Charging by Induction PDF

11.6 Charging by Induction You have learned that objects can be charged by conduction when they come in contact with a charged object. However, the same charged...

11.6 Charging by Induction You have learned that objects can be charged by conduction when they come in contact with a charged object. However, the same charged object can also be used to charge a neutral object without contact. This process is charging by induction charging a called charging by induction. Objects can be temporarily or permanently neutral object by bringing another charged by induction. charged object close to, but not touching, the neutral object Charging Objects Temporarily by Induction Recall from Section 11.1 that when a charged object is brought near a neutral object it causes (induces) the electrons to shift in position, resulting in an uneven distribution of charges. This will only be temporary as the electrons will move back to their original positions once the charged object is taken away. Figure 1 shows a negatively charged balloon that is brought near a neutral wall. The electrons in the balloon repel the electrons in the wall, causing an induced charge separation in the wall (the electrons in the wall move away from the balloon). This creates a positive charge on the surface of the wall, which the negatively charged balloon is attracted to. The result is that the balloon moves toward the wall. The wall remains neutral because it still contains the same number of positive charges as negative charges. this side of the wall –– is positively charged – ++ + ++ – + – –– – – – –+ +– electrons in –+ + – +– + the wall move + +– – – –– – + – – – away from – – + +– – + –+ – – + – +– + +– –+ ++ + + – + + balloon –– –– – ++ ++ + – +– –– –– this side negatively negatively ++ +– of the wall charged charged balloon balloon attraction is negatively ++ +– charged –– (a) neutral neutral wallwall (b) neutral wall Figure 1 A negatively charged balloon (a) is brought near a neutral wall, causing an induced charge separation in the wall. (b) The wall becomes temporarily charged by induction. An everyday example of charging by induction occurs with the buildup of dust on the screen of a television or computer monitor (Figure 2). When a computer monitor or television screen is turned on it begins to build up a charge.C11-F21a-UDOS9SB.ai When a neutral dust particle comes near theC11-F21b-UDOS9SB.ai screen, the charge on C11-F21a-UDOS9SB.ai the screen induces an opposite charge on the near side of the dust particle and a charge, similar to that on the screen, on the far side. The result is that the dust is attracted to the screen. Figure 2 After being charged by induction, the dust is attracted to the computer screen. Ontario Science 9 SB Ontario Ontario Science Science 1010SBSB 0-17-635519-7 0-17-635528-6 0-17-635528-6486 Chapter 11 Static Electricity NEL FN C11-F21b-UDOS9SB FNFN C11-F21a-UDOS9SB C11-F21a-UDOS9SB CO CrowleArt Group COCO CrowleArt CrowleArt Group Group Deborah Crowle Deborah Deborah Crowle Crowle C11-F32-UDOS9SB.ai 6646_Sci9_ch11_pp478-501.indd 486 Pass 4th pass 8/3/09 2:20:39 PM  T RY THIS BENDING WATER SKILLS: Predicting, Observing, Communicating SKILLS HANDBOOK 3.B.3., 3.B.6., 3.B.9. You have read about charge interactions between solid objects. 4. Predict what you think will happen when you bring the charged In this activity, you will observe charge interactions between a balloon near, but not touching, the stream of water. Test your solid and a liquid. predictions and use a diagram to record your observations. Equipment and Materials: faucet; balloon 5. Try moving the charged balloon to the other side of the stream of water. Does the same thing happen? 1. Blow up the balloon. A. Does your observation from step 2 prove that both the 2. Run a gentle stream of water from a faucet. Place the balloon balloon and the water are neutral? Explain. T/I beside, but not touching, the stream of water. Record your observations in a diagram. B. Use your knowledge of electrons to explain your observations in step 4. T/I 3. Rub the balloon against your hair to charge it. C. Predict whether it is possible to bend water away from a charged object. Explain your reasoning. T/I Charging Objects Permanently by Induction An object can be permanently charged by induction by grounding the object. For example, consider a negatively charged ebonite rod and a neutral pith ball. When the rod is brought near but not touching the pith ball, the electrons in the pith ball are repelled by the electrons in the rod. As a result, the side of the pith ball closest to the rod becomes temporarily positively charged, while the side farthest from the rod becomes temporarily negatively charged (Figure 3(a)). If you then ground the negatively charged side with your hand, some of the electrons travel from the pith ball into your hand, and the pith ball is left with a positive charge. You could also remove the electrons by connecting a conducting wire to the ground (Figure 3(b)). When the conducting wire is disconnected from the pith ball, the pith ball is left with a permanent positive charge (Figure 3(c)). For the charge to To learn more about charging by be permanent, the ground must be disconnected or removed before the induction, charged object is removed. GO TO NELSON SCIENCE (a) (b) (c) Figure 3 (a) When a negatively charged ebonite rod is brought near a neutral pith ball, the electrons in the pith ball are repelled and it becomes temporarily negatively charged on its right side. (b) Attaching a ground wire to the pith ball conducts the repelled