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Chapter 20 Electrostatics Matter contains electric charges. The study of forces between charges at rest is called electrostatics. Electric charges Nucleus Neutron Proton Where charges come from Matter is made up of atoms (原子). An atom consists of a nucleus (原子核) at the center and electrons surrounding the Electron nucleus (Figure 20-1). The nucleus is All atoms are built of protons, made up of protons (質子) and neutrons electrons and neutrons. They (中子). A proton has a positive charge normally have equal numbers of (+). An electron has a negative charge (-),protons (+) and electrons(-) so they are neutral. which is equal in magnitude to the charge on the proton. A neutron has no charge. Usually atoms are electrically neutral (中性), that is, the number of protons is equal to the number of 1 electrons. The electrons sometimes get loose and can move around. When an atom loses an electron, the atom has a net (淨) positive charge because it now has one proton more than the number of electrons remaining in the atom (Figure 202-a). It becomes a positive ion (正 離子). Similarly, when an atom gains an electron, the atom has a net negative charge and is called a negative ion (負離子)(Figure 202-b). Therefore, ions are formed when atoms lose or gain one or more electrons. Nucleus Proton Neutron Electron (a) A positive ion Nucleus Proton Neutron Electron (b) A negative ion 2 Quantity of Electricity The quantity of electric charge, symbol Q, is called coulomb (庫倫), written as C. The charge of an electron = - 1.6 x 10-19 C. The charge of a proton = + 1.6 x 10-19 C. 1 18 1 coulomb = charge on electrons = charge on 6.25 x 10 1.6 × 10 −19 electrons. Therefore, 1 coulomb is a lot of charge. METHODS OF CHARGING Charging By Friction 3 repel repel attract 4 repel attract attract to positive negative repel attract induced charges 5 Experiment: 1. Rub 2 acetate (醋酸酯) strips (條 片) with a dry woollen cloth (棉 布), and then held side by side (Figure 20-3(a)). They repel each other. The acetate strips are said to be charged (充電). Charged acetate strip repulsion Like charges repel (a) 2. Rub 2 polythene (聚乙烯) strips with a dry woollen cloth, and then held side by side (Figure 20-3(b)). They repel each other. Charged polythene strip repulsion Like charges repel (b) 6 3. Rub an acetate strip and a Charged polythene strip with a dry acetate woollen cloth, and then held side strip by side (Figure 20-3(c)). They attract each other. Conclusion: Charged polythene strip attraction Unlike charges attract 1. Two charged strips made of the same material repel each other. Two (c) charged strips made of different materials attract each other. ⇒ Two different kinds of charges are produced by friction on these materials. 2. Because charges on the same material are the same, and charges on different materials are different. Like charges repel, unlike charges attract. 7 Note: 1. The two kinds of charges are called positive (+) and negative (-). When plastic materials are rubbed with a dry woollen cloth, the charges on them depend on the nature of the materials as shown in Table 20.1. Material Charge Acetate + Perspex 不碎透明膠 + Polythene - PVC 聚氯乙烯 - 8 Explanation of charging by friction 1. Initially, a woollen cloth and an acetate strip both have equal number of electrons and protons. Therefore, they are neutral or uncharged (Figure 20-5(a)). 2. When they are rubbed together, some of the electrons from the atoms on the surface of the acetate strip are pulled away and transferred to the woollen cloth (Figure 20-5(b)). Then the acetate strip has a net positive charge and has fewer electrons than usual. The woollen cloth has a net negative charge and has more electrons than usual. rubbing acetate (a) woollen cloth acetate Both charged (b) after rubbing woollen cloth 9 rubbing acetate (c) Only net charges are shown woollen cloth acetate Both charged (d) after rubbing Only net charges are shown woollen cloth 3. Figure 20-5(c) and (d) show the same process but only the net charges are shown in the figures. rubbing polythene (a) woollen cloth Only net charges are shown polythene Both charged (b) after rubbing woollen cloth 4. When a polythene strip is rubbed with a dry woollen cloth, electrons are transferred from the cloth to the polythene strip (Figure 20-4). Thus, the polythene strip is negatively charge and the woollen cloth is positively charged. 