Transcript chapter 11 - Nutley Schools

CHAPTER 11
ELECTRICITY
Electricity
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11.1 – Electric Charges
11.2 – Static Electricity
11.3 – Electric Current
11.4 - Electric circuits
11.5 – Electric Power & Safety
11.1 Electric Charges
Objectives
• Identify two forces that result from electric charges.
• Explain why objects attract and repel each other
• Communicate how a positively charged object can be
used to determine the charge on another object.
• Infer how electric charges behave in everyday situations.
Charge and Force
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All matter is made of atoms that contain electrons, neutrons,
and protons.
•
Protons and electrons in atoms have an electric charge.
•
Electrons have a negative charge, and protons have a positive
charge. Neutrons have no electric charge.
•
When the atoms in an object have more electrons than
protons, the total charge of the object is negative.
•
When the atoms in an object have more protons than
electrons, the total charge of the object is positive.
•
When an object has an equal number of protons and
electrons, the object has no charge. It is neutral.
• Objects are forced together, or attracted,
when their charges are different.
• An object with a positive charge is attracted
to an object with a negative charge.
• When objects have the same electric charge,
they are pushed apart, or repelled.
• SO…
• Unlike charges attract
• Like charges repel.
Force of Attraction
• A cloth and the balloon are both
neutral objects.
• When two neutral objects are
rubbed together, electrons will leave
one object and move onto the other.
• The total, or net, charge of each
object will become either positive or
negative.
Force of Attraction
• When the cloth rubs against the balloon some
electrons in the cloth move to the balloon.
• Since the balloon now has more electrons, its net
charge is negative. Since the cloth loses electrons, its
net charge is now positive.
• Because unlike charges attract, the positively-charged
cloth attracts the negatively-charged balloon.
Force of Repulsion
• Imagine a second balloon is rubbed with the cloth.
• How will this balloon react to the negatively-charged
balloon when they are brought near each other?
• The second balloon now has more electrons than it has
protons.
• Like the first balloon, its net charge is negative.
• This force of repulsion occurs because the objects have
like charges.
• Why do you think the hairs on your head stand away
from each other when you comb your hair?
• What is the charge of the comb?
Electric Field
• How close do you think two charged objects must be in
order to be affected by each other?
• The forces of attraction and repulsion depend on the
electric field around each object.
• An electric field is the region surrounding a charged
object.
• The strongest part of the field is the area closest to the
charged object. The weakest part of the field is the area
farthest from the charged object.
• An object with a large amount of charge has a bigger
electric field than an object with a small amount of charge.
Electric Field
Check & Explain
1. Name the two types of forces that can result from electric
charges.
2. You bring two balloons together and they repel. Explain what
happens in terms of charges .
3. How can a negatively charged comb be used to determine the
charge of an object?
4. Why does the television screen attract dust after the screen
has been wiped clean?
11.2 Static Electricity
Objectives
1.
2.
3.
4.
Identify three ways static charge can build up.
Explain what causes lightning.
Compare electric conductors and insulators
Infer why lightning can be dangerous.
Static Electricity
• Lightning and the shock that you might get from a metal
doorknob are alike.
• Both are caused by static electricity.
• All electricity is energy that results from the movement of
electrons.
• Static electricity, however, consists of electric charges at
rest.
• The electrons that cause static electricity can move from
one object to another, and then remain at rest.
• When electrons gather in one location, a build-up of static
electricity results.
Static Build Up
• A neutral object builds up an electric
charge by gaining or losing electrons.
• Movement of static electricity depends on
the forces of attraction and repulsion and
the way charges act in different materials.
• The build-up of static charge can be
caused by friction, conduction, or
induction.
A simple electroscope detects
static charges
• An electroscope is a device that
detects static electricity by using thin
metal-like leaves, which separate
when charged.
• An object with a suspected static
electric charge is brought near the
metal ball of the electroscope.
• Electrical charges move to the metal
and down to the metal leaves, which
then repel each other.
Friction
• Charging by friction occurs when
objects rub together and electrons
move from the surface of one
object to the surface of the other.
• Friction separates electrons from
the surface of an object whose
atoms hold electrons loosely.
• Once separated, the electrons
move onto another object.
Conduction
• Charging by conduction occurs when electrons are
transferred from one material to another by direct
contact.
• Conduction causes the small electric shock you feel
when you rub your shoes on a carpet and touch
metal.
• The shock you feel occurs when electrons move from
you to the metal.
Induction
• When charges on an object are rearranged without
physical contact, charging by induction occurs.
Conductors
• A material through which electric charges move easily is
called a conductor.
• Most metals are good conductors.
• Good conductors have a large number of freely-moving
• electrons.
• Gold, silver, copper, aluminum, and mercury are the best
conductors of electricity.
• Metals that are good conductors can be
useful.
• Copper and aluminum are used in electric
wiring and telephone lines.
• Besides electrons, heat also moves easily
through conductors.
• Other materials are also good conductors.
• For example, the acid used in a car battery
conducts electric charges.
• Water is also a good conductor.
Insulators
• A material through which electric charges can't
move easily is called an insulator.
• Electrons move extremely slowly through good
insulators.
• Electrons are tightly bound to the atoms in good
insulating materials and the electrons are not
free to move around.
• Materials such as wood, ceramic,
rubber, glass, and plastic are
electric insulators.
• Charges don't flow easily through
insulators.
• But charges do collect on their
surfaces. The electrons that build
up on the surface of an insulator
tend to remain there as a static
charge.
Lightning
• Electrons that build up as static charge
eventually leave the object.
• The electrons may move to the surface of
another object, or they may move onto
molecules in the air.
• The loss of static electricity by an object that
occurs when electrons move is called a static
discharge.
• A static discharge may occur slowly and quietly,
or it may occur rapidly, converting electric
energy into sound, light, and heat energy.
• The lightning you see during a thunderstorm is
caused by the discharge of static electricity .
• During a storm, water molecules and particles of
matter in clouds are rubbed together by the
wind.
• As they move, the water molecules and particles
become charged by friction. As a result, areas of
positive and negative charges form in the clouds.
Check & Explain
1. Name three different ways static charges can build
up on an object.
2. Explain how lightning and a shock from static electricity
are related to each other.
3. Compare and Contrast - How do conductors and insulators
of electricity differ? Identify three materials that are good
conductors and three materials that are good insulators.
11.3 Electrical Currents
Objectives
1. Identify three sources of electric current.
2. Distinguish between current, voltage, and
resistance.
3. Calculate resistance using Ohm's law
4. Predict how the resistance of a wire changes with
wire temperature, length, thickness, and type of
material.
Sources of Electrical Current
• You know that electrons at rest produce static
electricity.
• However, electrons in a wire move, or flow.
• These moving electrons are called electric
current.
• Electric current flows through a closed,
continuous path, called a circuit.
• A source of electrons is needed to produce
electric current in the wire of the circuit.
• To keep electric current moving, the electron
source must also provide a difference in the
charges present at each end of the wire.
• One end of the wire must be negatively
charged. The other end must be positively
charged.
• The difference in the charges at each end of
the wire is called potential difference, or
voltage.
• The potential difference gives electrons
energy to move from the negative end of the
wire toward the positive end.
• A device that produces the potential
difference needed to move electrons
through a circuit is an energy source.
• Electrochemical cells and thermocouples
are sources of electric current.
• The two kinds of electrochemical cells are
wet cells and dry cells, often referred to
as batteries.
• An electrochemical cell changes chemical
energy into electric energy.
• Differences in
temperature are
also used to
generate electric
current with a
device called a
thermocouple.
Types of Currents
• When an electrochemical cell is connected to a
circuit, it causes a steady flow of electric current.
• Current that flows from an electrochemical cell
source moves in one direction.
• Electrons that flow in the same direction in a wire
produce direct current, or DC.
