12.4
Forms of Current Electricity
You may have noticed that some loads, such as an MP3 player, require a
battery in order to work, while others, such as a television, must be plugged
into a wall outlet. These two ways to access electrical energy are related to
how electricity is produced. Current electricity can be produced directly
(for example, from a battery) or it can be generated in a generating station
(for example, the electricity you obtain from your wall outlet). Each of these
methods produces a different type of current electricity.
There are two forms of current electricity: direct current and alternating
current. In direct current (DC), electrons flow in one direction only
(Figure 1). Direct current is produced by an electric cell, such as a battery,
to power portable electrical devices.
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direct current (DC) a flow of electrons in
one direction through an electric circuit
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electric cell
switch
electron flow
Figure 1 In a DC circuit, the electrons move in one direction only. The electrons move from the
negative end of the energy source, through the circuit, to the positive end of the energy source.
In alternating current (AC), electrons move back and forth, alternating
their direction (Figure 2). Alternating current is produced by generators at
electric generating stations. Alternating current is used in electric generating
stations because it is a more efficient method of distributing electrical energy
over long distances than DC. Wall outlets provide alternating current. Many
devices, such as lights, ovens, and clothes dryers, use alternating current.
alternating current (AC) a flow of
electrons that alternates in direction in an
electric circuit
DID YOU KNOW?
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switch
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switch
AC generator
AC generator
electron flow
electron flow
Figure 2 In an AC circuit, the direction of electron flow changes as often as 60 times per second.
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AC Circuits
In an AC circuit, as the electrons move
back and forth, there is an instant at
which the electrons stop to change
direction. During this instant of time,
there is no electric current. Although it
happens too quickly for you to see, the
lights in your home turn on and off 60
times per second.
12.4 Forms of Current Electricity
515
Generating Electricity
Electricity does not exist as a primary form of energy. It is produced when one
type of energy is converted into electrical energy. Electric generating stations
convert mechanical energy into electrical energy. An external energy source
is used to push on the blades of a fan-like device called a turbine, causing it to
turn. The turbine is connected to the movable parts of another device called
a generator. Inside the generator, a coil of wire is held between the two poles
of a magnet. The wire is made of copper because copper is a good conductor
of electricity. When the turbine rotates, the coil of wire in the generator also
rotates. Since the moving coil is held near a magnet, electrons begin to flow
in the wire. These electrons eventually move in transmission lines to the
electrical outlets in your home. The process of generating electrical energy in a
generating station is shown in Figure 3.
falling water
turbine
IN
OUT
external energy
electrical energy
coil in generator
current electricity
Figure 3 An electrical generator obtains energy from a non-electrical source of energy and
converts it into electrical energy. The external energy source spins a turbine consisting of a set
of fan-like blades. The turbine spins a coil of wire inside a magnet, producing electrical energy.
Figure 4 Sir Michael Faraday
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A generator makes it possible to produce a constant flow of electrons
whenever it is needed. Every time you plug a load into a wall outlet, you are
accessing electrical energy produced using a generator. This phenomenon
was first observed by Michael Faraday (Figure 4).
Electrical energy is special because, unlike other forms of energy, its
production can be controlled and it can travel long distances to where it
will eventually be used. Since the flow of electrons is controlled, engineers
can design devices that convert the electrical energy more easily into other
useful forms of energy, such as motion, thermal energy, or light. An electric
blender, for example, is a device that converts electrical energy into motion.
A toaster, for example, converts electrical energy into thermal energy.
Faraday’s discovery was so important that it revolutionized the way
humans use electricity. Before this discovery, electrical energy could not
be produced for long enough periods of time. The only available electrical
energy at the time came from early versions of batteries.
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direction only. Electric cells produce direct
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and forth. Alternating current is produced in
generating plants.
CHECK
YOUR LEARNING
1. What did you learn about the different types of current that
surprised you? Explain. K/U C
2. What is the difference between alternating current and
direct current? K/U
3. Draw a flow chart showing how a generator works. K/U
C
4. Each of the images shown in Figure 5 shows electrical
devices or electricity in some form. Identify the type of
current electricity represented in each image as being either
direct current (DC) or alternating (AC) current. K/U A
(a)
Figure 5
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(b)
(c)
c)
(d)
12.4 Forms of Current Electricity
517