10 rubbing acetate (c) Only net charges are shown woollen cloth rubbing polythene (a) woollen cloth Only net charges are shown acetate Both charged (d) after rubbing polythene Both charged (b) after rubbing Only net charges are shown woollen cloth woollen cloth Note: 1. Equal and opposite charges are produced by a redistribution (再分 配) of some electrons from one material to the other during rubbing. ⇒ Rubbing only separate charges, because the principle of conservation of charge said that: Charge cannot be created nor destroyed. ⇒ The net charge on the strip is equal to the net charge on the woollen cloth but they are of opposite sign. 11 2. An uncharged object becomes positively charge if it loses electrons and negatively charge if it gains electrons. Positive charge, i.e. proton, cannot be transferred by rubbing. Conductors (導電體) and insulators (絕緣體) Definitions: Conductors are materials which allow electrons to readily (容易地) flow through them. Insulators are materials, which do not allow electrons to readily flow through them. 12 free electron positive ion electron positive ion In metals, some electrons, called free electrons (自由電子), are loosely held by the atoms and can move freely between atoms (Figure 20-6). In an insulator, the electrons are tightly bound to the atoms and are not free to move from place to place (Figure 20-7). Table 20.2 lists some common conductors and insulators. Good conductors Poor conductors Insulators metals, especially gold, silver, copper, aluminium water human body earth rubber plastic, e.g. acetate, perspex, polythene, PVC carbon semiconductors, e.g. germanium, silicon glass moist air dry air 13 Note: 1. An insulator can be charged by friction. It is because the charge on the insulator remains on it and does not escape easily. 2. Conductors can be charged by friction. However, the charge cannot be detected, because it is immediately conducted from the conductor to the hand and then to the earth (Figure 20-8(a)). The flow of electrons to the earth can be prevented by adding an insulating handle on the conductor so that the charge cannot not flow away (Figure 20-8(b)). conductor conductor insulating handle (a) electron flows to earth (b) free electron 14 Attraction of uncharged Objects Conductors acetate conductor When a positively charged acetate rod attraction is placed near a light uncharged induced conductor, the free electrons in the charges conductor are attracted and move near (a) repulsion the positively charged rod (Figure 20attraction > repulsion 9(a)). Therefore, the near side Only net becomes negatively charged and the charges are shown far side becomes positively charged. The positively charged rod attracts the near side and repels the far side. Because the separation between positive charges on the rod and negative charges on the conductor is smaller, the attraction is stronger than repulsion. So there is a net attraction on the conductor and the conductor is pulled towards the charged object. 15 If a negatively charged polythene rod is brought near, the free electrons in the conductor are repelled to the far side (Figure 20-9(b)). Therefore, the near side becomes positively charged and the far side becomes negatively charged. However, the net force on the conductor is still attractive because the positive charges on the conductor is nearer to the rod than the negative charges. The conductor is thus pulled towards the charged object. polythene attraction (b) induced charges conductor repulsion attraction > repulsion Only net charges are shown 16 Film show time Charge Propelled Cylinder Start 17 Film show time Metal Rod Attraction Start 18 Insulators When a positively charge rod is placed near an uncharged insulator, the electrons and the positively charge nuclei of the atoms experience attraction and repulsion respectively (Figure 20-10). The electrons tend to move nearer to the rod and the whole atom is deformed (使成畸形). The two ends of each deformed atom bear ( 帶) opposite charges. Thus, the insulator behaves as if it has negative charges at the near side acetate attraction insulator electron positive ion repulsion attraction > repulsion and positive charges at the far side. Since the negative charges on the insulator is nearer to the rod, attraction is greater than repulsion. Thus, the insulator is attracted by the charged rod. 19 Note: 1. The charges appear on an uncharged object because of the presence of nearby charged object are called induced charges ( 感生電荷). 20 21 B 22 C Charging By An E.H.T. Power Supply An E.H.T. supply (超高壓電源) pulls electrons from the positive terminal and pushes the electrons to the negative terminal of the supply. 23 Film show time 20.1 Charging by an EHT supply Start 24 the same 25 the same as do do not 26 Experiment: wooden rods on retort stand 1. Connect the circuit as shown in Figure 20-11(a). aluminium Both aluminium strips are strip connected to the positive terminal and the negative terminal is earthed (接地 E.H.T. power supply ). When the E.H.T. supply is switched on, electrons repulsion are drawn from the strips Like charges repel connected to the positive (a) terminal to the negative terminal and then to the earth. The strips lose electrons and become positively charged. Hence, they repel each other. 27 2. Connect the circuit as shown in Figure 2011(b). Both aluminium strips are connected to the negative terminal and the positive terminal is earthed. When the E.H.T. supply is switched on, electrons are drawn from the earth through the positive terminal to the strips connected to the negative terminal. The strips become negatively charged and repel each other. wooden rods on retort stand aluminium strip E.H.T. power supply repulsion (b) Like charges repel 28 3. Connect the circuit as wooden rods on retort stand shown in Figure 2012(a). When the supply aluminium is switched on, strip electrons are drawn from the strip E.H.T. power supply connected to the positive terminal to the strip connected to the attraction negative terminal. The Unlike charges strip connected to the (a) attract positive terminal becomes positively charged and the strip connected to the negative terminal becomes negatively charged. Therefore, they attract each other. 29 4. When an aluminium strip wooden rods on is connected to the retort stand negative terminal and the rubbed positive terminal is polythene earthed, the aluminium strip strip becomes negatively E.H.T. power supply charged. A negatively charged polythene strip, aluminium strip which is charged by friction, is then brought Like charges repel (b) near the aluminium strip (Figure 20-12(b)). They repel each other. Therefore, electric charges are the same, whether produced by friction or a power supply. It is because electrons are identical (完全 相同), only the method of producing the charges changes. 30 Van de Graaff Generator (范德格拉夫起電機) A Van de Graaff generator (Figure 20-13) produces a large and continuous supply of electric charge. A rubber belt is charged by friction. The charge produced is carried by the belt to the metal dome (圓頂) where it is collected. metal dome receives and stores positive charge insulating column rotating belt polythene roller rubber motor metal base 31 Film show time Experiments with Van de Graaff Generator Start 32 Film show time 20.2 Van de Graaff Generator - electric forces Start 33 Being uncharged, the polystyrene ball is first attracted to the dome. On touching the dome, the ball acquires the same charge as that on the dome and so is immediately repelled. 34 Film show time Van der Graaff Generator Start 35 Film show time 20.3 Van de Graaff Generator - hair rising experiment Start 36 Film show time 20.4 Van de Graaff Generator - electric sparks Start 37 The hairs stand on ends. Carrying the same kind of charge, the hairs repelled one another and so stand on ends. Sparks are produced as electric charges pass from the dome through the 38 air to the metal sphere and then to the earth. An electric current passes through the microammeter. This is due to the flow of charges from the dome to the earth via the microammeter. electric charges current 39 Film show time Jumping Pie Plates Start 40 Experiment: 1. Suspend a polystyrene ball by a nylon thread from an insulating rod (Figure 20-14 (a)). Since the polystyrene ball is initially uncharged, it is attracted to the dome. On touching the dome, it acquires the same charge as that on the dome and so is repelled. Like charges repel (a) 41 2. Connect a small metal sphere S on a conducting stand to the base B of the Van de Graaff generator (Figure 20-14 (b)). With a continuous supply of charge to the dome D, sparks (火花 (b) ) are produced between D and S if their separation is small. The charges on the dome D jump across to S, flow down the conducting base and through the connecting wire to the base of the generator. D spark S The flow of positive charge Connecting wire Conducting base B 42 3. Put a head of hair on the dome and start the Van de Graaff generator (Figure -(a)). The hairs on the dome share some charges from the dome. Since the hairs carry the same type of charge, they repel each other and so stand on end (豎起). 4. Connect the Van de Graaff generator to a light beam galvanometer (光束電流計), which can measure a small current, with two wires (Figure (b)). When the generator is switched on, the light beam deflects showing that the flow of charge is the same as the flow of electric current. Like charges repel (c) Flow of electric charge is electric current (d) 43 Light beam galvanometer 44 Gold leaf electroscope (金泊檢電器) (Out of Syllabus) A gold leaf electroscope consists of a metal cap piece of gold leaf suspended (懸掛) from a metal plate with a metal cap on top. The gold leaf is enclosed inside a metal case with glass windows (Figure 20-15). A gold leaf electroscope is used to 1. detect charges, and insulation metal plate gold leaf glass window metal case 2. determine which type of charges an object has. 45 Detection of charges When a charged object (either positively or negatively charged) is brought near an uncharged electroscope, the leaf (a) (b) rises showing When the gold leaf of an uncharged electroscope charges on the object rises, a charged object is brought near. (Figure 20-16). When the charged object is removed, the leaf drops. 