1. Chapter 20 & 21 Electricity and the Nature of Matter Physics is Life http://www.stmary.ws/highschool/physics/home/videos/hyperphy sics/jenvan3.mov 1

2. 2 Objectives of Chapter 20 & 21 1. To study the evidence that all bodies of matter contain positive and negative electric charges 2. To learn how bodies acquire electric charges and how to detect and identify those charges. 3. To propose a theory to explain how bodies gain or lose electric charge. 4. To learn how to control the movement of electrons in charging and discharging bodies and setting up a continuous electric current. 5. To investigate how electricity is conducted through liquids and gases. 6. To examine the evidence leading to the discovery that there is a natural unit of electric charge equal to the quantity on the electron. 7. To study the evidence that supports the law of conservation of electric charge and learn to apply that law Physics is Life2

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4. Introduction You walk across the rug, reach for the doorknob and..........ZAP!!! You get a static shock. Or, you come inside from the cold, pull off your hat and......BOING!!! Static electricity makes your hair stand on end. What is going on here? And why do static problems only seem to happen in the winter?hat Physics is Life4

5. What is Static Electricity? Static electricity refers to the buildup of electric charge on the surface of objects.Static electricity electric charge The static charges remains on an object until they either bleed off to ground or are quickly neutralized by a discharge.ground Although charge exchange can happen whenever any two surfaces come into contact and separate, a static charge will only remain when at least one of the surfaces has a high resistance to electrical flow (an electrical insulator).electrical insulator The effects of static electricity are familiar to most people because we can see, feel and even hear the spark as the excess charge is neutralized when brought close to a large electrical conductor (for example a path to ground), or a region with an excess charge of the opposite polarity (positive or negative).electrical conductor The familiar phenomenon of a static 'shock' is caused by the neutralization of charge. Physics is Life5

6. Static electricity and electric current are two separate phenomena, both involving electric charge, and may occur simultaneously in the same object.electric currentelectric charge Static electricity is a reference to the electric charge of an object and the related electrostatic discharge when two objects are brought together that are not at equilibrium.electrostatic discharge An electrostatic discharge creates a change in the charge of each of the two objects. In contrast, electric current is the flow of electric charge through an object, which produces no net loss or gain of electric charge. Although charge flows between two objects during an electrostatic discharge, time is too short for current to be maintained. How does Static Electricity differ from Electric current? 6

7. Introduction A simple experiment will demonstrate the electrostatic phenomena. Nothing will. Take a polythene rod and place one end of it near some pieces of paper. Does anything happen? Experiment 1 Physics is Life7

8. Introduction Does the rod affect the paper after being rubbed? Rub the rod with a cloth and again place it near some pieces of paper as shown in the diagram below. When the rod is placed near the pieces of paper, some pieces of paper are attracted by the “rubbed” polythene rod. Experiment 1 Physics is Life8

9. Introduction This experiment tells us that the friction produced by rubbing the rod must have affected the rod in some way. We can do further experiments to discover the properties of such rods. Experiment 1 Physics is Life9

10. Introduction Further Electrostatics Experiments Experiment 2 Observation: The pith ball remains unaffected even when the uncharged glass rod is placed very near to it. Physics is Life10

11. Introduction Further Electrostatics Experiments Experiment 2 Observation: When the silk-rubbed glass rod is placed near the pith ball, the ball moves toward the rod. Deduction: The glass-rod is able to attract the pith ball once it is rubbed with silk. Physics is Life11

12. Introduction Further Electrostatics Experiments Experiment 3 Observation: When the fur-rubbed ebonite rod is placed near the pith ball, the ball moves toward the rod. Deduction: The ebonite-rod is able to attract the pith ball once it is rubbed with fur. Physics is Life12

13. Introduction Further Electrostatics Experiments Conclusion: The glass & ebonite rods are said to be charged after they are rubbed with silk & fur respectively. Only charged rods are able to attract the pith ball. Physics is Life13

14. Introduction Further Electrostatics Experiments Experiment 4 Observation: The angle displaced is less than the previous 2 angles in experiments 2 & 3. Deduction: The presence of the charged glass rod “weakens” the “charged state” of the charged ebonite rod. The presence of the charged ebonite rod “weakens” the “charged state” of the charged glass rod. Physics is Life14

15. Introduction Further Electrostatics Experiments Experiment 5 Observation: Repulsion occurs between 2 charged glass rods Deduction: All glass rods rubbed with silk are charged similarly. The charges in glass rods are thus identical & like charges repel each other. Physics is Life15

16. Introduction Further Electrostatics Experiments Experiment 6 Observation: Repulsion occurs between 2 charged ebonite rods Deduction: All ebonite rods rubbed with fur are charged similarly. The charges in fur rods are thus identical & once again like charges repel each other. ebonite Physics is Life16

17. Introduction Further Electrostatics Experiments Experiment 7 Observation: Attraction occurs between a charged ebonite rod & a charged glass rod. Deduction: Charges in the ebonite rod & glass rod are different. Unlike charges attract each other. Physics is Life17

18. Introduction Further Electrostatics Experiments Experiment 8 When you comb your hair and… … bring your comb over a pile of paper bits Physics is Life18

19. Introduction Further Electrostatics Experiments Experiment 8 What will happen? {A or B} A. B. 19

20. Introduction Further Electrostatics Experiments 4 A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects. Experiment 9 Physics is Life20

21. Introduction Further Electrostatics Experiments: Try this at Home! Light a light bulb with a balloon or rubber comb You Need: hard rubber comb or balloon a dark room fluorescent light bulb (not an incandescent bulb) SAFETY NOTE: DO NOT USE ELECTRICITY FROM A WALL OUTLET FOR THIS EXPERIMENT. Handle the glass light bulb with care to avoid breakage. The bulb can be wrapped in sticky, transparent tape to reduce the chance of injury if it does break. What to do: Take the light bulb and comb into the dark room. Charge the comb on your hair or sweater. Make sure to build up a lot of charge for this experiment. Touch the charged part of the comb to the light bulb and watch very carefully. You should be able to see small sparks. Experiment with touching different parts of the bulb. Experiment 10 Physics is Life21

22. Introduction Actually, the thing we call static electricity is an imbalance in the amounts of positive and negative charges found on the surface of an object. Physics is Life22

23. AIR SPARK 4 Rubbing action redistributes charge (unbalanced) 4 If enough charge builds up, we get discharge 4 Air spark is actually due to “breakdown” of air –neutral air molecules separate into ions (electrons are stripped away) –current can then flow through the “plasma-field” air –In essence, air becomes a “wire” for a short bit –this happens at 3 million volts per meter 1 cm spark then at 30,000 volts typical finger-spark may involve a few billion electrons Things you can do to reduce shock Physics is Life23

24. Lightning 4 Lightning is an unbelievably huge discharge 4 Clouds get charged through air friction 4 1 kilometer strike means 3 billion volts! 4 Main path forms temporary “wire” along which charge equalizes –often bounces a few times before equal 4 Thunder is the sound made by lightning. Depending on the nature of the lightning and distance of the listener, it can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder.lightning sonic shock wave 4 Lightning strikes in the U.S. Lightning strikes in the U.S. 4 More Information (Internet Link) More Information (Internet Link) Physics is Life24

25. Lightning Rods 4 Perform two functions –provide safe conduit for lightning away from house –diffuse situation via “coronal discharge” Charges are attracted to tip of rod, and “electric field” is highly concentrated there. Charges “leak” away, diffusing charge in what is sometimes called “St. Elmo’sFire”, or “coronal discharge ”“St. Elmo’sFire”coronal discharge Physics is Life25

26. Van de Graaff electrostatic generatorVan de Graaff electrostatic generator: simulates lightning from cloud to ground How this Works Physics is Life26

27. Physics is Life Van de Graaf Generator Demonstrations 1.Lightning/spark distance 2.Jumping balls in a box 3.Hair Raising 4.Deflect a Flame 5.Electric Wind 6.Blowing Bubbles 7.Encased in wire mesh 27

28. TRIBOELECTRIC SERIES 4 When we rub two different materials together, which becomes positively charged and which becomes negative? Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown here. Under ideal conditions, if two materials are rubbed together, the one higher on the list should give up electrons and become positively charged. You can experiment with things on this list for yourself. 4 Example#1: Rubbing rubber with fur makes rubber negative and rabbit fur positive. 4 Example#2: Rubbing glass with silk makes glass positive and silk negative. 4 Example#3: Rubbing your hand with scotch tape makes the tape negative and your hand positive. TRIBOELECTRIC SERIES your hand Rabbit fur glass your hair nylon wool fur silk aluminum paper cotton Wood Amber hard rubber polyester styrofoam polyethylene (scotch tape) teflon Physics is Life28

29. 4 If you take two pieces of tape-one on top of another and rub them with your hand, the tapes will be negatively charged. 4 But if you were to separate the tapes one strip of tape will be negative and the other will be positive. 4 What will happen to the charge if you were to stick these tapes back together? 4 Due to conservation of energy, the net charge of the tapes will still be negative. TRIBOELECTRIC SERIES Physics is Life29

30. Static Electricity; Electric Charge and Its Conservation In conclusion, Charge comes in two types, positive and negative; like charges repel and opposite charges attract. Physics is Life30

31. Static Electricity; Electric Charge and Its Conservation Benjamin Franklin (1706-1790) is credited for naming the two types of charge. He argued that whenever a certain charge is produced on one body in a process, an equal amount of the opposite type of charge is produced on another body.The positive and negative charges are to be treated algebraically, so that during any process, the net change in the amount of charge produced is zero. This is an example of a law that is now well established: the law of conservation of electric charge, which states that the net amount of charge produced in any process is zero. Physics is Life31

32. Electric Charge in the Atom To understand electrostatics it is first important to understand the basic structure of an atom. An atom is made up of 3 different sub-atomic particles. This is demonstrated in the following diagram showing an atom of beryllium. - - - - + + + + electron nucleus proton neutron Physics is Life32

33. Electric Charge in the Atom Atom is electrically neutral. Sometimes however, an atom may lose one or more of its electrons, or may gain extra electrons. In this case the atom will have a net positive or negative charge, and it is called an ion. In solid materials the nuclei tend to remain to fixed positions where as some of the electrons move quite freely. The charging of a solid object by rubbing is explained mainly by the transfer of electrons from one material to another. The electric force between the electrons and protons supplies the centripetal force to keep electrons in the atom. We will discuss the equation for the electric force in detail later in this presentation. Physics is Life33

34. Electric Charge in the Atom Polar molecule: neutral overall, but charge not evenly distributed Normally when objects are charged by rubbing, they hold their electrons only for a limited time and eventually return to the neutral state. The excess charges may be ‘leaked off” onto water molecules in the air. What do you think is going to happen if you bring a charged rubber rod to a steady stream of water? Physics is Life34

35. The electrons moving around the nucleus can be moved from an atom to another atom, and from object to object. These electrons will move depending on whether the material is a conductor or an insulator Some of the electrons in a conductor are held loosely by the atom. Such electrons move freely from atom to atom within the material. (Example: Metals) In insulators, the electrons are held tightly to the atom and are not able to move freely within the material. (example: Wood, fur, glass, etc.) Insulators and Conductors Physics is Life35

36. Insulators Materials that do not allow electrons to move freely inside them are called electrical insulators.electrical insulators An electrical insulator has electrons that are all in fixed positions. The addition or removal of electrons at any one part of the insulator does not result in the electrons in other parts of the same insulator to move. Thus, we say that the charge is localised (or confined) to the region. Insulators and Conductors Physics is Life36

37. Examples of insulators are wood, plastics, ebonite, glass, fur, silk. The method of charging by friction will only work when two insulators are rubbed against each other. When an insulator is charged by the friction method the charge remains on the surface of the material. This is because the charge cannot move through the insulator.charging by friction Insulators and Conductors Physics is Life Insulators keep electricity from leaving power lines. Glass, plastic, or ceramic insulators high up on power poles keep electricity from traveling down the pole to the ground. If an insulator breaks, or a power line becomes disconnected from the insulators that hold it up, the line can fall to the ground and energize the area around it with a lot of electricity. If you touch a downed line — or even the ground near the line — you could be hurt or killed. If a power line falls on a car and you touch the car and the ground at the same time, you would also get a shock. 37

38. Insulators A positively-charged insulator can be discharged (to lose all its charges) by passing it quickly over a flame. The air above a flame consists of many ions (both positive & negative). When a positively-charged insulator (excess positive charge) passes over a flame, the negatively-ions will be attracted to the positive charges in the insulator. This causes the positive charges to be neutralized by the negative ions. + + + - - - - - - + + + + + + Insulators and Conductors Physics is Life38

39. Insulators A positively-charged insulator can be discharged (to lose all its charges) by passing it quickly over a flame. The air above a flame consists of many ions (both positive & negative). When a positively-charged insulator (excess positive charge) passes over a flame, the negatively-ions will be attracted to the positive charges in the insulator. This causes the positive charges to be neutralized by the negative ions. + + + - - - - - - + + + + + + Insulators and Conductors Physics is Life39

40. Insulators A positively-charged insulator can be discharged (to lose all its charges) by passing it quickly over a flame. The air above a flame consists of many ions (both positive & negative). When a positively-charged insulator (excess positive charge) passes over a flame, the negatively-ions will be attracted to the positive charges in the insulator. This causes the positive charges to be neutralized by the negative ions. - - - + + + + + + Insulators and Conductors Physics is Life40

41. Discharging Insulators Summarising, all charged insulators can be discharged by passing them over a flame. Ions present in the air above the flame will be attracted towards the charges present in the charged insulators. These ions will neutralize the charges in the insulators, thus discharging them. Insulators and Conductors Physics is Life41

42. 42 Conductors Some materials allow electrons to move about easily inside them. These are called electrical conductors. - - - - - - Insulators and Conductors Physics is Life All metals are conductors of electricity. All conductors can be discharged easily by a method known as Grounding). Grounding 42

43. Conductors In electrical conductors, the outer electrons (also known as valence electrons) are loosely bound. They are relatively free from individual atoms. We say that these electrons are delocalized. When electrons are gained by the conductors, the other electrons will flow automatically so that electron re-distribution in the conductors occur. When electrons are lost by the conductors, the other electrons will also flow automatically so that electron re-distribution in the conductors occur. Insulators and Conductors Physics is Life43

44. Semi conductors vs. Super conductors 4 Semiconductors are materials which are good insulators in pure form, but their conducting properties can be adjusted over a wide range by introducing small amounts of impurities. Examples are silicon and germanium Semiconductors 4 Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273  C) Superconductors 4 Recently, “high temperature” (Above 100 K) superconductors have been discovered. Physics is Life44

45. Conduction in Liquids Liquids vary in ability to conduct electricity. A crude device for testing this ability is the electrolytic cell. As current passes through the electrolytic cell, the positive ions are attracted toward the cathode and the negative ions are attracted toward the anode. 45

46. Conduction in Gases Gases can be made to conduct electricity under conditions that give valuable information about the composition of their atoms. This can be done in a gas discharge tube consisting of two metal electrodes sealed into its opposite ends. The air in the tube is removed by a vacuum pump and is replaced by a small amount of gas to be studied. The electrodes of the tube are then connected source of potential difference. Current now passes through the tube and the gas emits light. 46Physics is Life

47. Induced Charge Metal objects can be charged by conduction: Physics is Life47

48. Induced Charge They can also be charged by induction: Physics is Life48

49. Induced Charge Nonconductors won’t become charged by conduction or induction, but will experience charge separation: Physics is Life49

50. Induced Charge 4 Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. 4 Like charges in the neutral object are repelled by the charged object. 4 Unlike charges in the neutral object are attracted by the charged object. 4 The neutral object returns to normal when the charged object is removed. 4 An object that is electrically neutral overall, but permanently polarized, is called an electric dipole. An example is the water molecule. Physics is Life Click here for simulation 50

51. Induced Charge; the Electroscope The electroscope can be used for detecting charge:electroscope Physics is Life thin gold leaves conducting rod earth insulator conducting disc earth metal case with glass window 51

