Electric Charge & Stuff: Understanding Coulombs Law and the Electric Field

1. Welcome to PHY2054 Chapter 17 Electric Charge & Stuff

2. A Slide Like This Every Day • Today we begin chapter 17 – Electric Charge, Coulombs Law and the Electric Field • There will be NO QUIZ this week. • Clicker use will begin on Friday. • Bring your i-clicker to class every session • Bring a scientific calculator ($13.00 – COSTCO) • If you have your clicker with you, you may use it today. • There WILL be a Quiz next Friday

3. Probable First Observation Electricity

4. Idiot! If lightening had actually traveled down the kite string, old Ben Franklin would have been toast! Probably never happened, but good story!

5. A Quick Experiment

6. Allowable Predictions(Use your clicker if you have one.) • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

7. motion Rubber rod Pivot Rubber rod Experiment #1 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

8. Rubber rubbed with skin of dead rabbit Pivot Rubber rubbed with skin of dead rabbit Experiment #2 Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

9. The charges on the two rods are .. • Since we treated both rods in the same way, they should be of the same type • ……. different types • I have no idea what you are asking for. • Leave me alone … I’m napping!

10. If you rubbed the rods longer and/or harder, do you think the effect that you see would be • Stronger • Weaker • The same

11. If the two rods are brought closer together, the force acting between them will get … Stronger Weaker The same

12. Definition of sorts We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE. We will try to understand what charge is and how it behaves. We add to the properties of materials: Mass Charge

13. Pivot Experiment #3 Glass rubbed with wool Glass rubbed with wool Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

14. Pivot Experiment #4 Rubber rubbed with skin of dead rabbit Glass rubbed with wool Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

15. What’s Going On? • All of these effects involve rubbing two surfaces together. • Or pulling two surfaces apart. • Something has “happened “to each of these objects. • These objects have a new PROPERTY • Other properties are mass, color • We call this NEW PROPERTY .………. ………CHARGE. • There seems to be two types of charge.

16. We call these two types of charge • Positive • Negative An object without either a (+) or (-) charge is referred to as being NEUTRAL.

17. Example - Tape

18. Separation

19. An Example Volunteer Please

20. Effect of Charge

21. We have also observed that there must be TWO kinds of charge. • Call these two types • positive (+) • negative(-) • We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE. • The charge on the glass rod or the dead cat is consequently defined as POSITIVE.

22. Old Ben screwed up more than once!! ++++++++++--------- ----+++---++---+-++-

23. From whence this charge??? Easily Removed - +

25. AXON

27. Signal propagation in neurons.Neurons are components of the nervous system of the body that transmit signals as electrical impulses travel along their length. These impulses propagate when charge suddenly rushes into and then out of a part of the neutron called an axon. Measurements have shown that, during the inflow part of this cycle, approximately (sodium ions) per meter, each with charge +e enter the axon. How many coulombs of charge enter a 1.5 cm length of the axon during this process?

29. Materials • Two kinds of materials: • Insulators • Electrons and Protons are tightly bound to their positions. Hard to move them around. • Conductors • Electrons are easily removed and moved around. • Electrons are said to be MOBILE charges. • There are other kinds of materials that we will not discuss: semiconductors, semi-metals

30. What about a charged rod and a piece of wooden dowel?? Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

31. Neutral (Conducting) Object - POLARIZATION Positive charge attracts negative charges. Rod becomes “polarized. Negative end is closer to positive charge Distance effect causes attraction.

32. Contact Transfer Q Q/2 Q/2

33. A Question Two iron spheres contain excess charge, one positive and the other negative. (a) Show how the charges are arranged on these spheres if they are very far from each other. (b) If the spheres are now brought close to each other, but do not touch, sketch how the charges will be distributed on their surfaces. (c) In part (b), show how the charges would be distributed if both spheres were negative.

34. Ways to charge an object • Rubbing or bond breaking (same thing) • Transfer • Direct transfer • Polarization • Induction

35. Quickie: How many kinds of charge are there? • Two: +,- • Three: +,-.neutral

36. Induction

37. Polarize

38. Ground

39. Remove Ground

40. Positive !

41. Balloon Physics

42. Same as before: Polarization

43. What happens when two surfaces touch or rub? Bonding!

44. The Triboelectric Series When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged.MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE No! No! No! No!

45. SUMMARY: So far we have found? • There are TWO types of charge. • Positive • Negative • Like Charges Attract • Un-Like charges repel • The force between charges increases as they are brought closer together. • This charge separation results from chemical bonds which are severed.

46. Getting down to business: • We will discuss the inverse square law that describes how electric charges interact via forces. • This is COULOMB’S LAW • We will discuss the FACT that Coulomb's law is a VECTOR equation. • We will add forces acting on a charge from an assembly of other charges. • We will discuss the concept of FLUX and use it to “derive” a useful law for continuous distributions of charge with high symmetry. This is Gauss’s LAW

47. Forces Between Charges • The force between charges is along the line between them. • The direction depends on the type of charges- • Like Charges Repel • Unlike Charges Attract • The Magnitude of the force is proportional to the inverse square of the distance between the charges.

48. Coulomb’s Law The magnitude F of the force that each of two point charges q1 and q2 a distance r apart exerts on the other is directly proportional to the product of charges (q1q2) and inversely proportional to the square of the distance between them (r2). The relationship is expressed symbolically as This relationship is called Coulomb’s law.

49. Coulomb’s Law

50. Coulomb’s Law Force is a Vector ! The Unit of Charge is called THE COULOMB Smallest Charge: e ( a positive number) 1.6 x 10-19 Coul. electron charge = -e Proton charge = +e