46 Determining the sign of charge on an object To determine the type of charge on an object, an electroscope is charged so that charges are distributed on the cap, metal plate and the gold leaf. Since like charges repel, the charges on the metal plate repel the charges on the gold leaf and therefore the gold leaf rises. If the electroscope gains more charges, the leaf rises more due to larger repulsion. 47 Induced charges on an initially uncharged object (a) (b) (c) Induced charges on an initially uncharged object (d) (e) (f) When the gold leaf of a charged electroscope rises, a charged object with the same kind of charge as the electroscope is brought near. 48 From Figure 20-17, the only sure test of charge on a body is to obtain a rise of the gold-leaf of a charged electroscope. The charge on the electroscope and the test charge are of the same kind. Table 20.3 summarizes the result in Figure 20-17. Charge on electroscope Charge brought near electroscope Effect on gold-leaf + + rises - - rises + - drops - + drops + or - Uncharged body drops 49 Film show time Charging By Sharing (授受起電) Start 50 Experiment: 1. Charge two electroscopes with the same amount of charge with an E.H.T. supply. metal plate insulating handle (a) 2. Use two metal plates of different sizes to touch the electroscopes and observe the drops of the gold leaves when the plates are removed. Result and conclusion: 1. After the metal plate is removed, the leaf of the electroscope drops showing that charges flow from the electroscope to the metal plate (Figure 20-18). ⇒ Charges flow between charged conductors in contact giving them both a share of the same charge. The electroscope is said to be discharged (放電) and the metal plate is charged (充電). ⇒ Charges are shared between the two charged conductors. 51 2. If a larger metal plate touches the electroscope, the leaf falls more (Figure 20-18(b)). ⇒ A larger conductor gets a larger share of charge. If the conductor is very large, there is almost no charge left on the electroscope. Note: 1. If the size of an uncharged metal ball B is equal to the size of a charged metal ball A (Figure 20-19), both balls will share equal amount of charge. (b) B A A B Size of A = size of C ⇒ Charge on A = Charge on C 52 2. When two equal size but oppositely charged conductors are brought together (Figure 20-20), the negative charges are neutralized by the positive charges. The excess positive charges are then shared equally between the two conductors. Y X X Y Size of X = size of Y ⇒ Charge on X = Charge on Y Earthing (接地) Definition: Earthing is the process of sharing charge with the earth. Earth a Negatively Charged Electroscope Stop 53 Earth a Positively Charged Electroscope Play To remove the net charge on a conductor, an extremely large conductor is needed to share the charges with the conductor. The largest conductor is the earth. When a charged conductor is connected to the earth by a metal wire (Figure 20-21), the net charge flows from the conductor to the earth, and so practically no net charge remains on the conductor. Therefore, the conductor becomes uncharged. Charged metal sphere electron flow connected to earth Earth 54 Discharging Through Air Air contains some positive ions and electrons. When charged objects are left in air, charges of opposite kind will be attracted towards the charged objects (Figure 20acetate 22). The charged objects are then gradually neutralized by the charges in air and are electron discharged. Charges with the same kind to that of the charged objects are also repelled by the objects and so moved away from them. A charged object can be discharged easily when: polythene Only Net charges on the rods are shown positive ion 1. the air contains more ions which can be done by heating the air with flame or put a radioactive source in air, 2. the air contains a lot of water, that is, the humidity is high. 55 Charge Distribution On A Conductor The net charge on a conductor 1. stays on the surface of the conductor, and 2. is mostly concentrated at the places where the surfaces are sharply curved (Figure 2023). spherical conductor pear shaped conductor insulating stands Note: 1. The above results only apply to the surfaces of conductors and not to insulators because the charge cannot flow to establish any particular distribution in an insulator. 56 Charging By Induction (感應起電) Definition: Electric induction (電感應) is the change in the charge distribution of an object under the influence of a nearby charged object. Charging By Induction: Separating Conductors Play 57 Charging by induction: separating conductors 1. Place two insulated metal spheres A and B in contact and thus they form a single conductor (Figure 20-24(a)). 2. Bring a negatively charged rod near to A (Figure 20-24(b)). The rod pushes free electrons from A to B. Therefore a positive charge is induced on A and a negative charge is induced on B. spherical conductor B A insulating stands Sphere A and B are put in contact. (a) free electrons flow B A Free electrons in sphere B are pulled towards sphere A. (b) 58 3. Move B a short distance away from A while keeping the charged rod in position (Figure 20-24(c)). Therefore, the excess electrons on B cannot return to A. 4. Remove the charged rod (Figure 20-24(d)). A ends up with positive charge and B with negative charge. The charges on A and B spread over their conducting surfaces so that they are far enough apart not to affect each other. B A Sphere A and B are separated while the charged rod is held in position. (c) B A Remove the charged rod. (d) 59 e.g. Two metal spheres can be charged by using a positively charged rod. Use Figure 20-25 to show how this can be done. spherical conductor B A free electrons flow B A insulating stands (a) (b) B (c) B A A (d) Simulation Programs 20.1 Charging by induction and separation 60 Note: 1. The method of charging conductors without contact is called charging by induction. The charges on the spheres are called induced charges. 2. Sphere A has received charge of opposite sign to that on the charged rod. Sphere B has received the same kind of charge as that on the charged rod. 3. Induction only separates charges. It does not create charges. The charges on sphere A are equal in magnitude to the charges on sphere B but of the opposite sign. B B A A 4. No charge is loss from the charged rod. 61 Charging By Induction: Earthing a Conductor Play 62 Charging by induction: earthing a conductor free electrons flow spherical conductor insulating stands Electron B A Bring a charged rod near an insulated sphere. (a) Electrons move from B to A. flow B A Earth the sphere with a finge r. Electrons are (b) attracted from earth to replace the missing electrons on B. 1. Bring a positively charged rod near an insulated metal sphere (Figure 20-26(a)). The free electrons in the sphere are pulled to the side A. Therefore negative and positive charges are induced on side A and B respectively. 2. Earth the sphere by touching it with a finger (Figure 20-26(b)). Some electrons are attracted from the earth to the sphere to replace the missing electrons on side B. Therefore side B becomes uncharged. 63 B (c) A Remove the finger. Electrons cannot flow back to earth. B A (d) Remove the charged rod. 3. Remove the finger from the sphere so that electrons cannot flow back to the earth (Figure 20-26(c)). Therefore, the sphere has a net negative charge. 4. Take away the charged rod and the sphere is left with a net negative charge (Figure 20-26(d)). The negative charges spread over the surface of the sphere to reduce the force of repulsion. 64 e.g. A metal sphere can be charged positively by using a negatively charged rod. Use Figure 20-27 to show how this can be done. spherical conductor B insulating (a) stands (c) B A B A (b) A (d) B A Simulation Programs 20.2 Charging by induction and earthing 65 Film show time Charging an electroscope by induction Start 66 Film show time Charging an electroscope by induction Start 67 Charging an Electroscope Positively By Induction Play 68 Charge a Metal Plate Positively By Induction Play 69 Film show time Pitch Balls Start 70 Film show time Beer Can Pitch Balls Start 71 Note: 1. The induced charge on the sphere is always opposite to that of the charged rod. B 2. Insulators cannot be charged by induction, because the electrons are not free to move in B an insulator. Compare different methods of charging Table 20.4 compares different methods of charging. A A Methods of charging Conductor Insulator By friction By E.H.T. supply By sharing By induction 9(with insulating handle) 9 9 9 9 x x x 72 1. For an insulator, the charges cannot move. Once the insulator is charged, the charges stay in the place where they are produced. Therefore, insulators can be charged by friction. 