Static Electricity

1. Static Electricity

2. Learning objectives In this section, you’ll be able to: • State that there are positive and negative electric charges and charges are measured in coulombs (C). • State that like charges repel, unlike charges attract

3. Electric charge • Electric charge is measured in coulombs (C) • E For info: • A proton carries a charge of 1.6×10-19 C • An electron carries a charge of -1.6×10-19 C

4. Electric charge repel attract Unlike charges Like charges + + - + - -

5. Attraction between charged objects and neutral objects • A charged object can also attract a neutral conducting object. • This is because electrons in a neutral object can be attracted or repelled by a charged object and move to the side closer to the charged object. • Charging by induction works on this principle, which you will learn in later part of this topic.

6. Attraction between charged objects and neutral objects The object on the insulating stand is neutral, with equal number of positive and negative charges. A charged rod is brought near it. negatively Electrons are repelled to the side furthest from the negatively charged rod. The unlike charges attract. Electrons are attracted to the side nearest to the positively charged rod. The unlike charges attract.

7. Example M, N, O and P are small spheres which behave as follows: M repels N; M attracts O and O repels P. If P is positively charged, _____________. A N must be positively charged B N must be negatively charged C N must be positively charged or neutral D N must be negatively charged or neutral

8. Lesson from the example: A charged object can attract an uncharged/neutral object But a charged object cannot repel an uncharged/neutral object

9. Learning objectives In this section, you’ll be able to: • Describe an electric field as a region in which an electric charge experiences a force • Draw the electric field of an isolated point charge • Draw the electric field pattern between two isolated point charges

10. Electric Field • The attractive or repulsive forces between charges is known as an electric force. • An electric force is an example of a non-contact force. • An electric field is a region where electric charge experiences an electric force.

11. Representing an electric field • An electric field can be represented by lines with arrows • The direction of the field lines gives the direction of the force that acts on a small positive charge

12. Things to note when drawing field lines • Lines are to originate from the charge • Lines not to cross each other • Arrows are to be drawn on centre of each line • Arrows are to point away from a positive charge, and towards a negative charge • The lines should be close to each other when near the charge, and get more apart as the lines get further from the charge. (see next slide)

13. Strength of electric Field The strength of an electric field is indicated by how close the field lines are to each other. Closer field lines indicate a stronger electric field Field lines further apart indicate a weaker electric field

14. Electric field between 2 isolated point chargesEg. Between two unlike charges

15. Between two unlike charges

16. Between two positive charges

17. Between two positive charges X Note region X where an electric charge will not experience any electric force if placed there.

18. Between two negative charges

19. Summary An electric field is a region in which an electric charge experiences a force You must be able to draw the electric field of a point charge and between two point charges.

20. Learning objectives In this section, you’ll be able to: • show understanding that electrostatic charging by rubbing/friction involves a transfer of electrons

21. Charges in objects • All matter are made of atoms, and electrons are present in the region of space outside the nucleus. • Electrons can be delocalised from the atom.

22. Charges in objects Which of the following statements is/are true? • A positively charged object has lost electrons • A positively charged object has gained protons • A negatively charged object has lost protons • A negatively charged object has gained electrons and lost protons Note: Only electrons can move or be transferred from one object to another. • When an electron is removed from an atom, we say that the atom is positively charged. • When an electron is added to an atom, we say that the atom is negatively charged.

23. Electrostatic Charging • Electrostatic charging means to give an object a net charge • There are two ways: • Electrostatic charging by rubbing – for insulators • Electrostatic charging by induction – for conductors What are insulators and conductors?

24. Insulators • Insulators are materials where electrons are not free to move about inside the material. E.g. glass, silk, perspex and wool • Insulators do not conduct electricity and are charged by rubbing. The charge remains at the region where it was transferred.

25. Conductors • Conductors are materials that allow electrons to move freely within them. E.g. metals like copper, iron or steel • They are able to conduct electricity and are charged by induction. When electrons are gained or lost in a conductor, the electrons will be redistributed.

26. Videos https://www.youtube.com/watch?v=aO-phqmyqdY

27. Demonstration Comb and paper pieces

28. Examples of static around us…

29. Electrostatic charging by rubbing/ friction • Rubbing transfers electrons from one object to another. • When a glass rod and a piece of silk are rubbed together, some electrons from the surface atoms of glass is transferred to the silk.

30. Electrostatic charging between some common materials

31. Learning objectives In this section, you’ll be able to: • Describe experiments to show electrostatic charging by induction

32. Electrostatic charging by induction • Induction is the process of charging without any contact with the charging body • Insulators cannot be charged by induction.

33. Attraction between charged objects and neutral objects • Recall an earlier example • The conducting object is neutral and on an insulating stand. • Electrons are attracted to the side nearest to the positively charged rod. • We say that a negative charge is induced by the rod.

34. Animation

35. Charging a conductor by induction • Step 1: • Bring a positively charged glass rod near the metal conductor on an insulating stand. • The free electrons in the metal will be drawn towards the side nearer the positively charged glass rod.

36. Charging a conductor by induction Step 2: • Without removing the glass rod, earth the positively charged side of the metal conductor by touching it with your hand. (Earthing will be explained in next section) • Electrons will flow from the ground into the conductor. conductor

37. Charging a conductor by induction Step 3: With the glass rod still in place, remove your hand from the conductor.

38. Charging a conductor by induction • Step 4: • Remove the glass rod • The conductor is now negatively charged

39. Discharging charged conductors (not specifically in syllabus but good to know)

40. Discharging a charged conductor • A charged conductor can be neutralised by earthing it. • To earth a charged conductor is to provide a path for the excess electrons to flow away or flow to the conductor.

41. Discharging a charged conductor Earthing

42. Summary • There are two ways to charge an object: • Rubbing/friction – for insulators • induction – for conductors • In both ways, the net charge is always due to movement of electrons. • Charging through rubbing/friction involves transfer of electrons through contact, whereas for induction, no contact is needed.

43. Learning objectives In this section, you’ll be able to: • describe examples where electrostatic charging may be a potential hazard

44. Hazards of electrostatics • Lightning • Thunderclouds are charged by friction between the water molecules in the thunderclouds and air molecules. • It then ionises the air and the ionised air provides a conducting path for electric charge to be discharged to the nearest or sharpest object on the ground.

45. Hazards of electrostatics • If a negatively charged cloud passes overhead, it induces a positive charge at the top of the lightning conductor. • The point then repels positive ions to the cloud to neutralise it,so it is less likely to produce a lightning flash • The electrons that are attracted to the conductor travel down it to the earth. An electric current flows through the conductor.

46. Hazards of electrostatics • Electrostatic discharge • Excessive charges may build up due to friction • Electronic equipment, such as computer boards and hard drive, can be easily damaged. • Such equipments are usually packed in antistatic packaging materials.

47. Video https://www.youtube.com/watch?v=T6VKxmUPb3g

48. Learning objectives In this section, you’ll be able to: • describe the use of electrostatic charging in a photocopier, and apply the use of electrostatic charging to new situations.

49. Practical applications of electrostatics • Photocopier (Compulsory to know) • Laser printer (Refer to textbook) • Spray painting • Electrostatic precipitator

50. Photocopier https://www.youtube.com/watch?v=MJ5ghlTdF9k