Transcript Electric current

Electricity
Physical Science
Static Electricity
 Static electricity is an excess or lack of
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electrons
Lightning, a spark jumping from your
fingertip to a metal doorknob, and the
clinging together of articles of clothing
when they are removed from a clothes
dryer are all forms of static electricity
Normally all atoms are electrically neutral
Each atom has the same number of
negative electrons and positive protons
Because electrons are found whirling
about the nucleus at high speeds they
may be removed from the atom by friction
Static Electricity
 When certain materials are rubbed together, electrons
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are transferred from one surface to another
Excess electrons on the surface of a substance produces
a negative charge
A lack of electrons produce a positive charge
One surface gains electrons and acquires a negative
charge
The other surface has lost electrons and is left with a
positive charge
Unlike electric charges attract, and like charges repel
Static Electricity
 When a surface has acquired a
strong negative charge, the extra
electrons may jump to a neutral or
positive object
 You see this jump of electrons
when you see a spark
 A spark is a rapid movement of a
number of electrons through the air
 Lightning is a giant spark that
sometimes occurs when clouds
that have acquired a charge
suddenly discharge electrons
Electric Current
 Normally, electrons move about atoms at random
 However, under certain conditions, some of the
free electrons may be made to migrate in the
same general direction
 When this happens, electrons within a conductor
are migrating in the same direction, an electric
current is flowing in the conductor
Electric Current
 In a copper wire, some electrons will move freely
and randomly among the copper atoms making
up the wire
 But suppose that in some way many electrons
are brought close to one end of the wire
 Repulsive forces will be exerted upon the
electrons in the wire
 The electrons in the wire will tend to migrate
away from the area of electron concentration
Electric Current
 If a path is provided on the other end of the
copper wire, permitting the electrons to move to
an area that absorbs them, electrons will continue
to migrate along the wire
 The movement is away from the source of
electrons
 The movement of the electrons through the wire
constitutes an electric current
Electric Current
 Electric current is the movement of electrons
through a conductor
 The rate at which the electrons move through the
conductor is called current
 To produce an electric current requires a source of
electrons, a conducting path, and a place for the
electrons to go-a ground or sink
Conductors
 Conductor is any
material that electrons
can move through
easily
 Such a material offers
little resistance to the
flow of electrons
 Examples of materials
that are good
conductors include
copper, silver, gold, and
aluminum
Insulators
 Insulators are substances that do not carry electrical energy
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well
Because these materials are poor conductors, they offer high
resistance to the flow of electrons
Wood, rubber, plastic, and dry air are examples of insulators
Some substances-germanium, silicon, and selenium- are neither
conductors nor insulators and are called semiconductors
They can be used to make tiny electrical devices to control the
flow of electrons
Semiconductors are widely used in the making of computer
chips
Chemicals Can Produce
Electricity
 The tendency to yield electrons varies from one
metal to another
 The electromotive series lists metals in
descending order, according to this tendency
 When any two metals are connected by a wire
and immersed in an electrolyte (a conducting
liquid), electrons will flow through the wire from
the metal that is higher in the electromotive series
to the metal that is lower
Chemicals Can Produce
Electricity
 This principle is the basis for the construction of
dry cells, voltaic cells, lead storage cells,
flashlight cells, and other common portable
methods of producing electric current
 The electrolytes used in storage batteries are
commonly acids such as sulfuric acid
 However, common salts such as table salt or
ammonium chloride will also work in an electric
cell
Magnetism Can Produce
Electricity
 Each electron in an atom is a tiny magnet and will tend to
align itself with external magnetic fields
 When the external magnetic field changes direction or
magnitude, force is exerted upon the electrons
 In metals that are subjected to a changing magnetic field,
the electrons that are loosely held by atoms will tend to
migrate in the direction that reduces the stress placed on
them
 Since metals tend to have an abundance of loosely held
electrons moving about among their atoms, a changing
magnetic field will result in a flow of electrons within the
metal
Magnetism Can Produce
Electricity
 When a current is caused to flow in a conductor
by means of a fluctuating magnetic field, the
process is called electromagnetic induction
 Commonly coils of copper wire are rotated within
a magnetic field to produce an electric current
 Example: generator
Commercial Generators
 The commercial electricity used in homes,
businesses, and industries is produced by
generators
 A generator is any device that permits a magnet
to be rotated rapidly inside a coil of copper wire
and so produces an electric current
 The motion of generators is produced by a
variety of means- water power; steam engines
that burn oil, gas, or coal; nuclear power; wind
power; etc.
