Direct Curr~n~ ~ an induced current, charges may flow in
one direction only, or they may alternate dlrec~ions. The direction of an induced current depends on file direction in whick,~i :i~
the wire or magnet moves.You probably no~iced in Figure 12 on ~ ’
the previous page that when the direction o£ the motion o£ the
wire coil changed, the ~rec~ion o£ the current reversed.
A current consisting o£ charges that flow in oIl~ direction
only is called direct currel~t, or DC. A direct current c~ be
induced ~om a changing magnetic field or produced £ro~ an
~nergy source such as a battery. Whe~a a battery is placed in a
circuit, charges flow away £rom one end o£the bakery, aroond
the circuit, and into the other end o£ the battery. Thomas Eclison used direct currel~t in his first electric generatisag plan~.
Several scientists were responsible for bringing electricity
~rorn the laboratory into everyday use.
A~ternating Current What would happen if a wire in a
r~a~netic field were moved up and down repeate~y? The
" ~,:ed current ~ the wire would reverse dire~on repeatedly
a~~eI!. This ~d of current is called ~ternat~g current, or
AC. ~ Mt~na~g c~re~t consis~ of charges ~at move back
md forth in a c~cuk. The electric c~rrent ~ the circuits in
homes, schools, a~d other buildings is Mternating c~r~t.
Mtema~g current has a’ major adv~tage over direct c~s-’
rent. An AC voltage c~ be easily r~sed or lowered to a higher
.or lower voltage. This means ~at a high vol~ge can be used to
send electricM e~er~ over great ~stances: Then ~e voltage
~n be reduced to a safer level for everyday use.
Joseph Henry, or Hans
Christian Oersted. Write a
letter to a friend in which
you describe your work as a
research assistant for the
~ Reading ~ What s the advantago of using alternating
scientist you choose.
~ Che~po~t~ cur~nt?
~"
include descriptions of his
e xperimentalproceduresand
the equipment he uses. Tell
how his work has led to
...... ] surprising discoveries. "
I
Chaptar3 N ~ 97
e~ergy ~to dectricaI ener~. ~ electric generator is the o~posite of an elecb’ic too*or. ~ e]e~ic tootor uses an electric current in a ~agnet field to produce moron. A generator uses
’
magnets
Armature ....
--
Current
FIGURE 14
Turbines
In most generators, a source of
mechanical energy turns huge
turbfnes such as this one. The
turbine is attached to the
armature of a generator, which
produces current.
Using Generators TI~e electric company uses giant generamrs to produce most~of the electrical energy you use in your
home and school. But, instead of using a crank to supl?ly the
toechanical energy to tuitn the armature, a turbine is used. Tur~
bh~es are large circ~laz devices made nt~ of many blades.
Figure 14 shows how a turbine is attached to the armature in a
generator.. The turbine spins as the water flows by it. As a
result, the artoatare spins and generates electric current. .
Reading ~
~ What is a turbine?
Primary coil
Secondary coil
The electrical energy generated by electric companies is transmitted over tong distance at very high v01tages. However in
your hotoe, electrical energy is used at much lower voltages.
What changes the vottage of the electrical energy? The answer
is tralxsfortoers.
What is a Trans~iormer7 A transformer is a device ~hat
increases or decreases voltage. A transformer consists of two
separgt~ coils of insulated ~a~re wrapped around an iron core.
One coil, called the primary coil, is connected to a circuit with
a voltage source and alter~ating current. The other coil, the
secondary c~fl, is connected to a separate drcuit that does
FIGURE 15
A Transformer
The primary co!! of a transformer
is connected to a wltage source,
The secondary coil is not
F~ua~ 16
Chancding Voltage
Transformers are involved in the
transmission, of elec~Hcal energy
~rom an electric plant to a ho~e,
Relating Cause and Effe~ Ho~.
does the number of loops in th~~ ,
primary and secondary colb
the voltage of the induced
current?
A Transformer at Work W~en a current is in the primary
coil of the transformer~ it produces a magnetic field. The magnetic field changes as the current alternates. This changing
magnetic fidd is like a moving magnetic field. It induces a current in thq secondary coil. A transformer works only if the current in the primary coil is changing. If the current does not
change, the magnetic field does not change. No current will be
induced ha the secondary coil. 8o a transformer will not work
with direct current.
~ypes of Transformers If the number of loops in the primary and secondary coils of a transformer is the same, the
voltage of the induced current is the same as the original voltage. But if the secondary coll has more loops than the primary
coil, thevoltage in the secondary coil will be greater. A transformer that increases voltage is called a step-up transformer.
Step-up Transformer
A step-up transformer increases voltage.
The secondary coil has more loops than
the primary coil.
Step*do~vn Transformer
A step-down transformer decreases
voltage, The primary.c, oi! has mere loops
than the secondary
Low Voltage
High Vol~age
High Voltage
@
Primary
coil
Step-up
transformer
plant
11,000 V
Low Voltage
@
Secondary
coil
Step~down
":kransformer
Primary
coil
Secondary
coil
Step-down
.transformer "
Suppose there are fewer loops in the secondary coil than in
i fhe primary coil. The vokage in the secondary coil will be less
~ itan in the primary coil A transformer that decreases voltage
i .s called a step-dm,m transformer, Figure 16 shows both types
~f transformers,
Uses of ~ransformers An important use of transformers is
in the transmission of electrical energy from generating pla~ts’.
The most efficient way to transmtt current over long dlstan es
is to maintain high voltages--about 11,000 volts to 765,000
volts. But the high voltage must be decreased to be used safely
in your home, The use of step-up and step-down transformers
al!ows safe transmission of electrical energy from generating
plants to the consumer.
. ,.; ,~
. ..
T~ansformers are also used m some elecmca! dewces.
"}escant lights, teIe,~slons, and X,-r~y machines require higher
voltages than the current in your home, which is about 120
volts, These devices contain step-up transformers. Other
: devices, such as doorbells, electronic games, and portable CD
pIayers, require lower voltages, about 6 to 12 volts. They con’i~ain step-down transformers.
:~~~eading
What is the voltage in your house?
1, What is the difference between AC and DC? Explain[
2. Which one do you beheve we use today. Why is this an advantage?
3. What does a ger~erat~r do? How does a tarbine make a generator work?
4. What is a transformer?
5. Observe the image on page 100 in you~ packet. How does energy move ~omthe
generating plant to our houses? What happens to the voltage? Explain!
(Use step-up and step-down transformers in your answer)
What’s the voltage in an average household?