22.1. IntroductionOrdinary matter consists of atoms. Each atom consists of a nucleus, consisting of protons and neutrons, surrounded by a number of electrons. The masses of the electrons, protons and neutrons are listed in Table 22.1. Most of the mass of the atom is due to the mass of the nucleus.
|electron||9.11 x 10-31|
|proton||1.673 x 10-27|
|neutron||1.675 x 10-27|
The precise magnitude of the electric force that a charged particle exerts on another is given by Coulomb's law:
" The magnitude of the electric force that a particle exerts on another particle is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The direction of the force is along the line joining the particles. "
|electron||- 1.6 x 10-19|
|proton||1.6 x 10-19|
q1 and q2 are the charges of particle 1 and particle 2, respectively
r is the distance between particle 1 and particle 2 (see Figure 22.1)
[epsilon]0 is he permittivity constant: [epsilon]0 = 8.85 x 10-12 C2/(N . m2)
This formula applies to elementary particles and small charged objects as long as their sizes are much less than the distance between them.
|particle-particle||Fg (N)||Fc (N)|
|electron - electron||-5.5 x 10-51||2.3 x 10-8|
|electron - proton||-1.0 x 10-47||- 2.3 x 10-8|
|electron - neutron||-1.0 x 10-47||0|
|proton - proton||- 1.9 x 10-44||2.3 x 10-8|
|proton - neutron||- 1.9 x 10-44||0|
|neutron - neutron||- 1.9 x 10-44||0|
22.2. Charge Quantization and Charge ConservationAn important experiment in which the charge of small oil droplets was determined was carried out by Millikan (details of this experiment will be discussed in Chapter 23). Millikan discovered that the charge on the oil droplets was always a multiple of the charge of the electron (e, the fundamental charge). For example, he observed droplets with a charge equal to +/- e, +/- 2 e, +/- 3 e, etc., but never droplets with a charge equal to +/- 1.45 e, +/- 2.28 e, etc. The experiments strongly suggested that charge is quantized.
Another important property of charge is that charge a conserved quantity. No reaction has ever been found that creates or destroys charge. For example, the annihilation of an electron and an anti electron (positron) produces two photons: