Lab 1: Electrostatics Edited 9/19/14 by Joe Skitka, Stephen Albright, DGH & NET

Transcription

1 Lab 1: Electrostatics Edited 9/19/14 by Joe Skitka, Stephen Albright, DGH & NET Figure 1: Lightning Exhibit, Boston Museum of Science Objective Students will explore the manifestation of the laws of electrostatics in a variety of simple devices. Unlike subsequent labs, there will be minimal quantitative analysis and no lab report. Instead, students must undergo an oral check out with TA s individually or in pairs before the end of the lab section. TA s will ask basic comprehension questions about the physical processes involved in the experiments. Students should still keep notes of observations for reference, as the procedure suggests. Introduction As long ago as 600 B.C., the Greek philosopher Thales knew that amber, when rubbed, would attract bits of paper and other light objects. Early experiments revealed that various materials became electrically charged when rubbed together. Ben Franklin defined two types of charge: +, the charge acquired by a glass rod when rubbed with silk, and, the charge acquired by a hard rubber rod when rubbed with fur. These charges can be detected and crudely measured with an electroscope. An electroscope consists of a pair of delicate conductors electrically and mechanically connected at one end. When a charged object touches the electrode of an electroscope, both leaves become charged with the same sign (polarity) and repel each other by a distance relative to their charge. Over time it became convention that red denotes positive charges, black negative charges, and green ground or zero charge. Some modern materials are more efficient charge generators than those used by Franklin. Glass rods, Teflon rods, silk & shrink-wrap will be provided in this lab. Other electrostatic devices provided in this lab include: Electroscopes, Van de Graaff generators, Faraday Ice Pails, a Wimshurst Machine, a Kelvin Water Dropper, and Franklin s Electrostatic Motor.

2 Theory Three primary theoretical concepts undergird the phenomena observed in this lab. First is that opposite charges attract. Second is that this force falls off in space as the inverse of the square of the distance between two charged point-like objects, resulting in Gauss s law, or that the flux of the electric field leaving a volume is proportional to the charge enclosed. The third is that in electrostatics, conductors are defined as objects which allow the free motion of charges and, therefore, cannot have electric fields within them (because electrostatics implies the system has had time to equilibrate). In addition to these fundamental concepts, it is useful to understand how charge differences might arise dynamically. One process, which is exploited in this lab, is that, when rubbed together, different materials will give up or retain electrons and become charged. This is because dissimilar materials will have distinct affinities for electrons based on their molecular orbital structure. All of the exercises in this lab involve clever ways to either generate a difference in charge or a way to make electrons do something interesting on their quest to find a position home. Procedure There are four stations setup in this lab, each with an electroscope, a Van de Graaff, and an assortment of charging materials. In addition, there are other devices around the room. Use this equipment to perform the following exercises. Part 1: Electroscope NOTE: In addition to a rubber rod, cats fur, a glass rod, and silk, each set up also contains a Teflon rod and shrink-wrap. Before starting any electroscope exercises, it is important to discharge the electroscope. To do this, hold a finger on the metal ball (electrode) sticking out of the top of the electroscope (see Figure 4), while touching a different finger to the case of the electroscope. In a fraction of a second, the electroscope will be discharged. Figure 2: Electroscope

