SUMMARY
Before going on to chapter 2, read the following summary of the material in chapter 1.
This summary will reinforce what you have already learned.
DC AND AC  Direct current flows in one direction only, while alternating current
is constantly changing in amplitude and direction.
ADVANTAGES AND DISADVANTAGES OF AC AND DC  Direct current has several
disadvantages compared to alternating current. Direct current, for example, must be
generated at the voltage level required by the load. Alternating current, however, can be
generated at a high level and stepped down at the consumer end (through the use of a
transformer) to whatever voltage level is required by the load. Since power in a dc system
must be transmitted at low voltage and high current levels, the I^{2}R power loss
becomes a problem in the dc system. Since power in an ac system can be transmitted at a
high voltage level and a low current level, the I^{2}R power loss in the ac system
is much less than that in the dc system.
VOLTAGE WAVEFORMS  The waveform of voltage or current is a graphical picture of
changes in voltage or current values over a period of time.
ELECTROMAGNETISM  When a compass is placed in the vicinity of a currentcarrying
conductor, the needle aligns itself at right angles to the conductor. The north pole of
the compass indicates the direction of the magnetic field produced by the current. By
knowing the direction of current, you can use the lefthand rule for conductors to
determine the direction of the magnetic lines of force.
Arrows are generally used in electrical diagrams to indicate the direction of current
in a wire. A cross (+) on the end of a crosssectional view of a wire indicates that
current is flowing away from you, while a dot (·) indicates that current is
flowing toward you.
When two adjacent parallel conductors carry current in the same direction, the magnetic
fields around the conductors aid each other. When the currents in the two conductors flow
in opposite directions, the fields around the conductors oppose each other.
MAGNETIC FIELD OF A COIL  When wire is wound around a core, it forms a COIL.
The magnetic fields produced when current flows in the coil combine. The combined
influence of all of the fields around the turns produce a twopole field similar to that
of a simple bar magnet.
When the direction of current in the coil is reversed, the polarity of the twopole
field of the coil is reversed.
The strength of the magnetic field of the coil is dependent upon:

The number of turns of the wire in the coil.

The amount of current in the coil.

The ratio of the coil length to the coil width.

The type of material in the core.
BASIC AC GENERATION  When a conductor is in a magnetic field and either the field
or the conductor moves, an emf (voltage) is induced in the conductor. This effect is
called electromagnetic induction.
A loop of wire rotating in a magnetic field produces a voltage which constantly changes
in amplitude and direction. The waveform produced is called a sine wave and is a graphical
picture of alternating current (ac). One complete revolution (360°) of the conductor
produces one cycle of ac. The cycle is composed of two alternations: a positive
alternation and a negative alternation. One cycle of ac in one second is equal to 1 hertz
(1 Hz).
FREQUENCY  The number of cycles of ac per second is referred to as the FREQUENCY.
AC frequency is measured in hertz. Most ac equipment is rated by frequency as well as by
voltage and current.
PERIOD  The time required to complete one cycle of a waveform is called the PERIOD
OF THE WAVE.
Each ac sine wave is composed of two alternations. The alternation which occurs during
the time the sine wave is positive is called the positive alternation. The alternation
which occurs during the time the sine wave is negative is called the negative alternation.
In each cycle of sine wave, the two alternations are identical in size and shape, but
opposite in polarity.
The period of a sine wave is inversely proportional to the frequency; e.g., the higher
the frequency, the shorter the period. The mathematical relationships between time and
frequency are
WAVELENGTH  The period of a sine wave is defined as the time it takes to complete
one cycle. The distance the waveform covers during this period is referred to as the
wavelength. Wavelength is indicated by lambda (λ) and is measured from a point
on a given waveform (sine wave) to the corresponding point on the next waveform.
PEAK AND PEAKTOPEAK VALUES  The maximum value reached during one alternation of
a sine wave is the peak value. The maximum reached during the positive alternation to the
maximum value reached during the negative alternation is the peaktopeak value. The
peaktopeak value is twice the peak value.
INSTANTANEOUS VALUE  The instantaneous value of a sine wave of alternating voltage
or current is the value of voltage or current at one particular instant of time. There are
an infinite number of instantaneous values between zero and the peak value.
AVERAGE VALUE  The average value of a sine wave of voltage or current is the
average of all the instantaneous values during one alternation. The average value is equal
to 0.636 of the peak value. The formulas for average voltage and average current are:
Remember: The average value (E_{avg} or I_{ avg}) is for one
alternation only. The average value of a complete sine wave is zero.
EFFECTIVE VALUE  The effective value of an alternating current or voltage is the
value of alternating current or voltage that produces the same amount of heat in a
resistive component that would be produced in the same component by a direct current or
voltage of the same value. The effective value of a sine wave is equal to 0.707 times the
peak value. The effective value is also called the root mean square or rms value.
The term rms value is used to describe the process of determining the effective value
of a sine wave by using the instantaneous value of voltage or current. You can find the
rms value of a current or voltage by taking equally spaced instantaneous values on the
sine wave and extracting the square root of the average of the sum of the instantaneous
values. This is where the term "RootMeanSquare" (rms) value comes from.
The formulas for effective and maximum values of voltage and current are:
SINE WAVES IN PHASE  When two sine waves are exactly in step with each other,
they are said to be in phase. To be in phase, both sine waves must go through their
minimum and maximum points at the same time and in the same direction.
SINE WAVES OUT OF PHASE  When two sine waves go through their minimum and maximum
points at different times, a phase difference exists between them. The two waves are said
to be out of phase with each other. To describe this phase difference, the terms lead and
lag are used. The wave that reaches its minimum (or maximum) value first is said to lead
the other wave. The term lag is used to describe the wave that reaches its minimum (or
maximum) value some time after the first wave does. When a sine wave is described as
leading or lagging, the difference in degrees is usually stated. For example, wave E_{1}
leads wave E_{ 2} by 90°, or wave E_{2} lags wave E_{ 1} by 90°.
Remember: Two sine waves can differ by any number of degrees except 0° and 360°. Two
sine waves that differ by 0° or by 360° are considered to be in phase. Two sine waves
that are opposite in polarity and that differ by 180° are said to be out of phase, even
though they go through their minimum and maximum points at the same time.
OHM'S LAW IN AC CIRCUIT  All dc rules and laws apply to an ac circuit that
contains only resistance. The important point to remember is: Do not mix ac
values. Ohm's Law formulas for ac circuits are given below: