4. From long view of the history of mankind – seen from, say, ten thousand years from now – there can be little doubt that the most significant event of the 19 th century will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade. Richard P. Feynman Nobel Prize in Physics, 1965 1918-1988

5. Consider a force like gravitation which varies as but 1) billion-billion-billion-billion times stronger; 2) there are two kinds of “matter”: positive and negative; Like kinds repel and unlike kinds attract (unlike gravity where there is only attraction) ++ + __ _

6. Matter is effected by forces or interactions (the terms are interchangeable) There are four fundamental forces in the Universe: gravitation (between particles with mass) electromagnetic (between particles with charge) strong nuclear force (between quarks) weak nuclear force (that changes quark types)

9. The atom contains a nucleus surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and positively charged protons. The opposite charge of the electron and proton binds the atom together with electromagnetic forces.

10. Atom is mostly empty space! Size of proton or neutron: ~10 -15 m Size of an electron cloud: ~10 -10 m (1 Angstrom) Proton mass: 1.7x10 -27 kg Electron mass: 9x10 -31 kg Magnify a hydrogen atom by the diameter of the electron cloud about 4.5 times longer than a football field

12. What holds a negatively charged electron together ? (since it has no nuclear force) 1909: Robert Millikan famous oil drop experiment The smallest charge we ever observed is the “elementary charge”: By convention, the electron has negative sign, the proton positive.

13. Electrostatics (the interactions of electric charges that are at rest in our frame of reference) There are two kinds of charge, positive and negative. Charges of the same sign repel each other; charges of the opposite sign attract. All ordinary matter is made of protons, neutrons, and electrons. The positive protons and electrically neutral neutrons in nucleus of an atom are bound together by the nuclear force.

14. Continue …. The negative electrons surround the nucleus at distances much greater than the nuclear size. Electric interactions are chiefly responsible for the structure of atoms, molecules, and solids.

15. Coulomb’s Law Charge Conservation of electric charge Charge is conserved: in any isolated system, the total charge cannot change. If it does change, then the system is not isolated: charge either went somewhere or came in from somewhere is the permittivity of free space

16. qq L mg T x y

17. qq L T x y If the length is 1 meter, q = 1Coulomb, what is mg? Your weight? –

18. qq L mg T x y If we measure and know q and m, we can determine

19. Exercise: If two electrons are placed meters apart, what is the magnitude of the Coulomb force between them? Compare this to the gravitational force between them. Solution: The magnitude of electric force The magnitude of gravitational force (no matter what the separation is)

20. Principle of Superposition The presence of other charges does not change the force exerted by point charges. One can obtain the total force by adding or superimposing the forces exerted by each particle separately. Suppose we have a number N of charges scattered in some region. We want to calculate the force that all of these charges exert on some test charge.

21. Problem 2 page 9 Two equal, positive charges of magnitude are positioned along the x-axis as shown. What would be the force on a positive charge,, placed on the y-axis a distance H from the x-axis? H q q q0q0 x y a a

22. Problem 6 page 10 Suppose a charge were fixed at the origin and an amount of charge Q were uniformly distributed along the x-axis from x=a to x=a+L. What would be the force on the charge at the origin?

23. Have a great day! Please don’t forget your pictures Reading: Chapters 1, 2