108 Practical MATLAB® Applications for Engineers
where lbf denotes pound-force. The law or principle of conservation of energy for
an electric circuit can simply be restated as one of the following:
The total energy in a closed system is zero.
Energy in, equals energy out.
In a close loop system the energy consumed is equal to the energy generated.
R.2.37 When electrical energy is supplied to a device, and the device dissipates that energy
in the form of heat, then the device is called a “resistor” (denoted by R), with the
unit given in ohms, denoted by the Greek character Ω.
R.2.38 When electrical energy is supplied to a device, and the device stores that energy in
its magnetic fi eld, then the device is an inductor (denoted by L), with the unit given
in henries (H).
R.2.39 When electrical energy is supplied to a device, and the device stores that energy in
its electric fi eld, then the device is a capacitor (denoted by C), with units given in
faradays (F).
R.2.40 A practical device usually has more than one way to deal with the energy it receives.
There are no pure inductors in nature, rather a combination of a resistor and an
inductor (RL circuit). The same statement is valid for capacitors. There are no pure
capacitors in real life, rather a combination of a resistor with a capacitor (RC circuit).
R.2.41 The standard and widely accepted symbols used to represent resistors, capacitors,
inductors, and sources are shown in Table 2.1.
R.2.42 Ohm’s law relates the voltage (V) across and the current (I) through a resistor (R) by
means of the following equation:
V = R * I
TABLE 2.1
Electrical Symbols
Variable Symbol UnitsR Ohms (Ω)C Farads (F)L Henries (H)V+ +
− − DC volts (V)v(t) AC volts (V)I DC amp (A)i(t) AC amp (A)