Direct Current and Transient Analysis 105
R.2.13 By a process that is known to scientists, but is beyond the scope of this discussion,
energy may be absorbed by some electrons in the outer orbits and migrate outside
its natural orbits, and in this way, become free electrons.
R.2.14 In metals such as silver and copper, some electrons are very loosely held, since they
are not strongly attached to any atom, and they can be shaken off to become free
electrons.
R.2.15 In nature as well as in the physical sciences, only forces are capable of making or(^) creating changes.
R.2.16 Gravity is nature’s force.
R.2.17 Recall from physics (basic mechanics) that force (Newtons) = mass (kilograms) *
acceleration (meters/second^2 ).
R.2.18 An electric charge may induce or create an electrical force.
R.2.19 Coulomb’s law states that the force F between two electrically charged points Q 1
and Q 2 is given by
F
kQ Q
r
**^122
where F is given in Newton (N) (1 N = 1 kg * m/s^2 ), Q in coulombs (C; where a cou-
lomb is a measure of the amount of electric charges given in terms of electrons; the
relationship is 1 C = 2.64 * 1018 electrons), and k a proportionality constant given byk1
4
9199
2
∈ 0 *^2N*m
Cwhere ∈ 0 = 8.85 * 10 −^12 C^2 /N. m^2 is the permittivity of free space, and r is the dis-
tance between Q 1 and Q 2 in meters. In this discussion, it is assumed for simplicity
Q 1 and Q 2 are stationary, otherwise additional forces must be considered.
R.2.20 The charged electric points Q 1 and Q 2 induce a radial electric fi eld around itself.
The electric fi eld is known as a force fi eld.
R.2.21 The following can be said about the nature of the induced electric force F:
a. Like charges repel each other (creating a repulsion force).
b. Unlike charges attract each other (creating an attraction force).
c. The closer the charges the stronger the force.
d. The larger the charges the stronger the force.
R.2.22 As mentioned earlier, the smallest charged (negative) particle in nature is the elec-
tron. The charge of a single electron is given by
1 electron = 1.602 * 10 −^19 C
Therefore, it takes 6.242 * 1018 electrons to generate a charge of 1 C.
R.2.23 Some materials (in particular metals) contain free electrons in their natural state.
The movement or fl ow of free electrons creates an electric current.
R.2.24 Materials can be classifi ed according to their capacity to conduct current or possess
free electrons as
Conductors (including superconductors)
Semiconductors
Insulators