2. A conductor readily shares the charges with other conductors in contact because the charges in a conductor are free to move. However, an insulator does not share charges with other conductors or insulators. Therefore an earthed conductor discharges immediately, but an earthed insulator does not. 73 3. The advantages of charging by induction over charging by sharing are shown in Table 20.5. Charging by induction Charging by sharing Charge on the original charged rod remains constant. Charge on the original charged conductor is shared. Can produce many charged conductors by repeating the process of charging. Charge on the original charged conductor is getting less and less as more conductors are charged. For charging by induction using single conductor and earthing, the charges on the charged conductor always have the opposite polarity to that of the original charged rod. The charged conductor produced has the same type of charge as the original charged conductor. 74 C B 75 ELECTRIC FIELDS When a charged object B is brought near another charged object A (Figure 20-28), B may move away from or towards A depending on the charges on B. Such effects are the result of an electric force, which acts on any charged object B by A. This electric force can act at a distance (遙 距作用). We call this influence around a charged object A its electric field. B repulsive force A attractive force B 76 Definitions: An electric field is a space or a region where an electric charge experiences an electric force. F F The direction of an electric field at a particular place is the direction of the force it produces on a positively charged object (Figure (a) 20-29(a)). Therefore, the force acting on a negatively charged object will be in the opposite direction to that of the electric field causing the force (Figure 20-29(a)). P The thin arrow shows the direction of the electric field and the line itself is an electric field line. The arrows labelled F show the direction of the force on a charge in the electric field. 77 Electric Field Lines Definition: An electric field line is a line drawn in an electric field such that its direction at any point gives the direction of the force on a small positive charge placed at that point (Figure 20-29). F F F P (a) N F (b) The thin arrow shows the direction of the electric field and the line itself is an electric field line. The arrows labelled F show the direction of the force on a charge in the electric field. 78 If a small positive charge were free to move, it would move along an electric field line. However, the force acting on a negative charge is opposite to the direction of the electric field lines (Figure 20-29). F F F P (a) N F (b) Java Applet* Electric field Pattern Properties of Electric Field Lines 1. Electric field lines start at a positive charge (Figure 20-29(a)) and end at a negative charge (Figure 20-29(b)). They do not start or end in empty space. 79 2. The direction of the electric field lines gives the direction of the electric field. Strong field region Weak region P (a) field Triple the magnitude of the charge is represented by triple the electric field lines. Density of the field lines is proportional to the magnitude of electric field at a place. P (b) 3. The number of electric field lines attached to a charge is proportional to the magnitude of the charge (Figure 20-30). 4. The density of the electric field lines is proportional to the magnitude of the electric field (Figure 20-30(b)). 80 5. Electric field lines do not have branches and do not cross each other. It is because electric field has only one direction at a point (Figure 20-31). P Java Applet* Electric field Pattern 81 Field Patterns Metal electrodes of different shapes E.H.T. power supply Experiment: Shallow glass dish containing caster oil to a depth about 5 mm, covering the electrodes 1. Place a pair of metal electrodes in a shallow glass dish so that they just covered by a layer of castor oil (菎油) (Figure 20-32). 2. Connect the electrodes to an E.H.T. power supply, which is set at about 4000 V. 3. Sprinkle (灑) tiny grains or needles of an insulating material onto the surface of the oil. (Grass seed, semolina powder (粗粒小麥粉) or hairs work quite well.) 4. Repeat the experiment with electrodes of different shapes 82 Film show time 20.5 Electric field patterns Start 83 84 electric field positive negative 85 Results: The needles receive induced opposite charges at their ends. As the electric field between the electrodes causes forces to act on these charges (Figure 20-33), the needles become aligned (排列) in the direction of the electric field. The electric field line and the shape of the electric field are made visible as the needles link together forming lines between two electrodes. metal electrodes semolina powder 86 Figure 20-34 shows the electric field patterns produced by different electrodes. Java