Alternative current and voltage (EM) #1 Reading

ALTERNATING CURRENT AND VOLTAGE

An alternating current or voltage is defined as a current or voltage in which the

direction changes periodically. In other words, the current flow is first in one

direction in the circuit and then in the other, this reversal occurring at regular

intervals.

The frequency with which a complete change occurs may be 60 times a second

(as in the case of electric power supplied to most residences), from 20 to 15,000 times

per second (for voice and music waves in communication systems), or up to millions

and billions of times per second (as in the case of the radio or TV signals that are

used in communications and other signal purposes). In many of applications of

alternating current the variation with time is smooth and regular, following the

variation of the sine of a constantly varying angle. Such an alternating current or

voltage is said to vary sinusoidally with time or to be a sine wave.

A tuning fork produces sound or variation in the atmospheric pressure that

varies sinusoidally with time. The pendulum of a clock is sifting energy back and

forth from kinetic to potential in its sinusoidal movement. If a hacksaw blade is

clamped in a vise, a weight placed on the end of it may be made to oscillate with a

sinusoidal movement that also shifts energy back and forth kinetic to spring, or strain,

energy.

The vibration of a gasoline engine, caused by unbalances in the rotating

elements and uneven forces exerted on the pistons, appears in the form of sinusoidal

movements of the engine itself. The oscillations in the tuned circuit of a highfrequency

electric heating unit are also sinusoidal.

In electric power equipment every effort is made to assure a sinusoidal voltage

at the power outlet. This is so nearly achieved in power circuits that a sinusoidal

voltage may be assumed without appreciable error.

In high-frequency heating units, such as are used for preheating thermoplastics

for molding operations and for surface heating and hardening of small gears, the

oscillations assume the sinusoidal form by the very nature of the energy interchange

between the magnetic and electric fields.

Since such sinusoidal variations of current and voltage are so extensively used

in all a-c equipment, their characteristics have to be studied in considerable detail.

When a-c quantities pass through zero and reach the maximum positive value

at the same time these quantities are said to be in phase. When alternating currents

and voltages are not in phase, the quantity that reaches the maximum value first is

said to lead the other quantity. Similarly, the one which reaches its maximum later is

said to lag the other a-c current or voltage.