Download I. Electric Charge

Ch. 20 - Electricity
I. Electric Charge
 Electricity
 Static Electricity
 Conductors
 Insulators
 Electroscope
A. Electricity
 Everything in the world is made up of
atoms. Each atom has smaller parts in
it. One of those parts is called
electrons. Electrons can move from
atom to atom. When an electron moves
to a different atom, it causes another
electron to have to move. When
electrons move quickly from one atom to
another is it called Electricity!
A. Electricity
 When you talk to your friends on the telephone, the
microphone inside of the phone’s handset changes your
sound waves into electric signals. The light shines
throughout your homes when you flip a switch. And if
you stump your toe, your nerves send signals between
your brain and muscles.
 All of these everyday occurrences are powered by
electrical pulses. A charge is a measure of the extra
positive or negative particles that an object has.
 The only reason that we are able to use electricity in our
modern world is that it is possible to separate positive
and negative charges from each other.
A. Electricity
2 rules of electric charge:
1. Like charges repel.
2. Opposite charges attract
B. Static Electricity
 The study of the
behavior of electric
charges
 How the electric
charge is transferred
between objects:
 by friction,
conduction, and
induction.
C. Transferring Charge
Friction
Conduction
Induction
Transfer of charge by
rubbing two different
materials together.
Transfer of charge when Transfer of charge
two objects come into
without direct contact
direct contact with
each other.
Rubbing a balloon in
your hair.
Touching a metal
Being struck by
sphere that has been lightning b/c you are
charged b a Van de
standing near a tree.
Graff generator.
C. Transferring Charge
Friction
Example 1
Example 2
C. Transferring Charge
Conduction
Transfer by
direct contact
C. Transferring Charge
Conduction
 Conductor
 material that allows electrons to
move through it easily
 e- are loosely held
 ex: metals like copper and silver
C. Transferring Charge
Conduction
 Insulator
 material that doesn’t allow electrons
to move through it easily
 e- are tightly held
 ex: plastic, wood, rubber, glass
C. Transferring Charge
Induction
Ch. 20 - Electricity
II. Electric Current
 Circuit
 Electric Current
 Resistance
 Ohm’s Law
B. Electric Current
 Current
 flow of electrons through a conductor
 depends on # of e- passing a point in
a given time
 measured in amperes (A)
B. Electric Current
The continuous flow of electric charge is an electric current. There
are 2 types of electric current: direct current and alternating current.
Flow of charges in only one
direction
Flow of charges that
regularly reverses
direction
D. Resistance
 Resistance
 opposes the flow of electrons
 electrical energy is converted to
thermal energy & light
 measured in ohms ()
Copper - low resistance
Tungsten - high resistance
D. Resistance
 Resistance depends on…
 the conductor
 wire thickness
• less resistance
in thicker wires
 wire length
• less resistance in shorter wires
 temp - less resistance at low temps
D. Resistance
Ohm’s Law
 Most of the electrical
appliances that you use in your
homes each day are designed
for 120 Volts (V).
 Light bulbs however are sold in
several varieties, from dim 40
Watt light bulbs to bright 100
Watt light bulbs.
 These bulbs shine differently
because they have different
amounts of current in them.
The difference in their current is
because of their resistance.
In other words, 40 W light bulbs have a
greater resistance (less current than
the 100 W bulbs) and therefore give off
less light.
Therefore greater resistance = less
current
D. Resistance
Ohm’s Law
 You have probably noticed that electrical
devices such as video games or radios become
warm after they have been on for a long amount
of time.
 As the electrons collide, with the atoms of the
material, some of their kinetic energy transfers.
 This transfer of energy causes the atoms to
vibrate, and then the material warms up.
 As the electrons are slowed by a resistor,
energy is lost in the form of heat.
 This means that current, resistance and
voltage must be linked.
D. Resistance
Ohm’s Law
Did you know that resistance depends on the
material used as well as the material’s length and
temperature?
 Longer pieces of material have greater
resistance (less current) than shorter pieces.
E. Calculating Resistance
Ohm’s Law
 Ohm’s Law - The SI unit of resistance
is the ohm, Ω,
V=I×R
V: potential
difference (V)
I: current (A)
R: resistance ()
• Voltage increases when current increases.
• Voltage decreases when resistance increases.
E. Calculating Resistance
Ohm’s Law
 A lightbulb with a resistance of 160  is plugged
into a 120-V outlet. What is the current flowing
through the bulb?
GIVEN:
WORK:
R = 160 
V = 120 V
I=?
I=V÷R
I = (120 V) ÷ (160 )
I = 0.75 A
V
I
R
Homework
Practicing Ohm’s Law
Ch. 21 - Electricity
III. Electrical Circuits
 Circuit components
 Series circuits
 Parallel circuits
 Household circuits
A. Circuit
 Circuit
 closed path through
which electrons can flow
Electrical Circuits
 If you’ve ever seen a house being built, you know that wires
hidden inside the walls connect to every electrical outlet and
to every light switch.
 The steady flow of electricity is called an electric current.
 A current will move along a wire or a path called a circuit.
Circuit means to “go around.”
 There are 2 types of circuits: series circuit and parallel circuit.
A. Circuit Components
A - battery
B - switch
C - light bulb
D - resistor
Circuit Symbols
B. Series Circuits
 Series Circuit
 current travels in a single path
• one break stops the flow of current
 current is the same throughout circuit
• lights are equal brightness
 each device receives a fraction of the
total voltage
• get dimmer as lights are added
C. Parallel Circuits
 Parallel Circuits
 current travels in multiple paths
• one break doesn’t stop flow
 current varies in different branches
• takes path of least resistance
• “bigger” light would be dimmer
 each device receives the total voltage
• no change when lights are added
Series Circuit vs. Parallel Circuit
 A series circuit is a circuit that has only one path for
the current.
 A parallel circuit has two or more paths for current to
travel.
Series Circuit
Parallel Circuit
Old Christmas Lights were once
Lights in our homes are wired in
wired in series. If one bulb went out, parallel circuit. That is why we are
they all went out.
to have lights on in one room, but
off in another.
Series Circuit vs. Parallel
Circuits1. Has two or more paths for
Series
1.
Has a single loop for electrons
Parallel
electrons to flow down
to travel round
2. Current is shared between the
branches
2. Components are connected one
after another
3. Current has to travel through all
components
D. Household Circuits
 Combination of parallel circuits
 too many devices can cause wires to
overheat
 Safety Features:
 fuse - metal melts, breaking circuit
 circuit breaker - bimetallic strip bends
when hot, breaking circuit
Ch. 21 - Magnetism
I. Characteristics of Magnets
 Magnetism
 Magnetic poles
 Magnetic field
 Magnetic domain
A. Magnetism
 Magnetism
 force of attraction or repulsion between unlike
or like poles
 due to the arrangement of electrons
 closely related to electricity
 Magnetism is the force of attraction or
repulsion of a magnetic material due to the
arrangement of its atoms, particularly its
electrons.
A. Magnetism:
The History of Magnets
 Magnets have been known for centuries.
The Chinese and Greeks knew about the
“magical” properties of magnets. The
ancient Greeks used a stone substance
called “magnetite.” They discovered that
the stone always pointed in the same
direction. Later, stones of magnetite
called “lodestones” were used in
navigation.
B. Magnetic Poles
 The ends of a
magnet are where
the magnetic effect is
the strongest. These
are called “poles.”
Each magnet has
 2 poles – 1 north, 1
south.
 If you cut a magnet in
half, you get two
magnets.