3 1. Charge a Teflon rod by rubbing it with silk. Touch the rod to the electrode of the electroscope. Now touch your finger to the electrode. Record and explain what you observe. Discharge the electroscope and repeat the above procedure using the glass rod with shrinkwrap. 2. Use the electroscope to devise an experiment that demonstrates there are two types of charge. Be able to explain your procedure and reasoning. 3. Hold a charged rod close to, but not touching, the electrode of the electroscope. Momentarily touch the electrode with a finger. Withdraw the charged rod from the electroscope. Use the procedure devised in step 2 to determine the charge on the electroscope relative the charged rod. Explain what you observe. This is called Charging by Induction. Let the TA know that you ve completed part 1 before moving on. Part 2: Faraday s Ice Pail 4. On the tables in the middle of the room are hollow aluminum cans connected to electrometers. These are shielded from the outside by larger metallic cylinders. The inner cans are meant to behave as Faraday ice pails, which are nearly closed metallic surfaces with a small orifice in them. An electrometer, an electronic version of an electroscope, measures charge on the Faraday ice pail very precisely and without discharging them. 5. Charge a rod and rub it several times against a proof-sphere (which are the small metal balls on plastic wands). Place this charged proof-sphere inside one of the hollow metal spheres. Do not let the proof-sphere touch the ice pail. Observe the charge and polarity of the electrometer. What happens? Without touching the inside of the ice pail, move the charged proof-sphere around. What happens to the electrometer? Withdraw the proof-sphere. What happens to the electrometer? What would happen with an ideal Faraday ice pail (one with a very small orifice)? Note: an electrometer is a very sensitive, expensive, and fragile device. Please use it with caution. 6. As in step 5, insert a charged proof-sphere into the Faraday ice pail. Touch the proof-sphere to the inside of the ice pail. Withdraw the charged proof-sphere from the ice pail. What happens to the electrometer? Is the proof-sphere still charged? Should you be able to add more charge to the ice pail by repeating this process multiple times? If you are not sure, try it and explain what you observe. Let the TA know that you have completed part 2 before moving on. Part 3: Van de Graaff Generator Van de Graaff Operating Instructions: Before starting a Van de Graaff, make sure there is a long banana cable connected to the base of the Van de Graaff and that the other end of this cable can be easily reached without getting close to the Van de Graaff. Alternatively this cable can be connected to the bottom of the grounding sphere. The Van de Graaff has an On/Off Switch and a speed control. Do not operate the Van de Graaff at speeds higher than necessary. Operating at higher speeds causes the belt to deteriorate more rapidly. If the speed control is turned all the

4 way down, the belt will not move. However, if left in this position, the speed control, including the base of the Van de Graff, will get very hot. Therefore, do not use the speed control to stop the belt of the Van de Graaff, always use the On/Off switch. Before shutting off the generator, be sure to discharge the sphere by touching it with a grounding sphere or grounding wire. Figure 3: Van de Graaff Generator 7. Find the insulated rod that has a string and pith ball attached. Use the rod to touch the suspended pith ball to the dome of the operating Van de Graaff. Be careful not to get your body too close to the dome. Explain what happens when the pith ball initially comes close to the sphere as well as what happens after it touches the sphere. The string is a dielectric and will become polarized; however, be sure to consider the case of an ideal small conducting sphere in the absence of the string to answer the above question. 8. Turn off the Van de Graaff and place a stack of aluminum pie plates on top of the sphere. Turn on the generator. Record and explain what happens. 9. On the center tables are a variety of static electric devices. As a final task, select at least 2 of these devices and use the Van de Graaff to make them work. Be able to explain how the selected devices work. Check in with the TA when you ve completed the final part of the group exercises. Part 4: Class Demonstrations 10. The Kelvin Water Dropper (see Figure 5) is an unusual static generator that uses drops of water to generate an electric charge. Watch as the TA demonstrates this device. Using words and a schematic diagram, explain how this device generates charge.

5 Figure 4: The Kelvin Water Dropper 11. Warning: The Wimshurst machine is a powerful electrostatic generator that can create a 3 spark between its electrodes. Given that a 1 spark through dry air requires ~70,000 volts, a 3 spark indicates a voltage greater than 200,000 volts. This is similar to the voltages generated by a Van de Graaff and provided the Leyden jars of the Wimshurst machine are not in use, both generators produce very little current and the sparks generated are reasonably safe. However, when the Leyden jars are in use, the Wimshurst produces significantly higher currents making this 3 spark dangerous. Watch as the TA demonstrates this device. Note the difference in charging time and spark intensity when the Leyden jars are used. Be careful not to touch any part of the Wimshurst machine while it is working or charged. Explain the difference in charging time and spark intensity. [Challenge: How does the Wimshurst machine work?] 12. Benjamin Franklin created one of the first electrostatic motors using spokes and brass thimbles. (Scientific American Oct. 74, p see ejournals on Josiah). This is still referred to as Franklin s Electrostatic Motor. Watch as the TA demonstrates this device. How does it work?