Electrification

1. Electrification

2. History • static electricity was recognized before the time of Christ • William Gilbert was among the first to experiment extensively with electrification

3. History • amber and glass rods • resinous: acted like amber • vitreous: acted like glass

4. History • Du Fay: two kinds of “electricks” • Franklin: one kind of electrick; some things had excess while others had deficiency

5. Law of Charges Unlike charges attract. Like charges repel.

6. The Electron • demonstrated that electric charge was neither fluid nor continuous • these negatively-charged particles have the same charge-to-mass ratio

7. The Electron • named by George Stoney • all charges must be integer multiples of the charge of an electron

8. The Fundamental Charge • Robert Millikan invented an apparatus that measured the electrical charge on single drops of oil.

9. The Fundamental Charge • qn = total charge on a drop of oil, in coulombs (C) • V = potential difference between the plates, in volts (V)

10. The Fundamental Charge • vT = falling terminal velocity of drop, in m/s • ρ = net density of oil drop in air, in kg/m³

11. The Fundamental Charge • The fundamental charge (e) is the charge of one single electron. • All electrical charges are multiples of e. • There are only two kinds of charge.

12. The Fundamental Charge • The charge on an electron was termed to be negative (-e). • The proton was discovered later and its charge was “positive” (+e).

13. Classification • Conductors • have free electrons • conduct electricity well • metals

14. Classification • Insulators • hold electrons tightly • do not conduct electricity well • porcelain, rubber, glass

15. Classification • Semiconductors • usually act like insulators but can act like conductors under certain conditions

16. When an object has a charge, the charge often “leaks” into the air via dust particles, etc.

17. Detecting Electrical Charge

18. Devices • lightweight balls made from cork or pith • electroscope • What happens when a charged object is brought near the knob of the electroscope?

19. Devices • electroscope • What happens when a charged object is made to touch the knob of the electroscope?

20. Devices • electroscope • How can you use two electroscopes to determine whether a material is a conductor?

21. Establishing a Charge • two ways to charge a body: • by contact • a charged object touches an uncharged object and electrons are transferred from one to the other

22. Establishing a Charge • two ways to charge a body: • by induction • when the electrons on the surface of an uncharged object are redistributed by a charged object that is brought near

23. Establishing a Charge • electrical ground: a large body whose charge is unaffected when electrons are added or removed

24. Coulomb’s Law • measuring electrostatic force: the torsion balance • Coulomb discovered that electrostatic force is inversely proportional to the square of the distance between the charges.

25. |q1||q2| Felect = k r² Coulomb’s Law • stated: • k is a proportionality constant whose value depends on the units used

26. |q1||q2| Felect = k r² Coulomb’s Law • q represents a variable quantity of charge with no physical dimension (point charge)

27. Coulomb’s Law • The electrostatic force on each charged object is equal and opposite to the force exerted on the other object.

28. |q1||q2| Felect = k Mm r² Fgrav = G r² • Identical in form

29. |q1||q2| Felect = k Mm r² Fgrav = G r² • Both measure a force

30. Key differences: • two kinds of charge for electrostatic force • one kind of mass that produces gravitational force

31. Key differences: • electrostatic forces may be attractive or repulsive • gravitational forces are only attractive

32. both are central forces • both are conservative forces

33. (1 esu)(1 esu) 1 dyne = kesu (1 cm)² dyne · cm² kesu ≡ 1 (esu)² Units of Charge • electrostatic unit (esu)

34. N · m² kC = 8.99 × 109 C² |q1||q2| ( ) N · m² Felect = 8.99 × 109 C² r² Units of Charge • SI unit: Coulomb (C)