Fundamental Engineering Concepts - Mathematics and Statistics for the Environment - page 287

The unit of intensity of magnetizing force per unit of length is designated as H and is sometimes
expressed as Gilberts per centimeter of length. Expressed as an equation:

NI
L

×

H = (11.45)

where
H = Magnetic field intensity (ampere-turns per meter, At/m).
N = Number of turns.
I = Current (A).
L = Length between poles of the coil (m).

Note: Equation 11.45 is for a solenoid. H is the intensity of an air core. For an iron core, H is the
intensity through the entire core, and L is the length or distance between poles of the iron
core.

11.7.11 aC t heory
Because voltage is induced in a conductor when lines of force are cut, the amount of the induced
emf depends on the number of lines cut in a unit time. To induce an emf of 1 volt, a conductor must
cut 100,000,000 lines of force per second. To obtain this great number of cuttings, the conductor is
formed into a loop and rotated on an axis at great speed (see Figure 11.53). The two sides of the loop
become individual conductors in series, each side of the loop cutting lines of force and inducing
twice the voltage that a single conductor would induce. In commercial generators, the number of
cuttings and the resulting emf are increased by: (1) increasing the number of lines of force by using
more magnets or stronger electromagnets, (2) using more conductors or loops, and (3) rotating the
loops faster.
How an alternating-current (AC) generator operates to produce an AC voltage and current is a
basic concept taught in elementary and middle school science classes. Of course, we accept techno-
logical advances as commonplace today. We surf the Internet, watch cable television, use our cell
phones, take space flight as a given—and consider production of the electricity that makes all these
technologies possible as our right. These technologies are bottom shelf to us today; they are avail-
able to us so we use them.
In the groundbreaking years of electric technology development, the geniuses of the science of
electricity (including George Simon Ohm) achieved their technological breakthroughs in falter-
ing steps. We tend to forget that these first faltering steps were achieved using crude and, for the

N

S

FIGURE 11.53

Loop rotating in a magnetic field produces an AC voltage.