Applet* Electric field Pattern (a) (c) Parallel electrodes with unlike charge Two like charges on point electrodes (b) (d) Two unlike charges on point electrodes Two like charges on point electrodes A point charge and a straight electrode (e) A point charge surrounded by circular electrode (f) 87 1. The electric field between two parallel electrodes are called parallel field (Figure 2034(a)) because the electric field lines are parallel to each other. The density of the electric field lines is constant and so the electric field is a uniform electric field. (a) Parallel electrodes with unlike charge (e) A point charge surrounded by circular electrode 2. In the space between a point charge surrounded by circular electrode, the electric field lines extend from the point charge to the circular electrode along the radii (Figure 20-34(e)). This electric field is called radial (徑向) field. 3. Electric field lines are perpendicular to metal surfaces (Figure 20-34(a), (e) & (f)) A point charge and a straight electrode (f) 88 B A 89 HAZARDS AND APPLICATIONS. Electrostatic Hazards Charges accumulate (積聚) easily on insulators due to friction. These charges can be very dangerous and may cause explosion. Electric shock when getting on or off a car When a car moves, air is rubbing it continually. The car is then charged up. A passenger may get an electric shock when he touches the car (Figure 20-35). The chance of getting an electric shock (電震) can be minimized if the car is earthed by connecting a metal strip to the car to pass the charge to the earth (Figure 20-36). For the same reason, an oil truck carries a metal chain at the back as it moves. 90 Wheels of aircraft are made of conducting rubber. Any charge built up on the body during flight is conducted to the ground on landing. A metal chain is connected to the wheel of an aircraft when it is being refuelled. If any charge is developed in the tyres, it will pass to the earth through the chain. 91 Nuisances in everyday life Static charges cause paper sheets to stick together. They also make dirt to stick on woollen clothes, fibres, carpets and television screens because of electrostatic attraction. Dirt attached to magnetic tapes by static charges can degrade (降級) the quality of sound and picture reproduced by tape recorders. Therefore, head of tape recorder should be cleaned regularly to remove this dirt. High quality tapes are anti-static, that is, they are slightly conducting and are hence not easily charged by friction. 92 Application of Electrostatics Electrostatic Precipitation (靜電沉澱法) Film show time Start 93 Film show time 20.7 Electrostatic Precipitator Start 94 positively charged An electrostatic precipitator (靜電 particles are 沉澱器) (Figure 20-37) removes deposited on the smoke and dust from the waste metal plates gases going up the chimneys of positively charged factories and power stations. A wire wire grid grid is kept highly charged so that a metal plates continuous stream of ions occurs between the grid and the earthed chimney metal plates. The ions attach themselves to the dust particles in the gas going up the chimney. The connection to earth charged dust particles are now repelled from the wire grid and smoke and dust particles attracted to the earthed plates. rising with the waste gases These plates are tapped (輕敲) from time to time so that the dust and smoke particles fall down the chimney and are removed. The removed particles are buried in nearby swamp (沼 澤) area and used to make bricks and cement. 95 PhotocopyingPhotocopying Photocopiers print by an electrostatic process. The main part of the machine is a rotating lightsensitive drum (鼓) onto which the image of the document is projected (Figure 20-38). The photocopying process is shown by steps in Figure 20-39. toner Light sensitive drum negatively charged toner Charged grid paper 96 (a) A charged grid moves over a light sensitive drum. (b) The drum is then positively charged. (d) Negative charged carbon particles (toner) is poured over the drum and stick to the positively charged image. (e) A sheet of paper is passed over the drum and receives a positive charge from the grid. (c) The document is projected on the drum. Positive charges disappear in area exposed to light. (f) The positively charged paper attracts toner from the drum and forms and image on it. (g) The paper is heated under infrared light for a few seconds to fixed the print. 97 Electrostatic paint spraying Makers of products like automobile bodies, file cabinets, and refrigerators have relied, almost only, on electrostatic paint spraying for years. 1. The paint droplets (小滴) emitted from the spray gun are charged with positive charges. The similarly charged droplets repel one another to form a large cloud (Figure 20-40 (a)). 