• Electrons that flow in different directions in a wire
produce an alternating current, or AC. The
electricity in your home is alternating current.
Flow Rate
• The number of electrons that pass a specific point in
a circuit in one second indicates the flow rate of
electric current.
• The flow rate increases as the number of electrons
passing a point each second increases. If fewer
electrons pass a specific point each second, the rate
of current flow decreases.
• In the SI system, the unit measure for current is
amperes.
• Amperes are written as "amps," or as the letter "A."
• An instrument called
an ammeter can be
connected to an
electric circuit to
measure the flow rate
of electric current.
Voltage
• Electrons need an energy source to force them through
a wire.
• The energy to move electrons depends on the potential
difference between the positive and negative terminals
of a dry cell.
• The positive terminal of a dry cell has high potential.
The negative terminal has low potential. The difference
between high and low potential is the voltage of the
dry cell.
• Voltage is measured in the SI system in volts. The
symbol for volt is the capital letter V.
• Voltage is measured
with an instrument
called a voltmeter.
• Notice how the
voltmeter is connected
to the circuit.
Resistance
• Electrons flow easily through a copper wire to the
filament in a light bulb.
• However, when electrons reach the filament, its
resistance is so great that the electric energy is
converted into heat and light energy.
• The force opposing the flow of electrons through
• the filament is called resistance.
• Good conductors have low resistance.
• Poor conductors, called resistors, have a high
• resistance.
• A wire's resistance depends on the material
from which it is made and its length, thickness,
and temperature.
• Metals, such as copper and aluminum, have
relatively low resistance.
• Long wires have more resistance than do short
wires.
• Thin wires have greater resistance than do
thick wires.
• An increase in temperature also increases the
resistance of a material.
• A long, thin, hot, wire would have more
resistance than a short, thick, cold one.
• The symbol for resistance is the
capital letter R.
• The SI unit of resistance is the
ohm (OHM), and its symbol is n,
the Greek capital letter ‘omega’.
• Resistance force can be
measured by connecting an ohm
meter to a circuit, as shown in
the Figure .
Ohm’s Law
• A German schoolteacher named Georg Ohm related
electric current to voltage and resistance.
• He discovered that when he divided the voltage (V) of the
circuit by the current, he always got the same number.
• Ohm identified this number as resistance.
• The relationship among current, voltage, and resistance is
known as Ohm's law.
• Ohm's law states that the current in a circuit is equal to
the voltage divided by the resistance.
• Ohm's law is written: I = V/R
Sample Problem’s Ohm’s Law
• The current in a closed circuit measures 6.0 amps. The
resistance through the circuit is 20 ohms. What is the
voltage in the circuit?
• V = IR
• Voltage = current x resistance
• = 6.0 A x 20 ohms
• = 120 V
• Do Practice Problems page 271. 1-4
Check & Explain pg. 273
1. Name three common sources of electric current. Name a
use for each source.
2. Explain how current, voltage, and resistance relate to one
another. Identify the symbol for each term.
3. Calculate Use Ohm's law to find the resistance of a radio that
uses a 9-V battery and carries a current of 3 amps.
11.4 Electrical Circuits
Objectives
1. Identify the parts of a circuit.
2. Trace the path of electrons through two types of
circuits.
3. Compare series and parallel circuits.
4. Predict how circuits are wired in your home.
Electrical Circuits
• An electric circuit
requires a source of
electrons and voltage
to force the electrons
along the circuit. If
there is a break in an
electric circuit, the
electrons stop
moving.
Parts of a Circuit
• An electric circuit is a complete,
closed path through which electrons
travel.
• In a circuit that uses a dry cell as its
electron source, the path of electrons
begins in a wire attached to the
negative terminal.
Parts of a Circuit
• For work to be done by the flowing electrons, there must
be a resistor connected in the circuit.
• A resistor is a device that uses electric energy to do work.
• Machines, computers, lights, appliances, and motors are
examples of
resistors.
Parts of a Circuit
• A wire connected from the resistor
to the positive terminal completes
the circuit.
• A circuit may have a switch to
open and close the circuit.
• Electrical engineers draw circuit
diagrams.
Path of a Circuit
Types of Circuits
• Most electric circuits contain more
than one resistor that uses electric
energy.
• The resistors in a circuit can be
connected in two ways.
• They can be connected in series or
in parallel.
Series Circuits
• A circuit in which the
current must pass
through all the resistors
is a series circuit.
• All the resistors in a
series circuit lie along a
single path.
Parallel Circuits
• The electrons in a parallel circuit can travel
through more than one path.
• Each path is separate.
• If there's a break in one path in the circuit,
electrons can still flow through the other paths
and maintain a complete circuit.
• The circuits in your home are connected in
parallel. What is the advantage of parallel
wiring?
Check & Explain pg. 278
1. Name and describe three parts of a circuit.
2. Draw a simple circuit. Label your drawing with arrows to
trace the path of electrons through the circuit.
3. Compare and Contrast How does the path of electrons
differ between a circuit connected in series and one
connected in parallel?
11.5 Electric Power & Safety
Objectives
1. Calculate electric power and energy.
2. Apply safety guidelines for household use of
electricity.
3. Calculate the amount of electricity used by an
appliance.
4. Infer about the proper use of safety devices
Electric Power
• Electricity is useful because it changes
easily into other forms of energy.
• For example, a toaster changes electricity
into heat energy.
• An electric mixer changes electric energy
into mechanical energy.
• Each of these devices makes work easier.
Electric Power & Energy
• The rate at which electricity does work or
provides energy is called electric power.
• The amount of electric power a device uses
to do work is determined by its resistance.
• You can calculate power (P) if you know the
voltage (V) an appliance uses and the
current (I) in the circuit.
• Use the formula: P = (V)(I)
• The amount of
power used by
different
appliances is
shown in the
table to the left.
• Recall the unit for power is watts (W).
• Power is measured in watts (W). Time is in
hours (h).
• The SI unit for energy is the joule. However,
energy is usually measured with a unit
called a watt-hour.
• Because a watt-hour is a very small unit of
energy, electric companies measure electric
energy in 1,000 watt hours, or kilowatthours (kWh). Kilowatt-hour meters
measure the electricity used in your home.
• Do Skills Builder Activity page 280
Electric Safety
• Electricity is useful, but it can also be
dangerous.
• Each year, many people are seriously
injured or killed from shocks or fire
caused by electricity.
• Most appliances and buildings have
safety features built into them.
• Many appliances are equipped with a
"ground" wire on the plug.
• The ground wire prevents electric
shocks from the outside of the
appliance.
• The ground wire constantly moves
static electricity from the appliance
to the ground.
• Broken wires or water can cause electric
appliances to short-circuit.
• A short circuit occurs when electricity
takes a short path and bypasses the
resistors in the circuit.
• As a result, the resistance in the circuit is
less, and the current in the wire
increases.
• The increased current can produce
enough heat to melt wires and start a
fire, or cause a serious electric shock.
• Overloading a circuit also may cause a
fire.
• A circuit can become overloaded if too
many electric devices are plugged into
it. Each added electric device increases
the electric current flowing through the
wire.
• When the electric current load is
greater than the capacity of the wire,
the wire is overloaded.
Circuit Protectors
• Fuses and circuit breakers protect
against overloaded circuits.
• When the current passes through
the fuse that goes above its
maximum, the metal in the fuse
melts. The fuse must be replaced.
Circuit Protectors
• Circuit breakers are often used instead of
fuses.
• A circuit breaker is a switch that opens
automatically when electric current in a circuit
reaches its maximum.
• When the switch opens, it breaks the flow of
electrons in the circuit.
• Circuit breakers can be reset when the switch
is closed again.
Check and Explain pg. 282
1. A radio has a current of 1.0 amp from a
120-V source. What is the power of the
radio?
2. What can you do to minimize the risk of
an electric shock or fire in your home
caused by electricity?