Download chapter 18-electricity

Chapter Introduction
Lesson 1
Electric Charge
and Electric
Forces
Lesson 2
Electric Current
and Simple
Circuits
Lesson 3
Describing
Circuits
Chapter Wrap-Up
How do electric circuits
and devices transform
energy?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1. Protons and electrons have opposite
electric charges.
2. Objects must be touching to exert a
force on each other.
3. When electric current flows in a wire,
the number of electrons in the wire
increases.
Do you agree or disagree?
4. Electrons flow more easily in metals
than in other materials.
5. In any electric circuit, current stops
flowing in all parts of the circuit if a
connecting wire is removed or cut.
6. The light energy given off by a flashlight
comes from the flashlight’s batteries.
Electric Charge and Electric
Forces
• How do electrically charged objects
interact?
• How can objects become electrically
charged?
• What is an electric discharge?
Electric Charge and Electric
Forces
• static charge
• polarized
• electric insulator
• electric
discharge
• electric
conductor
• grounding
Electric Charges
• Protons and neutrons
make up the nucleus
of an atom.
• Protons in the nuclei
of atoms have positive
charge.
• Electrons moving
around a nucleus
have negative charge.
Electric Charges (cont.)
• An atom is electrically neutral—
it has equal numbers of protons
and electrons.
• Electrically neutral objects do not attract
or repel each other.
• An unbalanced negative or positive
electric charge on an object is sometimes
referred to as a static charge.
Electric Forces
• The region surrounding a charged object
is called an electric field.
• An electric field applies a force, called an
electric force, to other charged objects,
even if the objects are not touching.
• Objects with opposite electric charges
attract each other, and objects with
similar electric charges repel each other.
Charged objects can attract or repel each
other. The arrows show the direction of the
objects’ motion.
Electric Forces (cont.)
How do electrically charged
objects interact?
The strength of the electric force between
two charged objects depends on the total
amount of charge on both objects and the
distance between the objects.
Transferring Electrons
• A material in which electrons cannot
easily move is an electric insulator.
• A material in
which electrons
can easily move
is an electric
conductor.
Transferring Electrons (cont.)
• Electrons transfer between objects by
contact, induction, or conduction.
• When objects made of different materials
touch, electrons tend to collect on the
object that holds electrons more tightly.
This is called transferring charge by
contact.
• Both insulators and conductors can be
charged by contact.
Transferring Electrons (cont.)
Transferring charge by induction is a
process by which one object causes two
other objects that are conductors to
become charged without touching them.
• A negatively
charged balloon
repels electrons in
a metal soda can.
• The can is not
charged because it
has not gained or
lost any electrons.
• When electrons
concentrate at one
end of an object,
the object is
polarized.
• When a charged
balloon is brought
near two cans that
are touching, the
balloon polarizes
the cans as if they
are one object.
• Electrons in both
cans move toward
the far end of the
can on the right.
Then, the two cans
separate.
• The cans that were
originally polarized
as a group are now
individually charged.
• The can on the right
has an unbalanced
negative electric
charge, and the can
on the left has an
unbalanced positive
electric charge.
Transferring Electrons (cont.)
• Another way that electrons transfer
between two conductors is called
transferring charge by conduction.
• When conducting objects with unequal
charges touch, electrons flow from the
object with a greater concentration of
negative charge to the object with a lower
concentration of negative charge.
Transferring Electrons (cont.)
The flow of electrons continues until the
concentration on charge of both objects is
equal.
Transfer by conduction is similar to water
flowing from a container with a higher water
level to a container with a lower water level.
Transfer by conduction is similar to water
flowing from a container with a higher water
level to a container with a lower water level.
Transferring Electrons (cont.)
What are three ways by which
objects can become electrically
charged?
Electric Discharge
The process of an unbalanced electric
charge becoming balanced is an electric
discharge.
electric
from Greek ēlektron, means
“beaming sun”
Electric Discharge (cont.)
• Lightning is a powerful electric discharge
that occurs in an instant.
• Providing a path for electric charges to
flow safely into the ground is called
grounding.
• Lightning rods provide a path for the
electrons released in a lightning strike to
travel into the ground.
Electric Discharge (cont.)
What is an electric discharge?
Within a storm cloud, warm air rises past falling cold
air. The cold air is filled with hail, ice, and partially
frozen water droplets that pick up electrons from the
rising air. This causes the bottom of the cloud to
become negatively charged.
The negatively charged cloud polarizes Earth’s
surface by repelling negative charges in the ground.
Thus, the surface of the ground becomes positively
charged.
When the bottom of the cloud accumulates enough
negative charge, the attraction of the positively
charged ground causes electrons in the cloud to
begin moving toward the ground.
As electrons approach the ground, positive charges
quickly flow upward, making an electric connection
between the cloud and the ground. You see this
electric discharge as lightning.
• Any object can be positively charged,
negatively charged, or neutral.
• Charged objects exert forces on each
other.
• Lightning is one form of electric
discharge.
Which term refers to a material in
which electrons can easily move?
A. electric conductor
B. electric field
C. electric insulator
D. lightning rod
In which process does one object
cause two other objects that are
conductors to become charged
without touching them?
