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 massenergy equivalence E=mc^{2}. 
Is charge conserved? YES! 

Quantization? (discrete irreducible packets) Not really the case for mass. 
YES!  
Rest mass of electron m_{e} is 9.11 x 10^{31} kg m_{proton} = 1836 m_{e} m_{neutron} = 1839 m_{e} 
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 10^{18}. 

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 freestanding particles? Charge always quantized in multiples of +/ e. Well established by Millikan in 1909.
Quark model  fractional charges 

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:  
F_{grav} = 3.6 x 10^{47} N 
F_{el} = 8.2 x 10^{8} N, so that F_{el} = 2 x 10^{39} F_{grav}. Observations:


Three questions for you to ponder:

We begin by studying electrostatics, charges at rest. Later in the semester, we will consider moving charges and the magnetic effects that accompany them.
"http://www.physics.udel.edu/~watson/phys208/clas0211.html"
Last updated March 30, 1998.
Copyright George Watson, Univ. of Delaware, 1997.