Like Poles: Repel
Opposite poles: Attract
B. Magnetic Poles
 Magnetic Poles
 like poles repel
 unlike poles attract
 a broken magnet creates new poles
C. Magnetic Field
 Magnetic Field
 area around a magnet where magnetic
forces act
 field lines show direction of field (NS)
Permanent Magnets vs.
Temporary Magnets
 Some magnets occur in nature. These magnets
are called natural magnets. Natural magnets
maintain their magnetic properties and therefore
are permanent magnets. (ex. lodestone).
 Some materials that are not natural magnets are
easy to magnetize (ex. Iron). A material that is
easily magnetized tends to lose its magnetism
quickly, and is called a temporary magnet.
Permanent Magnets vs.
Temporary Magnets
 An electric current passing through a wire causes a
magnetic field.
 An electromagnet is a temporary magnet made by
wrapping a current-carrying wire around an iron core.
 The center of an electromagnet is called the core, and
it is often made of iron.
 As long as current is flowing, an electromagnet has a
magnetic field.
 When current is turned off, there is no longer a
magnetic field.
Ch. 21 - Magnetism
II. Uses of Magnetic Fields
 Electromagnet
 Speaker
 Motor
A. Electromagnet
 Electromagnet
 strong, temporary magnet formed when
current is passed through a coil of wire
surrounding an iron core
 acts like a bar magnet when current is on
A. Electromagnet
 There are two ways to make an electromagnet
stronger:
1. increasing the number of coils
2. increasing the amount of current
 Electromagnets are useful because they can
be turned on and off. Electromagnets have
many important uses:
 ex. radios, telephones, computers
B. Speaker
 Speaker
 electrical energy  mechanical energy
 wire coil moves back &
forth as its magnetic
field interacts with the
field of a fixed magnet
 forced vibration causes
the cone to move 
sound
C. Motor
 Motor
 electrical energy  mechanical energy
 electromagnet
rotates between
the poles of a fixed
magnet
 commutator
reverses the poles
of the e’magnet
C. Motor
armature & commutator
assembled motor
brushes & wires to battery
field magnet
Ch. 22 - Magnetism
III. Producing Electric Current
(p.633-639)
 Electromagnetic Induction
 Electric Generator
 DC & AC
 Transformer
A. Electromagnetic Induction
 Electromagnetic Induction
 producing a current by moving a wire through a
magnetic field
 some microphones
work just like minispeakers in reverse
Coil
 sound waves cause
coil to move  current
Dynamic Microphone
B. Electric Generator
 Electric Generator
 mechanical energy  electrical energy
 armature is
rotated between
magnet poles
 magnetic field
induces a
current in the
wire coil
GENERATOR
MOTOR
B. Electric Generator
 Hydroelectric Dam
 PE of lake water is
converted to KE
 mechanical KE
turns the generator
shaft which creates
electrical energy
C. DC & AC
 Direct Current (DC)
 current flows in one direction
 dry cells
 Alternating Current (AC)
 current reverses its direction
at regular intervals
 electrical outlets
D. Transformer
 Transformer
 increases or decreases AC voltage
 primary coil AC produces a magnetic field that
induces AC in the secondary coil
 voltage ratio = ratio of turns in each coil
D. Transformer
 Step-up Transformer
 increases the voltage
 more turns
 power plants
 Step-down Transformer
 decreases the voltage
 fewer turns
 household appliances
(hairdryers, etc.)
Electric Motors vs. Generators
 An electric motor is a device that
changes electrical energy into
mechanical energy.
 An electric motor is made up of an
electromagnet and a permanent
magnet.
 So, what do windshield wipers, CD
players, VCR's, blenders, ice makers,
computers, and talking toys have in
Electric Motors vs. Generators
 A generator is a device that changes
mechanical energy into electrical
energy.
 Most of the energy we use every day
comes from generators.
 Mechanical energy for many generators
is supplied by turbines.
 A turbine is a large wheel that is turned
by moving steam or water.
Electric Motors vs. Generators
Motor or Generator?
Motor or Generator?