2 Laws of Electrostatics Electrostatic charging contentsElectric chargesLaws of ElectrostaticsElectrostatic chargingConductors and InsulatorsElectric fieldsApplications of ElectrostaticsChapter Review2012 Sec 4 12 Static electricity AJL
3 electric charges Two types of charges negative charges (-Q) include electrons and negative ions, and positive charges (+Q) include protons and positive ionsInsulators are discharged by passing them quickly through a bunsen flame.examples:Negative chargePositive chargepolythene (rubbed with wool)perspex (rubbed with wool)ebonite (rubbed with fur)glass (rubbed with silk)
4 electric charges and the electric forces between them has a SI unit of coulomb (C)charge carried by an electron is 1.6 x C
5 Law of electrostatics + + + - + - The law of electrostatics states thatlike charges repel and unlike charges attract.+strong repulsion+further weaker repulsion+-strong attraction+-further weaker attraction
6 simplified structure of neutral atom electrostatic charging+-epevery atom has a massive, positively-charged nucleuspositive charge on nucleus = total negative charge on electronsno net charge in the atom (neutral)simplified structure of neutral atomwhen one or more electrons are removed from an atom, atom is ionised and becomes a positive ionatom with excess electrons becomes negatively charged ionoppositely charged ions attract each other
7 each material with equal number of positive and negative charges A. Charging by rubbingWhen two different materials (especially insulating materials) are rubbed together, negative charges (electrons) will transfer from one object to another.before rubbingafter rubbingpolythene stripwooleach material with equal number of positive and negative chargescertain amount of negative (electrons) is transferred from the wool to the polythene strip
8 A. Charging by rubbingPolythene and perspex (cellulose acetate) can become charged with static electricity when rubbed with a dry woollen cloth.polythene (-)repelpolythene (-)perspex (+)attractTwo different types of charges can be produced by friction on the strips
9 Refer to resources from links at physics wiki A. Charging by rubbingballoon, clothing & wallcarpet, human body & door knobRefer to resources from links at physics wiki
10 B. Charging by induction 1. Using 2 conducting spheres & a charged rod: both given opposite charges (p. 320, Textbook)2. Using 1 conducting sphere, a positively charged inducing specimen (a rod) & earthing: to give sphere negative charges3. Using 1 conducting sphere, a negatively charged inducing specimen (a rod) & earthing: to give sphere positive charges
11 Step 1 Step 2 Step 3 Step 5 Step 4 2. Using 1 conducting sphere, a positively charged inducing specimen (a rod) & earthing: to give sphere negative chargesStep 5Step 4
14 B. Charging by induction No physical contact between the source of charge (inducing specimen) and the conductors.Can be repeated many times without loss of charge from the inducing specimen.Only electrons (negative charge) can flow.Earthing provides a path for electrons to flow:Away from a negatively-charged conductor, orTowards a positively-charged conductorE.g. touching with hand or a wire
15 C. Charging by contact Between conductors Charging pith balls Charged glass rod touches neutral pith ballsCharge transferred to pith balls of the same sign as on the rod repulsionAnother charged rod brought near pith balls, attraction opposite charge on rod compared to pith balls
16 E.g. 1Two metallic spheres are suspended by insulating threads as shown in the diagram below. They have the same number of opposite charges and they are brought together until they touch. As a result, they neutralize each other electrostatically.+-
17 Both spheres become electrically neutral (uncharged). E.g. 1(continued)Draw on the diagram below to show the correct charge distribution in the two spheres after neutralization.Both spheres become electrically neutral (uncharged).
18 A and B are identical metal-coated balloons. E.g. 2A and B are identical metal-coated balloons.AB–nylon thread(a) A and B attract when A is rubbed (–ve charge). Why? What is the resultant charge on B?+–+ve and –ve charges are induced on B as shown on the diagram.Attraction between A and +ve induced charge greater than repulsion between A and –ve induced charge they attract.B is neutral (has no resultant charge)
19 A and B are identical metal-coated balloons. –(b) If B is rubbed in the same way as A, what happens? Why?–––––––ABWhen B is rubbed, it has the same sign of charge as A.Objects of the same sign of charge repel A and B repel each other.
20 Van de Graff GeneratorHow the Van de Graff generator works
22 (a) Why do the girl’s hairs stand on-ends? The hairs of a girl stand on-ends when she touches the dome of Van de Graaff generator.(a) Why do the girl’s hairs stand on-ends?The ends of the girl’s hair are given the same sign of charge.They repel each other and stand on-ends.