A. transferring charge by induction
B. transferring charge by contact
C. transferring charge by conduction
D. electric discharge
When electrons concentrate at
one end of an object, that object
can be described as which of
these?
A. balanced
B. electric
C. grounded
D. polarized
Do you agree or disagree?
1. Protons and electrons have opposite
electric charges.
2. Objects must be touching to exert a
force on each other.
Electric Current and Simple
Circuits
• What is the relationship between
electric charge and electric current?
• What are voltage, current, and
resistance? How do they affect each
other?
Electric Current and Simple
Circuits
• electric current
• voltage
• electric circuit
• Ohm’s law
• electric
resistance
Electric Current and Electric Circuits
The movement of electrically charged
particles is an electric current.
current
Science Use the flow of electric
charge
Common Use occurring at the
present time
Electric Current and Electric Circuits
(cont.)
A closed, or complete, path which an
electric current travels is an electric
circuit.
circuit
from Latin circuire, means “to go
around”
Electric Current and Electric Circuits
(cont.)
• All electric circuits have one thing in
common—they transform electric energy
to other forms of energy.
• The number of electrons flowing into a
wire from a power source equals the
number of electrons flowing out of the
wire back into the source.
Electric Current and Electric Circuits
(cont.)
• An electric current
flows in a circuit if
the circuit is
complete, or
closed.
• Current will not
flow if the circuit is
broken, or open.
Electric Current and Electric Circuits
(cont.)
• Electric current is approximately
measured as the number of electrons
that flow past a point every second.
• Scientists count electrons by a quantity
called the coulomb.
Electric Current and Electric Circuits
(cont.)
• The SI unit for electric current is the
ampere, commonly called an amp (A).
• One ampere of current equals about one
coulomb of electrons flowing past a point
in a circuit every second.
Electric Current and Electric Circuits
(cont.)
What is the relationship
between electric charge
and electric current?
What is electric resistance?
• Electric resistance is a measure of how
difficult it is for an electric current to flow
in a material.
• The unit
of electric
resistance
is the ohm (Ω).
What is electric resistance? (cont.)
• A good electric insulator has high electric
resistance. Atoms of an insulator hold
electrons tightly, which prevents electric
charges from easily moving through
material.
• A material’s electric resistance depends
on the material’s length and thickness.
• Increasing the length of a conductor also
increases its electric resistance.
What is voltage?
• Voltage is the amount of energy the
source uses to move one coulomb of
electrons through a circuit.
• A circuit with a high voltage source
transforms more electric energy to other
energy forms than a circuit with a low
voltage source.
What is voltage? (cont.)
• Electric energy transforms to other forms
of energy in all parts of a circuit.
• The voltage measured across a portion
of a circuit indicates how much energy
transforms in that portion of the circuit.
Voltage can be different in different parts of
a circuit. The voltmeter shows where most
of the battery’s energy is used.
Ohm’s Law
• Named after the German physicist Georg
Ohm, Ohm’s law is a mathematical
equation that describes the relationship
among voltage, current, and resistance.
• Ohm’s law states that as the voltage of a
circuit’s electric energy source increases,
the current in the circuit increases, too.
Ohm’s Law (cont.)
As the resistance of a circuit increases,
the current decreases.
Ohm’s law can be used to calculate
unknown quantities in a circuit.
Ohm’s Law (cont.)
How do voltage, current, and
resistance affect each other?
Ohm’s law states that, with equal current,
the voltage is greater across the device
with the greater resistance.
• An electric current is the flow of
negative electric charges through a
conductor.
• Electrical devices function only when
they are connected in a closed
circuit.
• Ohm’s law shows the relationship
among voltage, current, and
resistance.
Which is the unit of electric
resistance?
A. amp
B. coulomb
C. ohm
D. voltage
Which term refers to the amount
of energy the source uses to
move one coulomb of electrons
through a circuit?
A. amp
B. coulomb
C. ohm
D. voltage
What property of a wire increases
when it is made longer?
A. charge
B. current
C. resistance
D. voltage
Do you agree or disagree?
3. When electric current flows in a wire,
the number of electrons in the wire
increases.
4. Electrons flow more easily in metals
than in other materials.
Describing Circuits
• What are the basic parts of an electric
circuit?
• How do the two types of electric
circuits differ?
Describing Circuits
• series circuit
• parallel circuit
Parts of an Electric Circuit
• Three common parts of most electric
circuits are a source of electric energy,
electrical devices that transform the
electric energy, and
conductors that
connect the
components.
• An energy source,
such as a battery,
produces an electric
current in a circuit.
Parts of an Electric Circuit (cont.)
What are the three basic
parts of an electric circuit?
Parts of an Electric Circuit (cont.)
• Some electrical devices are designed
to transform the electric energy of the
current to kinetic energy.
• As the chemicals in a battery react,
electrons in the battery concentrate
at the battery’s negative terminal.
• When a closed circuit connects the
terminals, electrons flow in the circuit
from the battery’s negative terminal
to the positive terminal.
Parts of an Electric Circuit (cont.)
• Electrical devices in a circuit transform
most of the electric energy to other
useful forms of energy.
