Charge needs a continuous path to flow. - ClassZone

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STANDARDS
SC.B.1.3.1: The student
identifies forms of
energy and explains
that they can be measured
and compared.
SC.H.1.3.3: The student
knows that science disciplines
differ from
one another in topic,
techniques, and outcomes
but that they
share a common purpose,
philosophy, and
enterprise.
FCAT VOCABULARY
circuit p. 375
VOCABULARY
resistor p. 376
short circuit p. 378
VOCABULARY
Use a frame game diagram
to record the term
circuit in your notebook.
KEY CONCEPT
Charge needs a
continuous path to flow.
BEFORE, you learned
• Current is the flow of charge
•Voltage is a measure of electric
potential
• Materials affect the movement
of charge
EXPLORE Circuits
How can you light the bulb?
PROCEDURE
1
2
3
Tape one end of a strip of foil to the negative
terminal, or the flat end, of the battery. Tape
the other end of the foil to the tip at the base
of the light bulb, as shown.
Tape the second strip of foil to the positive terminal,
or the raised end, of the battery.
Find a way to make the bulb light.
WHAT DO YOU THINK?
•How did you make the bulb light?
•Can you find other arrangements that
make the bulb light?
Electric charge flows in a loop.
MATERIALS
•2 strips of
aluminum foil
•electrical tape
•D cell (battery)
• light bulb
In the last chapter, you read that current is electric charge that flows
from one place to another. Charge does not flow continuously
through a material unless the material forms a closed path, or loop.
A circuit is a closed path through which a continuous charge can flow.
The path is provided by a low-resistance material, or conductor, usually
wire.Circuits are designed to do specific jobs, such as light a bulb.
Circuits can be found all around you and serve many different
purposes. In this chapter, you will read about simple circuits, such as
the ones in flashlights, and more complex circuits, such as the ones
that run toys, cameras, computers, and more.
check your reading How are circuits related to current?
NOW, you will learn
• About the parts of a circuit
• How a circuit functions
• How safety devices stop current
Chapter 11: Circuits and Electronics 375

eminder
Remember, a battery consists
of two or more cells.
Circuit Parts
1
The voltage
source supplies
electrical
energy to the
circuit.
376 Unit 3: Electricity and Magnetism
The Parts of a Circuit
The illustration below shows a simple circuit. Circuits typically
contain the following parts. Some circuits contain many of each part.
1
2
3
4
Voltage Source The voltage source in a circuit provides the electric
potential for charge to flow through the circuit. Batteries are
often the voltage sources in a circuit. A power plant may also be a
voltage source. When you plug an appliance into an outlet, a circuit
is formed that goes all the way to a power plant and back.
Conductor A circuit must be a closed path in order for charge
to flow. That means that there must be a conductor, such as wire,
that forms a connection from the voltage source to the electrical
device and back.
Switch A switch is a part of a circuit designed to break the closed
path of charge. When a switch is open, it produces a gap in the
circuit so that the charge cannot flow.
Electrical Device An electrical device is any part of the circuit that
changes electrical energy into another form of energy. A resistor is
an electrical device that slows the flow of charge in a circuit. When
the charge is slowed, some energy is converted to light or heat.
A light bulb is an example of a resistor.
The parts of a basic circuit include a voltage source, conductor, switch, and
one or more electrical devices.
2
The conductor
provides a path
through which
charge can flow.
The resistor is
an electrical
device that
converts electrical
energy into
another form
of energy.
Would the light bulb be lit if there were no switch in this circuit?
Why or why not?
3
A switch is used
to open and close
the circuit.
4

