Understanding Electrostatics: Charges, Laws, and Applications

1. Topic 12 Static Electricity

2. contents Electric charges Laws of Electrostatics Electrostatic charging Conductors and Insulators Electric fields Applications of Electrostatics Chapter Review

3. electric charges Two types of charges • negative charges (-Q) include electrons and negative ions, and positive charges (+Q) include protons and positive ions • Insulators are discharged by passing them quickly through a bunsen flame. • examples:

4. electric charges and the electric forces between them electric charge • has a SI unit of coulomb (C) • charge carried by an electron is 1.6 x 10-19 C

5. + + + + + - - + Law of electrostatics The law of electrostatics statesthat like charges repel and unlike charges attract. strong repulsion further  weaker repulsion strong attraction further  weaker attraction

6. p e + - - - - - - - - - - electrostatic charging • every atom has a massive, positively-charged nucleus • positive charge on nucleus = total negative charge on electrons • no net charge in the atom (neutral) simplified structure of neutral atom • when one or more electrons are removed from an atom, atom is ionised and becomes a positive ion • atom with excess electrons becomes negatively charged ion • oppositely charged ions attract each other

7. A. Charging by rubbing When two different materials (especially insulating materials) are rubbed together, negative charges (electrons) will transfer from one object to another. after rubbing before rubbing wool polythene strip certain amount of negative (electrons) is transferred from the wool to the polythene strip each material with equal number of positive and negative charges

8. attract repel polythene (-) polythene (-) perspex (+) polythene (-) A. Charging by rubbing Polythene and perspex (cellulose acetate) can become charged with static electricity when rubbed with a dry woollen cloth. Two different types of charges can be produced by friction on the strips

9. A. Charging by rubbing • balloon, clothing & wall • carpet, human body & door knob Refer to resources from links at physics wiki http://nyghsec3physics.pbworks.com

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 charges 3. Using 1 conducting sphere, a negatively charged inducing specimen (a rod) & earthing: to give sphere positivecharges

11. Step 3 Step 1 Step 2 2. Using 1 conducting sphere, a positively charged inducing specimen (a rod) & earthing: to give sphere negative charges Step 4 Step 5

12. B. Charging by induction

13. http://www.s-cool.co.uk/gcse/physics/static-and-current-electricity/revise-it/static-electricity

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, or • Towards a positively-charged conductor • E.g. touching with hand or a wire

15. C. Charging by contact • Between conductors • Charging pith balls • Charged glass rod touches neutral pith balls • Charge transferred to pith balls of the same sign as on the rod  repulsion • Another charged rod brought near pith balls, attraction  opposite charge on rod compared to pith balls

16. E.g. 1 Two metallic spheres aresuspended 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. 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 B E.g. 2 nylon thread A and B are identical metal-coated balloons. (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 B A and B are identical metal-coated balloons. (b) If B is rubbed in the same way as A, what happens? Why? – – – – – – A B When 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 Generator http://www.s-cool.co.uk/gcse/physics/static-and-current-electricity/revise-it/static-electricity • How the Van de Graff generator works

22. E.g. 3 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 flame • for conductors: touch the apparatus to allow the excess charges to flow through our body (called earthing).

26. Removing electric charges

27. conductors and insulators conductors • materials that allow electric charges to flow through them easily • ‘free electrons’ flow when an electrical force is applied to them • examples: metals, graphite (except diamond), solutions (acids, bases, salts), ionised gases and water • Uses: electrical circuits, lightning conductors

28. conductors and insulators insulators • materials that do not allow electric charges to flow through them easily • electrons are tightly bound to atoms; not free to move • examples: rubber, glass, most plastics, dry wood and dry air

29. 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).  http://www.siliconfareast.com/tribo_series.htm The triboelectric series

30. Electric field • An electric field is a region where an electric charge experiences an electric force. • Electric fields between 2 charges • Directions of electric field and force • Electric fields of all configurations • Electric field of combination of charges Download, install and use Electric field.exe 2.01 from http://www.physics-software.com/software.html

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 rubbing • people may pick up charges as they walk on carpets made of synthetic fibres • in some situations like dry environment, sparks may be produced and the clothing may catch fire

33. E.g. 4 A –ve charge is put at P. What is the direction of the electric force on the charge? P  uniform electric field A Towards the right. B Towards the left. C No electric force.

34. – – E.g. 5 What are the electric field lines due to a –ve charge? A B C There is no electric field if only one type of charge is present.

35. E.g. 6 How 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. positive stronger

36. applications of electrostatics photocopier • operates on the principles of electrostatics • makes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the dark charged metal rod selenium coated drum drum is charged as it rotates under the rod drum is evenly charged

37. applications of electrostatics photocopier • operates on the principles of electrostatics • makes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the dark toner particles are attracted to the charged part only the dark areas remain charged toner

38. applications of electrostatics photocopier • operates on the principles of electrostatics • makes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the dark toner is transferred to the paper heat is supplied to ‘fix’ the toner

39. applications of electrostatics electrostatic paint spraying Paint 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. chimney wall ash and dust collects on plate charged metal rod earthed metal plate near the charged rods, air is ionised; the dust and ash pick up the ions and are attracted to the earthed plates waste gases carrying ash and dust applications of electrostatics electrostatic precipitator The 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.

42. http://www.s-cool.co.uk/gcse/physics/static-and-current-electricity/revise-it/static-electricityhttp://www.s-cool.co.uk/gcse/physics/static-and-current-electricity/revise-it/static-electricity

43. - + - - + + + + - - + + - - + - Static Electricity can be produced by is used by consists of • Method of charging • friction • induction • contact Positive and negative charges • Application • photocopier • spraying of paint obey laws are found in form beware of Like charges repel Unlike charges attract • conductors • insulators Electric field looks like • Potential hazards • sparks • fires