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HIGH SCHOOL SCIENCE
Physical
Science 7:
Electricity &
Magnetism
WILLMAR PUBLIC SCHOOL
2013-2014 EDITION
C HAPTER 7
Electricity &
Magnatism
In this chapter you will:
1. Analyze factors that affect the strength and direction
of electric forces and fields.
2. Describe how electric charges are transferred.
3. Describe and identify electric current, conduction,
and resistance.
4. Explain how voltage produces electric current.
5. Describe the effects of magnetic forces and magnetic
fields and explain how magnetic poles determine the
direction of magnetic force.
6. Describe how a moving electric charge creates a magnetic field and determine the direction of the magnetic field based on the types of charge and the direction of its motion.
7. Describe how electric current is generated by electromagnetic induction.
8. Summarize how electrical energy is produced, transmitted, and converted.
S ECTION 7.1
A lightning bolt is like the spark that gives you a shock when
you touch a metal doorknob. Of course, the lightning bolt is
on a much larger scale. But both the lightning bolt and spark
are a sudden transfer of electric charge.
Electricity
O BJECTIVE :
1. Analyze factors that affect the strength and
direction of electric forces and fields.
2. Describe how electric charges are transferred.
3. Describe and identify electric current,
conduction, and resistance.
4. Explain how voltage produces electric current.
Vocabulary:
electric charge
electric force
electric field
static electricity
conduction
induction
static discharge
electric current
ampere
direct current
alternating current
electric potential energy
voltage
electrical conductor
electrical insulator
resistance
Electric charge is a property that causes subatomic particles
such as protons and electrons to attract or repel each other.
There are two types of electric charge, positive and negative.
Protons have a positive charge and electrons have a negative
charge. The unit of electric charge is the coulomb (C). It takes
about 6.24 × 1018 electrons to produce a single coulomb.
The atom is neutral because it has an equal number of positive
and negative charges. If an atom gains one or more electrons,
it becomes a negatively charged ion. If an atom loses
electrons, it becomes a positively charged ion. An excess or
shortage of electrons produces a net electric charge.
Like charges repel, and opposite charges attract. The force of
attraction or repulsion between electrically charged objects is
electric force. The electric force between two objects is
directly proportional to the net charge on each object and
inversely proportional to the square of the distance between
them.
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The effect an electric charge has on other charges in the space
around it is the charge's electric field. The strength of an
electric field depends on the amount of charge that produces
the field and on the distance from the charge. Because of their
force fields, charged particles can exert force on each other
without actually touching. Electric fields are generally
represented by arrows. The arrows show the direction of
electric force around a positive particle and a negative
particle.
When charged particles are close enough to exert force on
each other, their electric fields interact. An electric field
exerts forces on any charged object placed in the field. The
force depends on the net charge in the object and on the
strength and direction of the field at the object's position. The
more net charge an object has, the greater is the force on it.
The direction of each field line shows the direction of the force
on a positive charge.
Whenever electrons are transferred between objects, neutral
matter becomes charged. This occurs even with individual
atoms. Atoms are neutral in electric charge because they have
the same number of negative electrons as positive protons.
However, if atoms lose or gain electrons, they become charged
particles called ions.
Static electricity is a buildup of electric charges on objects.
Charges build up when negative electrons are transferred
from one object to another. The object that gives up electrons
becomes positively charged, and the object that accepts the
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electrons becomes negatively charged. There are several ways
that a net charge can build up on an object or move from one
object to another. Charge can be transferred by friction, by
contact, and by induction. Did you ever rub an inflated
balloon against your hair? Friction between the balloon and
hair cause electrons from the hair to “rub off” on the balloon.
That’s because a balloon attracts electrons more strongly than
hair does. After the transfer of electrons, the balloon becomes
negatively charged and the hair becomes positively charged.
The individual hairs push away from each other and stand on
end because like charges repel each other. The balloon and the
hair attract each other because opposite charges attract.
Conduction occurs when there is direct contact between
materials that differ in their ability to give up or accept
electrons. A van de Graff generator produces a negative
charge on its dome, so it tends to give up electrons. Human
hands are positively charged, so they tend to accept electrons.
Therefore, electrons flow from the dome to the man’s hand
when they are in contact. Induction is a transfer of charge
without contact between materials. Keep in mind that
whenever there is a charge transfer, the total charge is the
same before and after the transfer occurs. This is the law of
conservation of charge—the total charge in an isolated system
is constant. Static discharge occurs when a pathway
through which charges can move forms suddenly.
The continuous flow of electric charge is an electric
current. The unit of electric current is the ampere (A), or
amp, which equals 1 coulomb per second. The two types of
current are direct current and alternating current. Charge
flows only in one direction in direct current (DC). A
flashlight and most other battery-operated devices use direct
current. Electric current in your home and school is mostly
alternating current. Alternating current (AC) is a flow of
electric charge that regularly reverses its direction.
Resistance is opposition to the flow of electric charges in an
electric current as it travels through matter. The SI unit for
resistance is the ohm. Resistance occurs because moving
electrons in current bump into atoms of matter. Resistance
reduces the amount of electrical energy that is transferred
through matter. That’s because some of the electrical energy is
absorbed by the atoms and changed to other forms of energy,
such as heat. All materials have resistance. How much
resistance a material has depends on the type of material, its
width, its length, and its temperature. Resistance is greater in
a longer wire because the charges travel farther. As
temperature increases, a metal's resistance increases because
electrons collide more often. Resistance is a hindrance when a
material is being used to transmit electric current. Resistance
is helpful when a material is being used to produce heat or
light.
Ohm found a mathematical relationship between voltage,
current, and resistance. This relationship became known as
Ohm's law. According to Ohm's law, the voltage (V) in a
circuit equals the product of the current (I) and the resistance
(R). Increasing the voltage increases the current. Keeping the
same voltage and increasing the resistance decreases the
current.
Electric potential energy comes from the position of a
charged particle in an electric field. For example, when two
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negative charges are close together, they have potential energy
because they repel each other and have the potential to push
apart. For an electric charge to move from one position to
another, there must be a difference in electric potential energy
between the two positions. A difference in electric potential
energy is called voltage. Electrical potential difference is
measured in joules per coulomb, or volts.
Section Review:
1. What are the two types of electric charge?
2. What makes up positive charge? Negative Charge?
3. What happens to an atom if it loses an electron?
4. What produces a net electric charge?
An electrical conductor is a material through which charge
can flow easily. Metals such as copper and silver are good
electrical conductors. A metal is made up of ions in a lattice.
The ions are not free to move. But each ion has one or more
electrons that are not tightly bound to it. These free electrons
can conduct charge.
5. What is the unit for electric charge?
A material through which charge cannot flow easily is called
an electrical insulator. Wood, plastic, rubber, and air are
good electrical insulators. Most materials do not easily
conduct charge because they don't have free electrons.
9. What does the strength of the electric field depend on?
6. How do like charges behave?
7. How do opposite charges behave?
8. What is the relationship of electric forces between two
objects?
10.What does an electric field exert forces on?
11.What does the force depend on?
12.What causes a greater force?
13.How can charger be transferred?
14.What is the law of conservation of charge?
15.When does static discharge occur
16.What are the two types of electric current?
17.Where do you find direct current? Alternating current?
18.What is Ohm’s Law?
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Section Review Continued:
19.What affects resistance?
20.What is necessary for charge to flow?
21.What materials make good conductors? Insulators?
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S ECTION 7.2
Magnetism is the ability of a material to be attracted by a
magnet and to act as a magnet. Magnetism is due to the
movement of electrons within atoms of matter. When
electrons spin around the nucleus of an atom, it causes the
atom to become a tiny magnet, with north and south poles
and a magnetic field. In most materials, the north and south
poles of atoms point in all different directions, so overall the
material is not magnetic. Examples of nonmagnetic materials
include wood, glass, plastic, paper, copper, and aluminum.
These materials are not attracted to magnets and cannot
become magnets.
Magnetism
O BJECTION :
1. Describe the effects of magnetic forces and
magnetic fields and explain how magnetic
poles determine the direction of magnetic
force.
2. Describe how a moving electric charge creates
a magnetic field and determine the direction of
the magnetic field based on the types of charge
and the direction of its motion.
Vocabulary:
magnetism
magnet
poles
magnetic force
magnetic field
A magnet is an object that attracts certain materials such as
iron. All magnets have two magnetic poles: north and south
magnetic poles. The poles are regions where the magnet is
strongest. The poles are called north and south because they
always line up with Earth’s north-south axis if the magnet is
allowed to move freely. (Earth’s axis is the imaginary line
around which the planet rotates.) The direction of magnetic
force between two magnets depends on how the poles face. Like magnetic poles repel one another, and opposite magnetic
poles attract one another.
The force that a magnet exerts on certain materials, including
other magnets. Magnetic force is the force a magnet exerts
on another magnet, on iron or a similar metal, or on moving
charges. The force is exerted over a distance and includes
forces of attraction and repulsion. North and south poles of
two magnets attract each other, while two north poles or two
south poles repel each other. A magnet can exert force over a
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distance because the magnet is surrounded by a magnetic
field.
A magnetic field surrounds a magnet and can exert
magnetic forces. A magnetic field, which is strongest near a
magnet's poles, will either attract or repel another magnet
that enters the field. When two magnets are brought close
together, their magnetic fields interact. The lines of force of
north and south poles attract each other whereas those of two
north poles repel each other.
Earth is like a giant magnet surrounded by a magnetic field.
Earth acts as a giant magnet with magnetic poles and a
magnetic field over which it exerts magnetic force. Earth has
north and south magnetic poles like a bar magnet. Earth’s
magnetic poles are not the same as the geographic poles.