Open and Closed Circuits
Current in a circuit is similar to water running through a hose. The flow
of charge differs from the flow of water in an important way, however.
The water does not require a closed path to flow. If you cut the hose, the
water continues to flow. If you cut a wire, the charge stops flowing.
Batteries have connections at both ends so that charge can follow
a closed path to and from the battery. The cords that you see on
appliances might look like single cords but actually contain at least
two wires. The wires connect the device to a power plant and back to
make a closed path.
Switches work by opening and closing the circuit. A switch that is
on closes the circuit and allows charge to flow through the electrical
devices. A switch that is off opens the circuit and stops the current.
check your reading How are switches used to control the flow of charge through
a circuit?
Standard symbols are used to represent the parts of a circuit. Some
common symbols are shown in the circuit diagrams below. The diagrams
represent the circuit shown on page 376 with the switch in both
open and closed positions. Electricians and architects use diagrams
such as these to plan the wiring of a building.
Circuit Diagrams
reminder
Current requires a
closed loop.
Symbols are used to represent the parts of a circuit. The circuit diagrams
below show the circuit from page 376 in both an open and closed position.
Key
cell
2-cell
battery
4-cell
battery
open
switch
light bulb
open switch = off
Would charge flow through the circuit diagrammed on the left?
Why or why not?
closed switch = on
Chapter 11: Circuits and Electronics 377

OUTLINE
Add this heading to
your outline, along with
supporting ideas.
I. Main idea
A. Supporting idea
1. Detail
2. Detail
B. Supporting idea
RESOURCE CENTER
CLASSZONE.COM
Explore resources on
electrical safety.
378 Unit 3: Electricity and Magnetism
Current follows the path of least resistance.
Since current can follow only a closed path, why are damaged cords so
dangerous? And why are people warned to stay away from fallen power
lines? Although current follows a closed path, the path does not have
to be made of wire. A person can become a part of the circuit, too.
Charge flowing through a person is dangerous and sometimes deadly.
Current follows the path of least resistance. Materials with low
resistance, such as certain metals, are good conductors. Charge will
flow through a copper wire but not the plastic coating that covers it
because copper is a good conductor and plastic is not. Water is also a
good conductor when mixed with salt from a person’s skin. That is
why it is dangerous to use electrical devices near water.
Short Circuits
A short circuit is an unintended path connecting one part of a circuit
with another. The current in a short circuit follows a closed path, but
the path is not the one it was intended to follow. The illustration
below shows a functioning circuit and a short circuit.
1
2
Functioning Circuit The charge flows through one wire, through
the light bulb, and then back through the second wire to the outlet.
Short Circuit The cord has been damaged and the two wires inside
have formed a connection. Now the path of least resistance is
through one wire and back through the second wire.
coating
wires
1 2
In the second case, without the resistance from the lamp, there is
more current in the wires. Too much current can overheat the wires and
start a fire. When a power line falls, charge flows along the wire and
into the ground. If someone touches that power line, the person’s
body becomes part of the path of charge. That much charge flowing
through a human body is almost always deadly.
check your reading Why are short circuits dangerous?

Grounding a Circuit
Recall that when lightning strikes a lightning rod, charge flows into
the ground through a highly conductive metal rod rather than
through a person or a building. In other words, the current follows
the path of least resistance. The third prong on some electrical plugs
performs a similar function. A circuit that connects stray current safely
to the ground is known as a grounded circuit. Because the third prong
grounds the circuit, it is sometimes called the ground.
In this illustration, green
represents the path that
connects the appliance and
the outlet to the ground.
ground
wire
Normally, charge flows through one prong, along a wire to an
appliance, then back along a second wire to the second prong. If there
is a short circuit, the charge might flow dangerously to the outside of
the shell of the appliance. If there is a ground wire, the current will
flow along the third wire and safely into the ground, along either a
buried rod or a cold water pipe.
check your reading What is the purpose of a ground wire?
Safety devices control current.
Orange is used in this illustration
to represent the path that connects
the appliance’s circuit to a power
source and back.
connects to
ground wire
Suppose your living room wiring consists of a circuit that supplies
current to a television and several lights. One hot evening, you turn
on an air conditioner in the living room window. The wires that
supply current to the room are suddenly carrying more current than
before. The lights in the room become dim. Too much current in a
circuit is dangerous. How do you know if there is too much current
in a wire?
Fortunately, people have been using electric current for over a
hundred years. An understanding of how charge flows has led to the
development of safety devices. These safety devices are built into
circuits to prevent dangerous situations from occurring.
! SAFETY TIPS
• Never go near a
fallen power line.
•Never touch an
electrical appliance
when you
are in the shower
or bathtub.
•Always dry your
hands thoroughly
before using
an electrical
appliance.
• Never use an
electrical cord
that is damaged
in any way.
•Never bend or
cut a ground
prong in order to
make a grounded
plug fit into
an ungrounded
outlet.
Chapter 11: Circuits and Electronics 379

