Transcript Table of Contents

Chapter 16
Electricity
Section 1 Electrical Charge and Force
Section 2 Current
Section 3 Circuits
Chapter 16
Section 1 Electrical Charge and
Force
•
Indicate which pairs of charges will repel
and which will attract.
•
Explain what factors affect the strength
of the electric force.
•
Describe the characteristics of the
electric field due to a charge.
Chapter 16
Section 1 Electrical Charge and
Force
1.
Static electricity is a part of life. Can you
name at least five examples of static
electricity that occur in your home?
2.
Fabric softeners are commonly used
today because they eliminate static cling.
3.
Explain why clothes in the dryer get static
cling.
4.
Why can walking across a carpeted room
be a shocking experience?
Chapter 16
•
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Electric charge is an electrical property
of matter that creates electric and
magnetic forces and interactions.
Like charges repel, and opposite
charges attract.
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Section 1 Electrical Charge and
Force
The two types of charges are called positive
and negative.
An object’s electric charge depends on
the imbalance of its protons and
electrons.
•
Electrons are negatively charged, protons
are positively charged, and neutrons are
neutral (no charge).
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
•
Section 1 Electrical Charge and
Force
•
Negatively charged objects have more
electrons than protons.
•
Positively charged objects have fewer
electrons than protons.
The SI unit of electric charge is the
coulomb, C.
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A proton has a charge of 1.6  10–19 C
An electron has a charge of 1.6  10–19 C.
The net electric charge of a charged object
is always a multiple of 1.6  10–19 C.
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
•
Conductors allow charges to flow;
insulators do not.
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•
Section 1 Electrical Charge and
Force
An electrical conductor is a material in which charges
can move freely and that can carry an electric current.
An electrical insulator is a material that does not transfer
current easily.
Objects can be charged by the transfer
of electrons.
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The outermost electrons can be easily transferred from
one atom to another.
Charging by friction is when one material gains
electrons and becomes negatively charged, and the
other loses electrons and becomes positively charged.
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
•
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Section 1 Electrical Charge and
Force
Objects can also be charged without friction.
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One way to charge a neutral object without friction is
by touching it with a charged object.
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Objects charged in this manner are said to be charged
by contact.
Charges move within uncharged objects.
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The charges in a neutral conductor can be redistributed
without contacting a charged object.
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Although the total charge on the conductor will be
zero, the opposite sides will have an induced charge.
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This polarization of the atoms or molecules of an
insulator produces an induced charge on the surface of
the insulator.
Chapter 16
Section 1 Electrical Charge and
Force
When a negative rod touches a
neutral doorknob, electrons
move from the rod to the
doorknob.
The transfer of electrons to
the metal doorknob gives
the doorknob a net
negative charge.
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
•
•
Section 1 Electrical Charge and
Force
Electric force is the force of attraction or repulsion
between objects due to charge.
• The electric force at the atomic and molecular
level is responsible for most of the common
forces we can observe.
• The electric force is also responsible for effects
that we can’t see.
Electric force depends on charge and distance.
• The electric force between two objects is
proportional to the product of the charges on
the objects.
• The electric force is inversely proportional to the
square of the distance between two objects.
Chapter 16
•
Section 1 Electrical Charge and
Force
Electric force acts through a field.
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An electric field is a region in space around
a charged object that causes a stationary
charged object to experience an electric
force.
One way to show an electric field is by
drawing electric field lines.
Electric field lines point in the direction of the
electric force on a positive charge.
The electric field lines around a positive charge point
outward.
The electric field lines around a negative charge point
inward.
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
Section 1 Electrical Charge and
Force
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Electric field lines never cross one
another.
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Field lines show both the direction of an
electric field and the relative strength
due to a given charge.
• More lines are drawn for greater charges
to indicate greater force.
Chapter 16
Section 1 Electrical Charge and
Force
Chapter 16
Section 2 Current
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Describe how batteries are sources of
voltage.
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Explain how a potential difference
produces a current in a conductor.
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Define resistance.
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Calculate the resistance, current, or
voltage, given the other two quantities.
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Distinguish between conductors,
superconductors, semiconductors, and
insulators.
Chapter 16
1.
2.
3.
4.
Section 2 Current
Dry cell batteries are a source of mobile electrical
power. Name five devices that use dry cell
batteries.
Give reasons why copper is normally used to wire
a home for electricity.
Why is it important to unplug a device by pulling
the plug instead of by yanking it out of the socket
by pulling on the electrical cord?
Why are electrical appliances, such as razors, hair
dryers, and curling irons, not to be used in the
bathtub or shower?
Chapter 16
Section 2 Current
• Electrical potential energy is the ability
to move an electric charge from one
point to another.
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The electrical potential energy of the
moving charge decreases because the
electric field does work on the charge.
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The electrical potential energy depends on
the distance between two charged objects
for both an attractive and a repulsive
electric force.
Chapter 16
Section 2 Current
The electrical potential energy between two negative charges
decreases as the distance between them increases.
Chapter 16
Section 2 Current
The electrical potential energy of a charge depends on its position in
an electric field.
Chapter 16
Section 2 Current
Chapter 16
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Section 2 Current
Potential difference is measured in volts.
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The potential difference between any two
points, is the work that must be done
against electric forces to move a unit
charge from one point to the other.
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The volt, V, is equivalent to one joule per
coulomb (1 J/C).
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Potential difference is often called voltage.
Chapter 16
Section 2 Current
Chapter 16
Section 2 Current
Chapter 16
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Section 2 Current
There is a voltage across the terminals of
a battery.
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A cell is a device that is a source of electric
current because of a potential difference,
or voltage, between the terminals.
• One terminal is positive, and the other is
negative.
•
Batteries convert chemical energy into
electrical energy.
Chapter 16
Section 2 Current
Chapter 16
Section 2 Current
Chapter 16
Section 2 Current
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A voltage sets charges in motion.
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Current is the rate that electric
charges move through a conductor.
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The SI unit of current is the ampere, A.
1 amp = 1 C/s
A battery is a direct current source because the
charges always move from one terminal to the other
in the same direction.
Conventional current is defined as
movement of positive charge.
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The direction of current in a wire is opposite the
direction that electrons move in that wire.
Chapter 16
Section 2 Current
Chapter 16
Section 2 Current
Chapter 16
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Section 2 Current
Resistance is the opposition posed by a material
or a device to the flow of current.
• Resistance is caused by internal friction, which
slows the movement of charges through a
conducting material.
Resistance can be calculated from current and
voltage
V
voltage.
resistance =
current
R=
I
• The SI unit of resistance is the ohm, Ω. 1 Ω =
1 V/A
• A resistor is a special type of conductor used
to control current.
Chapter 16
•
Section 2 Current
Resistance The headlights of a typical
car are powered by a 12 V battery. What
is the resistance of the headlights if they
draw 3.0 A of current when turned on?
1. List the given and unknown values.
Given: current, I = 3.0 A
voltage, V = 12 V
Unknown: resistance, R = ? Ω
Chapter 16
Section 2 Current
2. Write the equation for resistance.
voltage
resistance =
current
3.
V
R=
I
Insert the known values into the equation, and solve.
V
12 V
R=
=
I
3.0 A
R = 4.0 
Chapter 16
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Conductors have low resistances.
Some materials become
superconductors below a certain
temperature.
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Section 2 Current
Certain metals and compounds have zero resistance
when their temperature falls below a certain
temperature called the critical temperature.
Semiconductors are intermediate to
conductors and insulators.
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The controlled addition of specific atoms of other
materials as impurities dramatically increases a
semiconductor’s ability to conduct electric charge.
Chapter 16
Section 3 Circuits
•
Use schematic diagrams to represent
circuits.
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Distinguish between series and parallel
circuits.
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Calculate electric power using voltage
and current.
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Explain how fuses and circuit breakers
are used to prevent circuit overload.
Chapter 16
Section 3 Circuits
1. Inexpensive electrical power is essential. List at least
ten electrical devices that you have used today.
2. In some strings of Christmas lights, none of the lights
work if one light is burned out. What is a possible
explanation for this?
3. A big feast is being prepared for several people in a
home. The cooks are using a turkey roaster, the oven,
an electric mixer, the blender, and the toaster. Every
light is on and so is the refrigerator. All at once the
power in the kitchen goes out. What is an
explanation for this, and how can it be corrected?
Chapter 16
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Section 3 Circuits
An electric circuit is a path through
which charges can be conducted.
• An electric circuit is a set of electrical
components connected such that they
provide one or more complete paths for the
movement of charges.
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The conducting path produced when a light bulb is
connected across the battery’s terminals is called a
closed circuit.
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Without a complete path, there is no charge flow and
therefore no current. This is called an open circuit.
Chapter 16
Section 3 Circuits
Chapter 16
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Section 3 Circuits
Switches interrupt the flow of charges in a circuit.
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You can use a switch to open and close a circuit.
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Schematic diagrams are used to represent
circuits.
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A schematic diagram is a graphical
representation of a circuit that uses lines to
represent wires and different symbols to represent
components.
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Each element used in a piece of electrical
equipment is represented by a symbol that reflects
the element’s construction or function.
Chapter 16
Section 3 Circuits
Chapter 16
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Section 3 Circuits
Series circuits have a single path for
current.
When appliances or other devices are
connected in a series circuit, they form a
single pathway for charges to flow.
In a series circuit, the components of a
circuit form a single path for current.
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The current in each device is the same.
The resistances may be different.
The voltage across each device in a series circuit can be
different.
If one element along the path in a series circuit is removed,
the circuit will not work.
Chapter 16
Section 3 Circuits
Chapter 16
•
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Section 3 Circuits
Parallel circuits have multiple paths for
current.
A parallel circuit is a circuit in which all of
the components are connected to
each other side by side.
• The voltage across each device is the same.
• The current in each device does not have to
be the same.
• A break in any one path in a parallel circuit
does not interrupt the flow of electric
charge in the other paths.
Chapter 16
Section 3 Circuits
Chapter 16
Section 3 Circuits
When bulbs are connected
in series, charges must pass
through both light bulbs to
complete the circuit.
When devices are connected
in parallel, charges have more
than one path to follow. The
circuit can be complete even
if one light bulb burns out.
Chapter 16
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Section 3 Circuits
Electrical energy is the energy that is
associated with charged particles because
of their positions.
Electric power is the rate at which electrical
energy is used in a circuit.
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The rate at which electrical work is done is
called electric power.
power  current  voltage
P = IV
• The SI unit for power is the watt (W).
• 1W=1A1V
Chapter 16
•
Section 3 Circuits
If you combine the electric power
equation above with the equation V
= IR, the power lost, or dissipated, by a
resistor can be calculated.
2
V
P = I 2R =
R
• Electric companies measure energy consumed in kilowatt-hours.
• 1 kW•h = 3.6  106 J.
Chapter 16
Section 3 Circuits
Chapter 16
•
Section 3 Circuits
Electric Power When a hair dryer is
plugged into a 120 V outlet, it has a 9.1 A
current in it. What is the hair dryer’s
power rating?
1. List the given and unknown values.
Given: voltage, V = 120 V
current, I = 9.1 A
Unknown: electric power, P = ? W
Chapter 16
Section 3 Circuits
2. Write the equation for electric power.
power = current × voltage
P = IV
3. Insert the known values into the
equation, and solve.
P = (9.1 A)(120 V)
P = 1.1 × 103 W
Chapter 16
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Section 3 Circuits
When electrical wires carry more than a
safe level of current, the circuit is said to be
overloaded.
A short circuit can happen if a wire’s
insulation wears down, two wires may
touch, creating an alternative pathway for
current.
Fuses melt to prevent circuit overloads.
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A fuse an electrical device that contains a
metal strip that melts when current in the circuit
becomes too great.
Chapter 16
•
Section 3 Circuits
Circuit breakers open circuits with
high current.
• A circuit breaker a switch that opens a
circuit automatically when the current
exceeds a certain value.
• The circuit breaker acts as a switch.
• Unlike fuses, circuit breakers can be reset
by turning the switch back on.
Chapter 16
Section 3 Circuits
Chapter 16
Section 3 Circuits
Chapter 16
Standardized Test Prep
1. A 12-volt battery is connected to a light
bulb that has a resistance of 240 ohms.
Calculate the amount of current in
amperes drawn by the circuit.
A. 0.05 A
B. 0.5 A
C. 2.0 A
D. 20.0 A
Chapter 16
Standardized Test Prep
1. A 12-volt battery is connected to a light
bulb that has a resistance of 240 ohms.
Calculate the amount of current in
amperes drawn by the circuit.
A. 0.05 A
B. 0.5 A
C. 2.0 A
D. 20.0 A
Chapter 16
Standardized Test Prep
2. Which of the following statements is
true?
F. Electrical forces exist between any
two neutral particles.
G.Electrical forces exist between any
two charged
particles.
H. Electrical forces exist only between
particles with
the same charge.
I. Electrical forces exist only between
particles with
opposite charges.
Chapter 16
Standardized Test Prep
2. Which of the following statements is
true?
F. Electrical forces exist between any
two neutral particles.
G.Electrical forces exist between any
two charged
particles.
H. Electrical forces exist only between
particles with
the same charge.
I. Electrical forces exist only between
particles with
opposite charges.
Chapter 16
Standardized Test Prep
3. What happens to an object that is
charged by induction?
A. It acquires excess electrons.
B. It remains electrically neutral.
C. It acquires a net charge from the
object that is
inducing the charge.
D. A current exists between it and the
object that is
inducing the charge.
Chapter 16
Standardized Test Prep
3. What happens to an object that is
charged by induction?
A. It acquires excess electrons.
B. It remains electrically neutral.
C. It acquires a net charge from the
object that is
inducing the charge.
D. A current exists between it and the
object that is
inducing the charge.
Chapter 16
Standardized Test Prep
4. A stream of water is deflected when a
negatively charged rod is held close to
it. Use the concept of induced charge to
explain what happens.
Chapter 16
Standardized Test Prep
4. A stream of water is deflected when a
negatively charged rod is held close to
it. Use the concept of induced charge to
explain what happens.
Answer: A charge is induced on the water
by the charged rod. Attraction between
the charged part of the stream and the
rod deflects the flow.
Chapter 16
Standardized Test Prep
5. How does a fuse protect a circuit from
overloading?
Chapter 16
Standardized Test Prep
5. How does a fuse protect a circuit from
overloading?
Answer: The fuse has a thin wire with a low
melting point. When electrical resistance
increases the temperature too high, it
melts and breaks the circuit.
Chapter 16
Standardized Test Prep
In 1909 Robert Millikan measured the
charge on an electron in what is known as
the oil-drop experiment. He sprayed oil
droplets into a chamber. Two plates with
opposite charges produced an electric
field. Some of the drops acquired a
negative charge. The field was adjusted so
there was an upward force equal to the
downward pull of gravity. From the strength
of this force, Millikan could calculate the
amount of the electric charge on the drop.
6. Analyze how the electric field could cause
an oil drop to float inside the chamber.
Chapter 16
Standardized Test Prep
6. Analyze how the electric field could
cause an oil drop to float inside the
chamber.
Answer: Electrical forces between the
charged particle an the charged plates
balance the downward pull of gravity.
Chapter 16
Standardized Test Prep
7. Which metal would be
the best choice for a
power line, based on
electrical resistance?
F. aluminum
G. iron
H. lead
I. silver
Chapter 16
Standardized Test Prep
7. Which metal would be
the best choice for a
power line, based on
electrical resistance?
F. aluminum
G. iron
H. lead
I. silver
Chapter 16
Standardized Test Prep
8. Which metal would most
likely be then best
substitute for the tungsten
filament in an
incandescent light bulb
based on resistance?
A. aluminum
B. iron
C. lead
D. silver
Chapter 16
Standardized Test Prep
8. Which metal would most
likely be then best
substitute for the tungsten
filament in an
incandescent light bulb
based on resistance?
A. aluminum
B. iron
C. lead
D. silver