughly 10−9 C, (which corresponds to a voltage relative to the case of about 1000V). Thus we have a semi- quantitative way to measure charge. The extremely fragile gold leaf has to be protected from drafts and dirt, so it is enclosed by a case with glass windows for viewing. The case and windows also must shield the leaves from undesired external electrical fields, which might be caused by nearby charged objects. For this reason, the case is made of metal, and the glass windows have been given an electrically-conductive coating. In order to prevent uncontrolled charge buildup on the case, one can ground it once in a while by touching it with a finger. The leaf and plate are electrically connected to a spherically shaped external terminal by means of a short metal support rod that passes through a plastic insulating bushing in the case. This external terminal provides the experimenter with a means of transferring charge to and from the leaf/plate, without opening the case. There is also a metal cap, which can be placed over the terminal ball without touching it, but by resting on the case, makes electrical contact with it. With the cap in place, the entire leaf, plate, and terminal system is completely surrounded by a conducting surface that shields it from external electrical fields. Boseman Science, Electrostatic induction: https://www.youtube.com/watch?v=dwJ-MM7yu4E Rimstar, Inductive Charging: https://www.youtube.com/watch?v=-JsVZwc1dOo GPB Education, Friction, Conduction, Induction: https://www.youtube.com/watch?v=7-tW9AOcXT4 Video by Professor Sergan, posted on Canvas: https://www.youtube.com/watch?v=JOnYVEV0csk&feature=youtu.be 1. Using the information in the passage above, your knowledge of charge transfer, and the four videos, explain in a few sentences, how to charge the electroscope to a positive net charge, via the two methods. Make sure to talk about the direction of electron movement. You should also draw some simple diagrams to illustrate charge transfer, polarity, etc. By direct charge transfer: By induction: 2. Describe a method that can be used to determine the polarity of charge on the electroscope. Some diagrams again might be helpful. 3. In Experiment 4 (Charge detection by an electroscope), the rubber rod was charged and slowly brought near the ball of the electroscope, but without making contact. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram. 4. In Experiment 5 (Charging the electroscope by contact), the rubber rod was charged, and the charge was transferred to the electroscope. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram. 5. In Experiment 6 (Determining the polarity of charge on an electroscope), the electroscope was once again charged using the rubber rod. Then a glass rod was slowly brought nearby. Describe what you observed. Explain what happened from individual charges perspective, and draw a diagram. 6. Where does the charge go when you touch the ball and the casing of the electroscope in Experiment 7 (Discharging the electroscope)? Answer with one sentence. 7. What does the aluminum cap do in Experiment 8 (Electrostatic shielding)? Answer in one or two sentences. 8. In Experiment 9 (Charging by induction), record your observations for each step (as described below), and write a one-sentence explanation for each, from the perspective of charges. “With one hand ground the scope terminal to the case (ie. Have one finger touch both the case and the terminal.) Keep it there while you bring a charged rubber rod towards the electroscope terminal, but not touching the rod to the terminal, with you other had. Observe the leaf. Now, while the rod is held near the terminal, remove your grounding hand from the electroscope completely (without changing the position of the charged rod). Observe the leaf. Finally, slowly withdraw the charged rubber rod. Observe the leaf. What happens? Is there a final charge on the scope? Test to determine the sign of the charge on the scope. What is it? Is it the same as the charge you would get b the direct contact method? Explain and draw diagrams showing what has happened. How did the charge get on the electroscope?”