2. As the charged droplets approach a target, they induce negative charges on the target’s surfaces. These negatively charged target surfaces pull the positively charged droplets toward them (Figure 20-40 (a)). Positively charged paint droplets follow the electric field lines Spray gun Electric field line Grounded target (a) 98 (b) 3. The charged droplets deposit uniformly on target surfaces because as each one is deposited on a target surface, the electrical charges balance at that place, making it no longer attractive to other incoming charged droplets. Therefore, the charged droplets following are pulled to remaining surface places that are still exerting attractive electrostatic forces (Figure 20-40 (b)) 99 4. A very desirable result of electrostatic spray systems is "electrostatic wraparound". By electrostatic charging of the paint, the individual paint droplets follow the electric field lines and encircle (包圍) the target (Figure 20-41). This means that, depending on the target, it is often necessary to spray only from one direction and the target is painted "all around". (a) (b) 100 Lightning conductor Action of Points Electrons attracted Positive ions repelled causes the electric wind Electron flow flame Dome of Van de Graaff generator Charges concentrate at the sharp point candle positive ion electron 101 Film show time 20.6 Point Action Start 102 Electrons attracted Positive ions repelled causes the electric wind Electron flow flame Dome of Van de Graaff generator Charges concentrate at the sharp point candle positive ion electron Ordinary air contains a certain number of positive and negative ions. Therefore, when a pin is connected to a Van de Graaff generator, ions of the same sign will be strongly repelled and ions of opposite sign will be strongly attracted (Figure 20-42). When the fast moving ions collide with air molecules, they often knock electrons out of them, creating more ions. Thus, a strong stream of ions may be created near a sharp point in a very short time. If a candle flame is put near the pin (Figure -), the flame is blown away from the point of the pin by the stream positive ions. Free electrons or negative ions in the air will also be attracted to the point of the pin and flow in the opposite direction along the wire back to the dome of the generator. This sometimes causes part of the flame to be 103 attracted to the pin. Lightning Conductor A lightning conductor (避雷針) is a very thick copper strip which connects to some sharp metal points fitted above the highest part of a building to a large metal plate buried deeply in the earth below the building. The conductor provides a path for electrons to flow easily in large numbers from the top of the building to the earth. 104 When a negatively charged thunder-cloud passes overhead (Figure 20-43), positive charges and negative charges are induced Positive ions spray off the on the sharp metal points and the points on the end of the earthed plate respectively. The conductor negative charges move to the earth. At the same time, negative ions in Induced positive charge at the points the air are attracted to the metal points and give up their excess electrons. These electrons then pass down the conductor and escape to the earth. The upward stream of positive ions spreads out and cancels some of the negative charges on the cloud. This lowers the chance of lightning striking the Large metal plate buried deeply in house. damp earth Negatively charged cloud induces positive charges on the ground and the buildings below Electrons attracted towards the points on the end of the conductor Electrons flow (theses come from the electrons above the points of the lightning conductor Thick copper strip fixed to the side of the building = the lightning conductor Electrons spread negative charge around in the earth 105 If lightning does occur, the charges pass harmlessly to the earth through the copper conductor. So damage to the building is prevented. 1 2 106 107 108 mirror 109 C A End of Chapter 20 110 Play Total internal reflection in water Simulation Programs 3.2 Total Internal Reflection Film show time diverging diverging behind Start 111 Initial Position -100 Initial velocity 400.00 Strobe time 2 0 Present Position Present velocity 100.00 No Acceleration 10 Present Time 10.00 Take Strobe Photo (Yes/No) Time interval 0.5 400.0 100.0 Start 112 lens (focus an image on the film) shutter (control the length of time the film is exposed) film (record the image) Go to shutter speed focusing ring (move the lens) aperture ring (control the size of the aperture) aperture diaphragm (adjust the amount of light entering the camera) Go to aperture 113 Stop 114