Electrostatics

1. Basic Definitions 1 Electrostatics Next Slide Daily phenomena • “Charge” plastic materials by friction • Experience of electric force and electric charge • Repulsive and attractive electric force • Different kinds of charge : positive (+) and negative (-) • General Rule : like charges repel; unlike charges attract

2. Basic Definitions 2 Electrostatics Next Slide Nature of charges • Structure of atoms and matter Diagram • Balance of charge : no. of electrons = no. of protons • Neutral : no. of electrons = no. of protons Diagram • Positively charged : no. of electrons < no. of protons • Negatively charged : no. of electrons > no. of protons • Charging by changing the no. of electrons only

3. Basic Definitions 3 Electrostatics Next Slide Nature of charges • Unit : Coulomb (C) • Insulators (like plastics) : do not allow electrons to flow through them or escape easily • Conductors (like metals) : allow electrons to flow through them or escape easily • Charging by friction Diagram • Induced charge Diagram

4. Ways of charging Electrostatics Next Slide Different ways of charging • Charging by extra high tension power (EHT) supply Photo • Charging by Van de Graaff generator Photo • Charging by sharing Diagram • Charging by induction and separation Diagram • Charging by induction and earthing process Diagram

5. Electric field Electrostatics Next Slide Nature of electric field • Presence of electric field : experience of electric force by a small charge around a charged object • Representation of electric field : field lines • Electric field pattern around a positive charge Diagram • Electric field pattern around a negative charge Diagram • Various electric field patterns Diagram • Experimental setup to demonstrate electric fields Photo

6. Applications 1 Electrostatics Next Slide Application of charges • Action of a point Diagram • Lightning conductor and action of a point Diagram • Electrostatic precipitation • Photocopying

7. Applications 2 Electrostatics Next Slide Electrostatic hazards • Sheets of paper stick together in industry • Wool, cotton and artificial fibres attract dust • Sparks produced in vehicles and planes

8. END of Electrostatics

9. nucleus electron : negative (-) charge proton : positive (+) charge neutron : no charge Back to Basic Definitions 2 Electrostatics Click Back to • Matter is made up of tiny particles called atoms • Model of an atom : • A proton has a positive charge and an electron has an equal negative charge while a neutron is uncharged

10. Electrically neutral (normal case) protons neutrons electrons 20000 20000 20000 • Positively charged protons neutrons electrons 20000 20000 18000 • Negatively charged protons neutrons electrons 20000 20000 23000 Back to Basic Definitions 2 Electrostatics Click Back to • Assume that we have a rod which is made of 10000 atoms and each atom contains 2 protons, 2 neutrons and 2 electrons

11. flow of electrons flow of electrons Back to Basic Definitions 3 Electrostatics Click Back to • When a plastic rod is rubbed with a dry cloth, we have either one of the following cases. Plastic rod : +ve charged Cloth : -ve charged Plastic rod : -ve charged Cloth : +ve charged

12. large attractive force Aluminium flow flow of electrons small repulsive force Back to Basic Definitions 3 Electrostatics Click Back to • A charged (either +ve or -ve) rod can attract a piece of aluminium foil which is neutral initially. • Since the electrons are attracted towards the upper end, the upper part becomes -ve while the lower part becomes +ve. • Since smaller separation means larger force, the attractive force is always larger than the repulsive force.

13. Back to Ways of charging Electrostatics Click Back to • Extra high tension (EHT) power supply

14. Back to Ways of charging Electrostatics Click Back to • Van de Graaff generator

15. Negatively charged sphere Uncharged sphere Insulating stands Ways of charging Electrostatics Next Slide • Consider two metal spheres which are supported by insulating stands as shown. • One of the sphere is negatively charged while the other one is uncharged.

16. flow of electrons Ways of charging Electrostatics Next Slide • They are brought into contact with each other. • Some of the electrons flow from the charged sphere to uncharged sphere. • Both objects are then negatively charged.

17. Back to Ways of charging Electrostatics Click Back to • Both spheres are then separated and negatively charged. • This is called charging by sharing.

18. Uncharged spheres Insulating stands Ways of charging Electrostatics Next Slide • Consider two metal spheres which are supported by insulating stands as shown. • Both spheres are neutral and they are in contact with each other as shown.

19. flow of electrons Insulating stands Ways of charging Electrostatics Next Slide • A positively charged rod is brought near one of the sphere. • Induced charge appears as electrons flow from one sphere to another.

20. Insulating stands Ways of charging Electrostatics Next Slide • The spheres are separated without removing the rod.

21. Insulating stands Back to Ways of charging Electrostatics Click Back to • Then we remove the rod. One sphere is negatively charged while the other one is equally positive charged. • No loss of charge from the charged rod • This is the method of charging by induction and separation.

22. flow of electrons Insulating stands Ways of charging Electrostatics Next Slide • We have a uncharged conducting sphere supported by insulating stand. • A positively charged rod is brought near the sphere. • Induced charge appears in the sphere.

23. flow of electrons earthing process Insulating stands Ways of charging Electrostatics Next Slide • Touch the sphere with your finger or collect the sphere to the earth with a conducting wire without removing the rod. • Electrons flow from the earth to the sphere and neutralize the +ve charge on the other side in the sphere.

24. Insulating stands Ways of charging Electrostatics Next Slide • Remove the finger or the wire without removing the rod.

25. Insulating stands Back to Ways of charging Electrostatics Click Back to • Remove the rod and we get a negatively charged sphere. • No loss of charge in the rod is involved. • This is the method of charging by induction and earthing process.

26. Back to Electric field Electrostatics Click Back to • The electric field pattern around a positive charge is shown. • The arrows and the lines represent the direction of force acting on a very small testing charge placed at that point.

27. Back to Electric field Electrostatics Click Back to • The electric field pattern around a negative charge is shown. • The arrows and the lines represent the direction of force acting on a very small testing charge placed at that point.

28. Electric field Electrostatics Next Slide • Various patterns are shown in the following figures:

29. Electric field Electrostatics Next Slide

30. Electric field Electrostatics Next Slide

31. Back to Electric field Electrostatics Click Back to

32. Applications 1 Electrostatics Next Slide • Action of a point : The charge has a tendency to concentrate on the sharp points of a conductor. Around these points, the electric field is so strong that the air molecules are ionized.

33. electric wind Back to Applications 1 Electrostatics Click Back to • Let’s take a negatively charged object with a sharp end as an example. Positive ions are attracted towards the point while negative ions are repelled. • Positive ions remove some electrons from the object and become neutral again. The charge on the object is reduced. • The negative ions are repelled and move away from the point. The stream of ion flow is called electric wind.

34. electric wind Back to Applications 1 Electrostatics Click Back to • Lightning conductor is a metal rod with sharp ends and installed as shown in the following figure. • Lightning conductor provides a route for electrons (lightning) to pass into the ground. • Electric wind reduces the charge on the cloud and the induced charge on the roof.