23 (b) Why does she stand on plastic stool? She is insulated from the ground when standing on a plastic stool.(c) What if she stands on ground?If she stands on the ground, she is earthed. The dome will be discharged through her body, giving her an electric shock.Her hairs will not stand on-ends.
24 (d) What if she stands on a plastic stool and touches a boy on ground? Both the girl and the boy will get an electric shock. Her hairs will not stand on-ends.
25 Removing electric charges To neutralise an accidentally charged apparatus, remove these charges by discharging it.for insulators: pass the apparatus quickly through a bunsen flamefor conductors: touch the apparatus to allow the excess charges to flow through our body (called earthing).
27 conductors and insulators materials that allow electric charges to flow through them easily‘free electrons’ flow when an electrical force is applied to themexamples: metals, graphite (except diamond), solutions (acids, bases, salts), ionised gases and waterUses: electrical circuits, lightning conductors
28 conductors and insulators materials that do not allow electric charges to flow through them easilyelectrons are tightly bound to atoms; not free to moveexamples: rubber, glass, most plastics, dry wood and dry air
29 The triboelectric series A list that ranks various materials according to their tendency to gain or lose electrons. It usually lists materials in order of decreasing tendency to charge positively (lose electrons), and increasing tendency to charge negatively (gain electrons).
30 Download, install and use Electric field.exe 2.01 from An electric field is a region where an electric charge experiences an electric force.Electric fields between 2 chargesDirections of electric field and forceElectric fields of all configurationsElectric field of combination of chargesDownload, install and use Electric field.exe 2.01 from
31 electrostatic hazards Sometimes objects get dangerously charged by accident.Trucks which transport petrol or other inflammable liquids usually have a metal chain or conductive strip at the rear end dangling from the metal body to the ground.
32 electrostatic hazards many synthetic fibres (like nylon and acrylic) used in clothing are good insulators and are easily charged by rubbingpeople may pick up charges as they walk on carpets made of synthetic fibresin some situations like dry environment, sparks may be produced and the clothing may catch fire
33 E.g. 4A –ve charge is put at P. What is the direction of the electric force on the charge?Puniform electric fieldA Towards the right.B Towards the left.C No electric force.
34 What are the electric field lines due to a –ve charge? E.g. 5What are the electric field lines due to a –ve charge?–A BC There is no electric field if only one type of charge is present.–
35 And the denser the field lines, the __________ is the electric field. E.g. 6How do the field lines show the direction and the strength of an electric field?The arrows of the field lines show the direction of the force acting on a ________ charge at that point.And the denser the field lines, the __________ is the electric field.positivestronger
36 drum is charged as it rotates under the rod applications of electrostaticsphotocopieroperates on the principles of electrostaticsmakes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the darkcharged metal rodselenium coated drumdrum is charged as it rotates under the roddrum is evenly charged
37 applications of electrostatics photocopieroperates on the principles of electrostaticsmakes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the darkonly the dark areas remain chargedtoner particles are attracted to the charged parttoner
38 toner is transferred to the paper heat is supplied to ‘fix’ the toner applications of electrostaticsphotocopieroperates on the principles of electrostaticsmakes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the darktoner is transferred to the paperheat is supplied to ‘fix’ the toner
39 applications of electrostatics electrostatic paint sprayingPaint droplets from an aerosol become charged by rubbing against the nozzle of the spray. The car body is earthed during spraying.
40 E.g. 7 Why the paint droplets spread out as they leave the nozzle? Why is the car body earthed during spraying?State one advantage of using electrostatic paint spraying over normal spraying.As paint droplets leave the aerosol, they become charged by rubbing against the nozzle of the spray. Like charges repel, so the droplets repel each other and spread out evenly.The droplets would be attracted to the earthed car body.This gives a more even coating, ensuring that the paint reaches even the most inaccessible parts.
41 ash and dust collects on plate waste gases carrying ash and dust applications of electrostaticselectrostatic precipitatorchimney wallash and dust collects on platecharged metal rodearthed metal platenear the charged rods, air is ionised; the dust and ash pick up the ions and are attracted to the earthed plateswaste gases carrying ash and dustThe electrostatic precipitator is commonly used to clean the smoke coming out from industrial chimneys by removing fine ash and other dust from the waste gases.
43 Unlike charges attract Static Electricitycan be produced byis used byconsists ofMethod of chargingfrictioninductioncontactPositive andnegative chargesApplicationphotocopierspraying of paintobey lawsare found informbeware ofLike charges repelUnlike charges attractElectric fieldconductorsinsulatorslooks likePotential hazardssparksfires+-