• Metal wires often are used to complete
circuits by connecting a circuit’s energy
source and a device.
As electrons flow through a wire, they
move in random directions, colliding with
the atoms that make up the wire.
Series and Parallel Circuits
• There are two types of electric circuits—
series circuits and parallel circuits.
• A series circuit is an electric circuit with
only one path for an electric current to
follow.
• According to Ohm’s law, when resistance
increases and voltage remains the same,
there is less current in the circuit.
Adding devices to a series circuit adds
resistance to the total circuit.
Series and Parallel Circuits (cont.)
• A parallel circuit is an electric circuit
with more than one path, or branch, for
an electric current to follow.
• A device in a parallel circuit connects to
the source with its own branch.
• Adding branches to a parallel circuit
does increase the total current through
the source.
Parallel circuits have more than one path
through which electric current can flow.
Series and Parallel Circuits (cont.)
How do the two types of
electric circuits differ?
Electric Circuits in the Home
• Electric energy used in most homes and
businesses is generated at large power
plants.
• A complex system of transmission wires
carries the electric energy to all parts of
the country.
Electric energy enters your home through
a main wire and travels through an electric
meter which measures your energy use.
Electric Circuits in the Home (cont.)
• Fuses and circuit breakers automatically
open a circuit when the current becomes
dangerously high.
• A fuse is a piece of metal that breaks
a circuit by melting from the thermal
energy produced by a high current.
• A circuit breaker is a switch that
automatically opens a circuit when the
current is too great.
Electric Safety
• An electric shock occurs when an
electric current passes through the
human body.
• Each year, more than 500 people die by
accidental electric shock in the United
States.
Electric Safety (cont.)
• There are many ways to help protect
yourself from electric shock.
• Never use electrical devices with
damaged power cords.
• Stay away from water when using
electrical devices plugged into an
outlet.
Electric Safety (cont.)
• Avoid using extension cords and
never plug more than two home
appliances into an outlet at once.
• Never climb utility poles or play on
fences surrounding electricity
substations.
• A battery can be the
source of electric energy
in a circuit.
• A series circuit has only one path for
all devices in the circuit.
• A parallel circuit
has a separate path
for each device in
the circuit.
Which is an example of an energy
source?
A. wire
B. device
C. circuit
D. battery
Which term refers to an electric
circuit with more than one path,
or branch, for electric current to
follow?
A. electrical device
B. fuse
C. parallel circuit
D. series circuit
Electric energy enters your home
through which of these?
A. circuit breaker
B. electric meter
C. fuse
D. main wire
Do you agree or disagree?
5. In any electric circuit, current stops
flowing in all parts of the circuit if a
connecting wire is removed or cut.
6. The light energy given off by a
flashlight comes from the flashlight’s
batteries.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
An electric energy
source changes the
motion of the electrons
in a circuit. Energy is
conserved as a circuit
transforms the energy
of moving electrons to
light, sound, and
thermal energy.
Lesson 1: Electric Charge and
Electric Forces
• Particles that have the same type of electric charge
repel each other. Particles that have different types
of electric charge attract each other.
• Objects become negatively charged when they gain
electrons, and become positively charged when they
lose electrons.
• An electric discharge is the
loss of static charge.
Lesson 2: Electric Current and
Simple Circuits
• An electric current is the flow of electrically charged
particles through a conductor.
• According to Ohm’s law, across
any portion of an electric circuit,
voltage (V), current (I), and electric
resistance (R) are related by the
equation V = IR.
Lesson 3: Describing Circuits
• Electric circuits have a source of electric energy to
produce an electric current, one or more electric
devices to transform electric energy to useful forms
of energy, and wires to connect the circuit’s
device(s) to the energy source.
• A series circuit has one path in
which current flows. A parallel
circuit has more than one path
in which current flows.
Which term describes an
unbalanced negative or positive
electric charge on an object?
A. electric conductor
B. electric insulator
C. polarized
D. static charge
Which term refers to when objects
made of different materials touch,
and electrons tend to collect on the
object that holds electrons more
tightly?
A. charge by conduction
B. charge by contact
C. charge by induction
D. electric discharge
Which term describes a closed,
or complete, path which an
electric current travels?
A. voltage
B. electric resistance
C. electric circuit
D. coulomb
What is the name that scientists
use to count electrons by a
quantity?
A. amp
B. circuit
C. coulomb
D. voltage
Which term describes an electric
circuit with only one path for an
electric current to follow?
A. battery
B. circuit breaker
C. parallel circuit
D. series circuit
Which is a material in which
electrons cannot easily move?
A. electric conductor
B. electric discharge
C. electric field
D. electric insulator
Which term refers to an
unbalanced electric charge
becoming balanced?
A. electric discharge
B. grounded
C. polarized
D. static charge
Which is the SI unit for electric
current?
A. amp
B. coulomb
C. ohm
D. voltage
Which term refers to the
movement of electrically charged
particles?
A. electric circuit
B. electric current
C. electric resistance
D. voltage
In an electric circuit, which
transforms most of the electric
energy to other useful forms of
energy?
A. battery
B. circuit breaker
C. electrical device
D. fuse