electrons on the right side into the ground. (c) After removing the ground wire, the pith ball remains permanently positively charged. Charging by induction always results in two objects with opposite charges. The object that induces the charge keeps its original charge, while the object whose charge was induced receives the opposite charge. NEL 11.6 Charging by Induction 487 6646_Sci9_ch11_pp478-501.indd 487 8/3/09 2:20:47 PM  Technological Applications of Charging by Induction Charging by induction has many useful applications, including forensics and air-cleaning technologies. Electrostatic Lifting Apparatus Footprints are often left behind at crime scenes. Investigators can use this important evidence to help determine who was present at the time of the crime. But how can you make a copy of a footprint if it is very difficult to see? Investigators use an electrostatic lifting apparatus (ESLA) (Figure 4). Figure 4 An electrostatic lifting apparatus is used in forensics to create Special film or foil is placed over the footprint. The black side of the film is a copy of a footprint from a crime scene. placed over the footprint. The film is then electrostatically charged. The dust and dirt particles from the footprint are attracted to the black side of the To learn more about being a film. The dust particles “jump” off the floor onto the black film, revealing the forensic investigator, details of the footprint. Now investigators have a copy of the footprint on the GO TO NELSON SCIENCE film that they can take to a laboratory to analyze. Electrostatic Speakers You have probably already heard the results of one application of charging by induction—that of electrostatic loudspeakers (Figure 5). These speakers are constructed of three thin layers. The outer two layers, called stators, are fixed in place and are made of a porous material. The inner layer is a flexible film called the diaphragm. In order to produce sound, the diaphragm must vibrate. This is accomplished using the principles of induction and the Law of Electric Charges. First, the inner surface is given a permanent electrical charge. Then an audio transformer is used to induce opposite charges in the two outer plates. This causes the diaphragm to move—as it is simultaneously attracted to one outer plate and repelled by the other (Figure 6(a)). The audio transformer then rapidly induces the static charges in the outer plates to reverse themselves. This causes the diaphragm to now move toward the opposite outer plate (Figure 6(b)). This reversing of charges on the outer plates happens repeatedly and at variable frequencies causing the diaphragm to rapidly vibrate back and forth between the plates, producing sound waves in the air. Figure 5 Electrostatic loudspeakers Electrostatic speakers have the advantage of being extremely thin and operate on the principles of induction. light weight. A disadvantage is their poor bass response. – – – – – – – +– + +– + + –+ +– –+ –+ –+ + – –+ –+ – +– transformer transformer + ++ + + + + + + ++ + + + + + + audio audio +– + +– + + –+ +– +–– –+ – +– –+–– + – –+ – +– – – diaphragm stator sound waves in air (a) sound waves in air (b) Figure 6 (a) In an electrostatic speaker, an audio transformer induces opposite electrical charges on two outer plates (stators) and then (b) reverses them. This process is repeated rapidly causing a flexible and charged inner membrane (diaphragm) to vibrate back and forth between the plates, producing sound waves in the air. C11-F33a-UDOS9SB.ai 488 Chapter 11 Static Electricity NEL C11-F33b-UDOS9SB.ai 6646_Sci9_ch11_pp478-501.indd 488 8/3/09 2:20:53 PM  IN SUMMARY A temporary charge imbalance can be induced Charging by induction produces a separation in a neutral object by bringing a charged object of charge in the object that is charged. near it. Charging by induction has many applications, Objects can be charged permanently by induction including electrostatic loud speakers and lifting by bringing a charged object near a neutral object footprints. and then grounding the neutral object. CHECK YOUR LEARNING 1. Identify a concept that you found particularly difficult or 3. Use diagrams to show how you would confusing. Have a classmate explain the concept to you. (a) induce a positive, temporary charge on the right side Write a brief description of the concept with your new of a metal ball understanding of it. Then help your classmate understand (b) induce a permanent negative charge on a metal ball a concept he or she found challenging. K/U C K/U C 2. Copy and complete Table 1 in your notebook. Assume that 4. Identify and explain one kind of technology that uses the object getting charged is neutral just prior to using the charging by induction. A charging method. K/U 5. The leaves on a metal leaf electroscope repel each other Table 1 Charges on Objects even though no other objects are near it. With the use of diagrams, explain what must have happened if no charged Object Object Explanation of Charging doing the getting the movement object ever touched the electroscope. T/I C method charging charged of charge 6. (a) What would happen if you charged a balloon by charging by positive rubbing it against your hair and then brought it near induction another balloon that is neutral? Explain. K/U (temporary) (b) What would happen if the charged balloon were charging by negative allowed to touch the neutral balloon? Explain. K/U induction (temporary) charging by positive induction (permanent) charging by negative induction (permanent) NEL 11.6 Charging by Induction 489 6646_Sci9_ch11_pp478-501.indd 489 8/3/09 2:20:53 PM