 PHYS208 Class 1

Bottom of page / Next Class

Introduction to Electric Charge

Attributes of Matter:

MASS (m) CHARGE (q) only positive masses no need for "net mass" masses attract via gravitational forces positive and negative charges must deal with "net charge" most matter is electrically neutral -- no net charge "Like charges repel; unlike charges attract." Newton's law of gravity Coulomb's law  Check sign. G is such that m(kg) and r(m) yields F(N) if: Signs of q's take care of direction. k is such that q in coulombs and r(m) yields F(N) if:  Cavendish - 1798 Coulomb - 1785 Is mass conserved? No, not really! However, in conjunction with energy it is through the mass-energy equivalence E=mc2. Is charge conserved? YES! Quantization? (discrete irreducible packets) Not really the case for mass. YES! Rest mass of electron me is 9.11 x 10-31 kg mproton = 1836 me mneutron = 1839 me charge on electron is -e, charge on protron is +e, charge on neutron is 0, where e is 1.60 x 10-19 C. Please note that the magnitude of the charge on the electron and the proton have been experimentally determined to be equal to one part in 1018. Also the upper bound on the charge of the neutron is 2 x 10-20 e.  Fundamental particles are the basic building blocks of matter, carry mass and charge. matter -> molecules -> atoms -> nucleus + electrons nucleus -> protons + neutrons -> quarks Other particles? Differing masses. Other free-standing particles? Charge always quantized in multiples of +/- e. Well established by Millikan in 1909. Quark model - fractional charges quarks: +2/3 e, -1/3 e anti-quarks: -2/3 e, +1/3 e Consider the forces between the proton and the electron in a hydrogen atom in the ground state.From the Bohr model, the separation between the two particles is 0.53 x 10-10 m. Using the equations for force above, along with the numerical values specified, you should be able to demonstrate that: Fgrav = 3.6 x 10-47 N Fel = 8.2 x 10-8 N, so that Fel = 2 x 1039 Fgrav. Observations: At the atomic level, one may typically ignore gravitational forces. Given such strong electrical interactions, atoms tend to remain uncharged. Matter prefers to be electrically neutral. Forces we experience, if not gravitational, are electrical in nature (even though the net charge may be zero). Three questions for you to ponder: If the electron and proton are so strongly attracted, why does the hydrogen atom not collapse? If protons repel so strongly, why does the nucleus not fly apart? For that matter, why are the fundamental particles so stable; i.e. why doesn't the electron break apart?
So what we set out to study here is the vast subject of electricity and magnetism. After studying some quantum physics in addition, we would be able to correctly describe the properties of bulk matter: atomic physics, solid state physics, chemistry, macromolecules.

We begin by studying electrostatics, charges at rest. Later in the semester, we will consider moving charges and the magnetic effects that accompany them.

Relevant Online Resources

The Discovery of the Electron -- an exhibit celebrating its 100th anniversary, sponsored by the American Institute of Physics