106 Practical MATLAB® Applications for Engineers
R.2.25 Conductors are materials in which an electric charge can be moved from one point
to another over a fi nite interval of time with little resistance.
A superconductor is a conductor that has no resistance to the fl ow of electric cur-
rent (no energy is converted to heat).
R.2.26 Insulators are materials that resist the fl ow of electrons by presenting a large resis-
tance to its current fl ow, and they require a large external amount of energy to
produce a measurable current.
R.2.27 Semiconductors are materials that exhibit characteristics that in some cases behave
as a conductor, whereas in other cases, act as insulators depending on the inten-
sity and polarity of an external excitement such as temperature, light, or voltage.
Examples of semiconductor materials are silicon (Si), germanium (Ge), and gallium
arsenate (GaAs).
R.2.28 An electric current, denoted by I, is defi ned as the rate of fl ow of an electric charge
(made up of electrons). Analytically, current is defi ned as
I
Q
tdQ
dt()
()
()
A
Charge
time(C/s)The unit of an electric current is the ampere (denoted by A or amp), defi ned as
the rate of fl ow of an electric charge of 1 C/s (coulombs per second). Since current
involves motion, magnitude and direction must be indicated. The positive direc-
tion for current is defi ned as the fl ow of a positive electric charge. Electrons consist
of negative charges and constitute the bulk of the electric charge. The direction of a
positive current is then defi ned as the opposite of the electron fl ow.R.2.29 The work required to move one unit of charge from one point to another is mea-
sured in volts. Therefore,
1 V (volt) = 1 J/C (Joule/Coulomb)
where J is the unit of energy. Electric voltage is measured between two points,
which is also referred as electrical potential or potential difference and is the elec-
trical force that causes electrons to fl ow creating an electric current. Observe that
voltage is the potential to move a charge even when no charge is moved. When
charges (current) move through the circuit’s elements, energy is transferred. The
volt is the energy transferred per unit of charge through the affected elements.
A voltage is always defi ned by assigning a polarity (positive–negative) indicating
the direction of energy fl ow.
If a positive charge moves from the positive polarity through the element toward
the negative polarity, then the device generates or supplies energy. However, if
a positive charge moves from the negative toward the positive polarity, then the
device absorbs energy.
R.2.30 An ammeter is an instrument used to measure an electric current. To measure
current, an ammeter must be inserted in the path of the current fl ow (electrons) by
physically opening the circuit in order to insert the ammeter.
R.2.31 A voltmeter is an instrument used to measure an electric potential between two
arbitrary points in an electric circuit. To measure a voltage difference, a voltmeter
must be inserted (attached) to the two referred points.
R.2.32 In its simplest version, a current fl ow can be generated in a circuit if a close path or
loop is established and a source is present in the loop.