1. Electrostatics

2. The basic unit of positive charge is the proton
The basic unit of positive charge is the proton. (Although protons are ultimately made up of quarks)
The basic unit of negative charge is the electron. It is almost always electrons that are moving when charge “flows”
The SI unit of charge is the Coulomb ( C).
Charge of 1e- = 1 proton = 1.6x10-19 Coulombs

5. Q

6. Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens?
Protons are removed from the rod
Electrons are added to the rod
The fur is also charged negatively
The fur is left neutral

7. Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens?
Protons are removed from the rod
Electrons are added to the rod
The fur is also charged negatively
The fur is left neutral

8. Conductors & Insulators
Conductors are materials in which charges are free to move. Metals are a good example.
Insulators are materials in which charges can not move. Glass, plastics, and wood, are examples. Q

9. Which of the following best characterizes electrical conductors?
Low mass density ) High tensile strength
3) Poor heat conductors 4) charges move freely
5) All of the above

10. Which of the following best characterizes electrical conductors?
Low mass density ) High tensile strength
3) Poor heat conductors 4) charges move freely
5) All of the above

11. Consider the “pith” ball…- + - - - + + + - + + -
Charges are balanced, so the ball is neutral

12. Why is the neutral pith ball attracted to the negative rod?
Consider the “pith” ball…
Why is the neutral
pith ball attracted
to the negative rod? - + - + - - - - - - - - - + + - - + + -

13. Why does it then bounce away? Consider the “pith” ball… + - - - + - -

14. Consider the electroscope

15. Consider the electroscope+ + + + + + + + + + + +

16. Charging by conduction—A physical transfer of charge+
Explain what happens in pictures & words +
Consider the electroscope

17. Polarized but Still neutral+ - +
Polarized but Still neutral

18. Electrons transfer from the scope to the rod+ - +

19. + Scope is left positive.
I can tell it is charged because the leaves repel each other.
Scope is left positive. +

20. Consider the electroscope
Charging by Induction - A transfer of charge, but only two neutral objects touch + + + + + + +
Be ready to explain what happens in pictures & words + + + + +
Consider the electroscope

21. Charging by Induction + + + + + + + + + + + + - +
Still neutral

22. Charging by Induction + + + + + + + + + + + + - - - - - - - - - + + + + + + + +

23. These charges remain held in place - -
Charging by Induction + + + + + + + + + + + + - - - - - - - - -
These charges remain held in place - -
These charges flow from the ground to the electroscope. + + + - - + - + + +

24. These charges now spread out.
Explain what happens in pictures & words
Charging by Induction + + + + + + + + + + + + - - - - - - - - -
These charges now spread out.
The electroscope is now charged. - - - - - - -

28. Q

29. An uncharged conductor is supported by an insulating stand
An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be…
Negative ) Positive ) Neutral
4) Attracted ) Depends on the materials.

30. An uncharged conductor is supported by an insulating stand
An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be…
1) Negative ) Positive ) Neutral ) Attracted ) Depends on the materials.

31. An uncharged conductor is supported by an insulating stand
An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be…
1) Negative ) Positive ) Neutral ) Attracted ) Depends on the materials. + + - + + - + + + - + - + + - + +

32. Two charges, +Q and –Q, are located two meters apart as shown
Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? 2 1 3 4 + -

33. Two charges, +Q and –Q, are located two meters apart as shown
Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? 2 1 3 4 + -

34. Two charges, +Q and –Q, are located two meters apart as shown
Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? + -

35. Two point charges, separated by 1. 5cm, have charges of +2 and -4C
Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that 10 field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines?
1) 20 radiate out ) 5 radiate out 3) 20 radiate in ) 10 radiate in ) 5 radiate in

36. Two point charges, separated by 1. 5cm, have charges of +2 and -4C
Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that 10 field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines?
1) 20 radiate out ) 5 radiate out 3) 20 radiate in ) 10 radiate in ) 5 radiate in

37. Shocking Fingers and Lightning Rods
On a regularly shaped object, charges are evenly spread.
On an irregularly shaped object, charge tends to accumulate at areas of largest curvature/smallest radii.
Small radius, large curvature
Big radius, small curvature

38. In other words,
charge accumulates at
sharp points

39. Ben Franklin invented the lightning rod.Q

40. At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object?
Where the surface curves inward
Where the surface is flat
Where the curvature is greatest (smallest radius)
Where the curvature is least (largest radius)

41. At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object?
Where the surface curves inward
Where the surface is flat
Where the curvature is greatest
Where the curvature is least

42. Faraday Cage

43. The Electric Field inside a conducting surface is zero.
Conducting Cup
Insulating Stand

44. + Pict. 1 The Electric Field inside a conducting surface is zero.
Charged Ball +
Pict. 1

45. + Pict. 2 The Electric Field inside a conducting surface is zero. + -
Polarized Charged + - + + - - + + - - - - - - + +
Pict. 2 + + +

46. Charge remains on the outside only
The Electric Field inside a conducting surface is zero.
The negative charges from the polarized inside get neutralized as the positive ball comes in contact with them. The charge from the positive ball is now left on the outside of the cup.
Charge remains on the outside only + + + + + + + + +
Pict. 3 + + + +

47. A negative charge comes close to the conductor and the conductor polarizes.
Pict. 1
But not all the negative charge can accumulate on the far side because then the far side would be “too” negative so some stays in the center leaving the center neutral. - + -

48. The rod touches, the electrons transfer, and the outside is left negative while the inside is still neutral.
Pict. 2 - + -

49. Pict. 3 - - - + + + - - - - - + + + - - + - - + - + + - + + - - - + -

50. A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral.

51. A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral.
Not only do Faraday cages block charge, they more importantly block electromagnetic radiation.
Applications of a Faraday Cage
Microwave Oven
Electronic shielding
Lightning protection

52. During a lightning storm you are relatively safe in…
A metal framed building.
A car. (not a convertible however)