How Fuses Work
How can you stop a current?
PROCEDURE
1
2
3
4
new
fuse
blown
fuse
If you turn on an air conditioner in a room full of other electrical
appliances that are already on, the circuit could overheat. But if the
circuit contains a fuse, the fuse will automatically shut off the current.
A fuse is a safety device that opens a circuit when there is too much
current in it. Fuses are typically found in older homes and buildings.
They are also found in cars and electrical appliances like air
conditioners.
A fuse consists of a thin strip of metal that is inserted into the circuit.
The charge in the closed circuit flows through the fuse. If too much
charge flows through the fuse, the metal strip melts. When the strip
has melted and the circuit is open, the fuse is blown. The photographs
on the left show a new fuse and a blown fuse. As you can see, charge
cannot flow across the melted strip. It has broken the circuit and
stopped the current.
How much current is too much? That varies. The electrician who
installs a circuit knows how much current the wiring can handle. He
or she uses that knowledge to choose the right kind of fuse. Fuses are
measured in amperes, or amps. Remember that amperage is a measure
of current. If a fuse has blown, it must be replaced with a fuse of the
same amperage. But a fuse should be replaced only after the problem
that caused it to blow has been fixed.
Fuses
Use the alligator clips to clip one end of each wire to the steel wool strand.
Place the steel wool strand in the jar. Tape the wires to the sides of the jar.
Clip the free end of one wire to the negative terminal of the battery.
What do you predict will happen when you complete the circuit?
Clip the free end of the remaining wire to the positive terminal
of the battery and observe the steel wool strand.
WHAT DO YOU THINK?
•What did you observe when you completed
the circuit? Why did that happen?
• How can you stop the current?
CHALLENGE How is the setup in this
activity similar to a fuse that would be
found in a home circuit? How does it differ?
380 Unit 3: Electricity and Magnetism
SKILL FOCUS
Making Models
MATERIALS
•2 pieces of insulated
wire with
alligator clips
• single strand of
steel wool
•glass jar
• tape
•6 V battery
TIME
15 minutes

Other Safety Devices
Most modern homes do not use fuses. Instead, they use safety
devices called circuit breakers. Circuit breakers, unlike fuses,
do not have to be replaced every time they open the circuit.
Like fuses, circuit breakers automatically open the circuit when too
much charge flows through it. If the circuit becomes overloaded
or there is a short circuit, the wire and the breaker grow hot.
That makes a piece of metal inside the breaker expand. As it
expands, it presses against a switch. The switch is then flipped to
the off position and the current is stopped. Once the problem is
solved, power can be restored manually by simply flipping the
switch back. The illustration on the right shows a circuit breaker.
check your reading How are circuit breakers similar to fuses?
The photograph at the bottom right shows another
safety device—a ground-fault circuit interrupter
(GFCI) outlet. Sometimes a little current leaks out of an
outlet or an appliance. Often it is so small you do not
notice it. But if you happen to have wet hands, touching
even a small current can be very dangerous.
GFCI outlets are required in places where exposure
to water is common, such as in kitchens and bathrooms.
A tiny circuit inside the GFCI outlet monitors the current
going out and coming in. If some of the current
starts to flow through an unintended path, there will be
less current coming in to the GFCI. If that happens, a
circuit breaker inside the GFCI outlet opens the circuit
and stops the current. To close the circuit again, you
push “Reset.”
KEY CONCEPTS
1. Describe three parts of a
circuit and explain what
each part does.
2. Explain the function of a
ground wire.
3. What do fuses and circuit
breakers have in common?
CRITICAL THINKING
4. Apply Suppose you have built
a circuit for a class project. You
are using a flat piece of wood
for its base. How could you
make a switch out of a paperclip
and two nails?
5. Communicate Draw a
diagram of a short circuit.
Use the symbols for the parts
of a circuit.
circuit breaker
open
circuit
ground-
fault
circuit
interrupter
CHALLENGE
6. Evaluate A fuse in a home
has blown and the owner
wants to replace it with a fuse
that can carry more current.
Why might the owner’s decision
lead to a dangerous
situation?
Chapter 11: Circuits and Electronics 381