Electricity and Magnetism Physics 208 Dr. Tatiana Erukhimova Lecture 2

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)

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.

Atom is mostly empty space! Magnify a hydrogen atom by 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 Figure 6.2: Magnifying a hydrogen atom by 1012 makes the nucleus the size of a grape seed and the diameter of the electron cloud about 4.5 times longer than a football field. The electron itself is still too small to see. the diameter of the electron cloud about 4.5 times longer than a football field

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.

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.

Coulomb’s Law Conservation of electric charge is the permittivity of free space Conservation of electric charge 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

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)

y L T q q x mg

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

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

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 .

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? y q0 H q q x a b

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?

Have a great day! Hw: All Chapter 1 problems and exercises Reading: Chapters 1, 2