Electrostatics

1. Electrostatics …the branch of physics that studies the nature of charges that’s not moving

2. Static Electricity & Electric Current • How are they related? • What’s the difference in them? • How do they relate do the study of electricity? • Where (in our daily lives) do we see examples of their use? • Can you measure them? How?

4. ATOM BASICS • Neutral atoms contain equal number of positive protons and negative electrons • Only electrons move to create charges • When atoms lose electrons they become positively charged ions • Like charges repel and opposite charges attract

5. So…. • When an object gains or loses electrons it becomes CHARGED!!!! • Charge is a property of particles • This force of attraction or repulsion between charged particles is called the ELECTRIC FORCE • And the fact that these + or – particles in matter carry electric charge, this is the basic idea of electricity

6. Unit of Charge • Charge of electron and proton are equal in magnitude and opposite in sign • Symbol for charge is q • SI unit of charge is the Coulomb (C) • 1C = the charge of 6.25 x 1018 electrons • 1 C = 6.25 billion billion electrons

7. Conservation of Charge • Net charge of an isolated system remains constant • When we charge something, no e- are created or destroyed, e- are simply transferred

8. Electric Charge is Quantized • An electrically charged object has an excess or deficiency of some whole number of electrons (cannot be divided into fractions) • Charge is said to be quantized, charge only occurs in discrete amounts

9. Charge interaction is a FORCE!

10. How much electrical force? • Coulomb’s Law • k = 8.99 x 109 N•m²/C² • If answer is negative: attracting force • If answer is positive: repelling force

11. Practice Problem! • Two balloons are charged with an identical quantity and type of charge: -6.25 nC. They are held apart at a separation distance of 61.7 cm. Determine the magnitude of the electrical force of repulsion between them. • Felect = 9.23 x 10-7 N

12. So how can an object get charged? Note: any 2 substances that are rubbed together can potentially become charged. • By Induction • By Friction & • By Contact

13. FRICTION • Can cause charge separation • Electrons are stripped from one material and added to the other

14. Walking across wool carpet, leather soled shoes have less desire for electrons than the carpet. Electrons get stolen from the shoe by the carpet.  When the positively charged person gets near the metal door he will actually attract charges from the door which jump in the form of a spark.

15. STATIC ELECTRICITY • Electric charges that can be confined to an object OR • Electricity at rest OR • The presence of electric charge +/- on the surface of a material

16. Electroscope • Can be used to determine whether an object is electrically charged

17. Conductor • Material through which electrons move freely • Examples: copper wire, copper penny, gold, aluminum foil • Metals are good conductors because they have free electrons

18. Insulator • Material though which electrons do not move freely • Ex. Glass, air, rubber, wood, hair

19. It is easy to put a static charge on a insulator because • they stay where they are put

20. GROUNDING • Removing a static charge by providing a path to the ground • Extra electrons move from negatively charged objects to ground until object is neutral • When positively charged object is grounded electrons move from the ground to neutralize positively charged object • Earth both accepts and furnishes electrons and remains neutral (symbol)

21. To put a charge on a conductor • Isolate it • Use an insulator to separate it from the ground

22. Pith balls and electroscopes detect electric charges. Both are made of metal to allow electrons to move freely, and both use insulators to isolate the metal parts from the ground.

23. WHAT DO YOU KNOW • A material through which electrons move easily is: • A. an insulator • B. isolated • C. a conductor • D. grounding

24. And… • Electrostatic charges are often produced by friction between objects made of materials with • A. the same attraction for electrons • B. different attraction for electrons

25. What about.. • Rubber has a greater attraction for electrons than wool. When rubber-soled shoes scuff across wool carpet, electrons are … • A. stripped from the carpet • B. stripped from the shoes • C. created • D. destroyed

26. And.. • A ___________ can still be statically charged, even though it has been touched, because it is a ______________. • A. metal ball; conductor • B. metal ball; insulator • C. balloon, conductor • D. balloon, insulator

27. Induction • Transferring a charge without touching • Side of object closest to charging rod receives a charge opposite that of the rod • Charge separation is usually temporary

28. Contact or Conduction • If one of the object is charged & touches something that’s not charged, the 2nd object becomes charged • This is charge by contact.

29. Conduction • Transferring a static charge by touching • Object receives a charge that is the same as the charging rod • Charge is permanent (if isolated) • Does object receive same charge or opposite charge of the object touching it?

30. Charge by conduction object ends up similarly charged to the object used to charge it. Charge by induction object ends up oppositely charged to the object used to charge it.

31. Van de Graff generator • Charge is transferred to a student by what which method of transfer? • He receives the _____ charge as the dome. • When he steps down, the charge goes through him to the earth. What is this called? • Great Explanation: http://videos.howstuffworks.com/hsw/18563-electricity-and-magnetism-the-van-de-graaff-generator-video.htm

32. WATER AND PAPER • How can water and paper be attracted to both positive and negative rods? • The polar molecules turn so that the side closest to the rod is the opposite charge and attracts.

33. Why does a balloon stick to the wall after being rubbed with your hair? • Polar molecules on the wall’s surface are charged by induction

34. Lightning • Wind rubs the surface of the ground & buildings, air molecules pick up electrons from them and carry them to the sky • Ground becomes positively charged and the bottom of clouds become negatively charged • The clouds ,mostly consist of water vapor a good conductor, build up charge

35. Lightning • When charges are strong enough to flow through air (an insulator) then it takes the shortest course to get rid of excess charge

36. Field Force • Force that is exerted by one object on another even though there is no physical contact • Gravity and electric force are 2 examples • Electric forces can be either attractive or repulsive • Electric force is much stronger than gravity

37. Electric Field • Since electric force is an action-at-a-distance force an electric field surrounds every arrangement of charges • Has both magnitude and strength • The magnitude at any point is the force per unit of charge • E = F/q (Electric field = force ÷ charge) • SI unit = N/C (vector quantity)

38. Electric Field • By convention, a positive test charge (qo) is used to measure the field • Therefore vector arrows point to center of negatively charged object and point away from center of positively charged ball

39. Electric Field Strength • E = Fe = kqqo = kq qo qor² r² Note that electric field is produced by nearby charge and not the TEST charge If q is positive, the field is directed outward If q is negative, the field is directed inward

40. Electric Field Lines • Lines that represent both the magnitude and direction of the electric field • # of lines is proportional to electric field strength • Always drawn leaving a positive charge to a negative charge

41. Electric Fields

42. Potential Energy