52. Induced Charge; the Electroscope The charged electroscope can then be used to determine the sign of an unknown charge. Physics is Life52

53. Induced Charge; the Electroscope The electroscope can be charged either by (a) induction or by (b) conduction. Physics is Life53

54. Unit of Electric Charge The flow of water passing through a pipe may be described in terms of the quantity of water that passes through a given cross section of the pipe per unit time. Electric current may also be expressed as the quantity of charge that flows through any given point of a circuit per unit time. The SI unit used to measure the quantity of electric charge is called the columb (C). The columb is defined as the quantity of charge that flows though an electric circuit in one second when the current in the circuit is one ampere. One ampere is the current in which 6.25 x 10 -18 electrons pass through the circuit per second.Electric currentampere The current for an electric circuit can be expressed as: I = Q/t where I is the symbol for curent, Q is charge and t is the time 54Physics is Life

55. Measuring Electric Charge Sample Problem How many coulombs of electricity pass through a light bulb in 2 minutes if there is a current of 2 A passing through the circuit? t = 2 min = 2 x 60 s = 120 s I = 2 A Therefore Q = I t = (2) (120) = 240 C 55Physics is Life

56. Elementary Unit Charge The existence of a natural unit of electric charge was first discovered by Michael Farady in his study of the electric currents. His experiments were able to show that the smallest value for an elementary charge (e) was 1.60 x 10 - 19 C. This value is confirmed by measurements made by Robert Millikan and many others and is recognized as the quantity of negative charge on the electron, and the quantity of positive charge on the proton. Michael FaradyRobert Millikan Michael Faraday (1791 – 1867) Robert Millikan (1868 – 1953) 56Physics is Life

57. Elementary Unit Charge The charge on 1 proton is 1.6 x 10 -19 C The charge on 1 electron is –1.6 x 10 -19 C Charge on proton + Charge on electron = 1.6 x 10 -19 C + (-1.6 x10 -19 C) = 1.6 x 10 -19 C - 1.6 x10 -19 C = 0 C This is why when 1 proton & 1 electron are placed together, the net charge is zero. 57Physics is Life

58. Elementary Unit Charge Sample problem A strong lighting bolt transfers about 25 C to Earth. How many electrons are found? Number of electrons = Q/e = 25 C/1.6 x 10 -19 C/electrons = 1.6 x 10 20 electrons 58Physics is Life

59. Difference of Potential We have seen the free electrons move whenever they are repelled by other electrons and negative charges or attracted to a positively charges. If a negatively charged body is connected to a positively charged body by a conducting wire, the electrons are repelled by the negatively charged body and attracted to the positively charged body. As a result, electrons flow through the wire from the negatively charged body to the positively charged body. The is flow of electrons is called a current. Bodies with different concentrations of electrons are said to be at different potentials or have a difference of potential between them. When such bodies are connected by a wire conductor, electrons flow from one to the other until both bodies have the same concentrations of electrons. The differences in potential are measured in volts. 59Physics is Life

60. Simple Electric Current Electrical current flows in a loop, or a circuit. The image above shows a simple electrical circuit. The circuit has four important parts: a power source (a battery, in this case), a conductor (the wire), the load (the light bulbs), and a switch. Current flows from the positive side of the battery, through the bulbs, and back to the negative side of the battery. As the electric current makes a complete loop, both bulbs will light. The complete loop condition is known as a closed circuit. A current that flows continuously in one direction is called a steady state current and is abbreviated DC. An open circuit condition occurs when the path of current flow is interrupted. This can happen if the wire is severed at any point on the circuit, or if the wire is disconnected from either side of the battery. With an open circuit, current will not flow, and the bulb will not light. A switch is a device that enables a person to open or close the circuit whenever they choose. 60Physics is Life

61. Two kinds of electric charge – positive and negative Neutral objects are attracted to charged objects. Charge is conserved Conductors: electrons free to move Insulators: nonconductors Elementary unit of charge is 1.6 x 10 -19 C Current is equal to the charge divided by time (I=Q/t) Summary of Chapter 20 & 21 Physics is Life61