Earth’s magnetic field is called the magnetosphere. It is
strongest at the poles. In the 1600s, William Gilbert
demonstrated that Earth is basically a spherical magnet, with
north and south poles and a magnetic field. In the 1900s,
scientists used earthquake data to determine that Earth has a
solid inner core and molten outer core. Scientists think that
Earth is a magnet because of charged particles moving
through the molten outer core as Earth spins on its axis.
The field lines begin near the magnet's north pole and extend
toward its south pole. The arrows on the field lines indicate
what direction a compass needle would point at each point in
space. Where lines are close together, the field is strong.
Where lines are more spread out, the field is weak.
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Section Review:
1. What are the two types of magnetic poles?
2. How do magnetic poles interact with each other?
3. Where is a magnetic field strongest?
4. How are the lines in the magnetic field drawn?
5. Why is the Earth a magnet?
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S ECTION 7.3
Electromagnetism
O BJECTIVES :
1. Describe how electric current is generated by
electromagnetic induction.
2. Summarize how electrical energy is produced,
transmitted, and converted.
Electromagnetism is magnetism produced by an electric
current. When electric current flows through a wire, it creates
a magnetic field that surrounds the wire in circles. The
direction of the magnetic field created when current flows
through a wire depends on the direction of the current. A
simple rule, called the right hand rule, makes it easy to find
the direction of the magnetic field if the direction of the
current is known. When the thumb of the right hand is
pointing in the same direction as the current, the fingers of
the right hand curl around the wire in the direction of the
magnetic field.
Vocabulary:
electromagnetism
electromagnetic force
electromagnetic wave
solenoid
electromagnet
electromagnet induction
Faraday’s law
generator
Electricity and magnetism are different aspects of a single
force known as the electromagnetic force. The electric
force results from charged particles. The magnetic force
usually results from the movement of electrons in an atom.
Both aspects of the electromagnetic force are caused by
electric charges.
transformer
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Moving electric charges create a magnetic field. These moving
charges may be the vibrating charges that produce an
electromagnetic wave.
The magnetic field lines form circles around a straight wire
carrying a current. A coil of current-carrying wire that
produces a magnetic field is called a solenoid. Any wire with
current flowing through it has a magnetic field. However, the
magnetic field around a coiled wire is stronger than the
magnetic field around a straight wire. That’s because each
turn of the wire in the coil has its own magnetic field. Adding
more turns to the coil of wire increases the strength of the
field. Increasing the amount of current flowing through the
coil also increases the strength of the magnetic field. A
solenoid is generally used to convert electromagnetic energy
into motion. Solenoids are often used in devices that need a
sudden burst of power to move a specific part.
An electromagnet is a solenoid with a ferromagnetic core. Changing the current in an electromagnet controls the
strength and direction of its magnetic field. You can also use
the current to turn the magnetic field on and off.The
combined magnetic force of the magnetized wire coil and iron
bar makes an electromagnet very strong. In fact,
electromagnets are the strongest magnets made.
Electromagnetic devices such as galvanometers, electric
motors, and loudspeakers change electrical energy into
mechanical energy.
Electromagnetic induction is the process of generating a
current by moving an electrical conductor relative to a
magnetic field. It occurs whenever a magnetic field and an
electric conductor, such as a coil of wire, move relative to one
another. As long as the conductor is part of a closed circuit,
current will flow through it whenever it crosses lines of force
in the magnetic field. According to Faraday's law, a voltage
is induced in a conductor by a changing magnetic field.
Moving the magnet in and out of the coil causes an electric
current first in one direction and then in the other. The same
alternating current occurs if you move the coil and keep the
magnet still. As long as the magnet and coil are moving
relative to one another, the galvanometer will record a
current.
Two important devices depend on electromagnetic induction:
electric generators and electric transformers. Both devices
play critical roles in producing and regulating the electric
current we depend on in our daily lives. Electric generators
use electromagnetic induction to change kinetic energy to
electrical energy. They produce electricity in power plants.
Electric transformers use electromagnetic induction to change
the voltage of electric current. Some transformers increase
voltage and other decrease voltage.
A generator is a device that converts mechanical energy into
electrical energy by rotating a coil of wire in a magnetic field.
Electric current is generated by the relative motion of a
conducting coil in a magnetic field. The two types of
generators are AC generators and DC generators.
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The electrical energy produced by power plants is transmitted
through power lines at very high voltages. These voltages are
too high to be used safely in homes. The voltage must first be
changed, or transformed. A transformer is a device that
increases or decreases the voltage and current of two linked
AC circuits. A series of transformers changes high-voltage
current in power lines into 240-volt current that can be used
safely in your home.
A transformer works only with alternating current because
only alternating current induces a constantly changing
magnetic field. A transformer changes voltage and current by
inducing a changing magnetic field in one coil. This changing
field then induces an alternating current in a nearby coil with
a different number of turns.
Section Review:
1. What causes the electromagnetic force?
2. What creates a magnetic field?
3. What happens when current is changed?
4. What happens when a magnet is moved in and out of a
coil?
5. What are the two types of magnetic poles?
6. Why must we transform electrical energy so homes can
use it?
7. Why do transformers only use alternating current?
8. How do transformers work?
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