Commercial Generators
 In most commercial generators, the electrons flow in one
direction through the copper wires during one half of a
rotation and in the opposite direction during the other
half of the rotation
 This produces alternating current (AC), as opposed to
current that flows always in the same direction, direct
current (DC)- ex. electricity from dry cells (batteries)
 In most commercial distributing systems, the direction of
the current changes 60 times per second
Light Can Produce Electricity
 The atoms of the metal oxides of potassium and cesium
contain electrons that, when struck by light energy,
become excited and jump from the surface of the
material
 A photoelectric cell may be constructed by placing the
electron-yielding substance in a vacuum and providing a
place for the excited electrons to go
 The stronger the light source, the more electrons will be
given off by the photosensitive coating of the cell
 Much of this same process takes place in the electric
tubes of radio and television sets
Heat Can Produce Electricity
 In these applications, the electron-yielding materials
are heated
 The energy provided causes electrons to be emitted
from the surface of the material in a process called
thermionic emission
 A thermocouple is a device in which 2 wires of
unlike metals are joined and heat is applied to the
junction
 Electrons will migrate from one metal to the other to
produce a small current
Electric Circuits
 Think of an electric circuit as a path provided by a conductor that
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enables electrons with a great deal of energy to leave a source of
energy (generator, battery, etc.), move through the conductor (a
wire), lose some energy due to resistance (a light bulb), and end
in an electron sink or an acceptor of electrons
As long as the switch is closed (the wires are connected), energy
is transmitted through the circuit
The light bulb- the resistance- converts some of the electrical
energy to light energy
Any time a region with excess electrons is joined by a conductor
to a region with fewer electrons, the electrons will flow from the
region of higher concentration to the region of lower concentration
The magnitude of the current in a circuit is defined by the number
of electrons that flow through the circuit in a given period of time
Electric Circuits
 Ampere-the unit used to express the number of electrons
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passing by a given point in a conductor
Voltage- the force or pressure exerted upon the electrons in the
conductor
The magnitude of the voltage depends on the difference in the
concentration of electrons at the opposite end of the conductor
The difference in concentration affects the number of electrons
that move and the energy that they expend as they pass through
the circuit
Volt-the unit used to express the pressure on the electrons in a
conductor, or the energy that they expend in the circuit
Resistance- is the opposition to the flow of current in the circuit
Electric Circuits
 The atoms of different materials vary in the ease with
which they permit their electrons to migrate
 Atoms of high-resistance materials hold on to their
outer electrons more strongly than do atoms of lowresistance materials
 Thus, even with identical voltages, materials vary in
the number of electrons they will pass among their
atoms
 The unit that defines the opposition to the flow of
current (resistance) in a circuit is the ohm
Ohm’s Law
 The relationships that exist among voltage,
resistance, and current:
 The magnitude of the current in a conductor is
directly related to the voltage (current will
increase when voltage increases); current is
inversely related to the resistance (current will
decrease when resistance increases)
 This is known as Ohm’s law I=V/R
 I=current V=voltage R=resistance
Water System Analogy of the
Electric Circuit
 Often for simplicity’s sake, electric circuits are
compared to a water system
 A water pump is analogous to the generator in that
the former produces water pressure and the latter
electrical pressure
 The pipes of the water system represent conducting
wires in the electrical circuit
 Just as the size of the water pipes affect the amount
of water that flows through a system, so the size of
the conducting wires affects the number of electrons
that flow through the electric circuit
Water System Analogy of the
Electric Circuit
 The constricting effect of the pipes or wires that
oppose the flow of water or electricity is resistance
 The amount of water that flows through the water
system depends on the pressure exerted by the
pump and the resistance offered by the pipes
 Likewise, in the electric circuit, the number of
electrons (current) that flow through an electric circuit
depends on the pressure (volts) exerted by the
generator and the resistance (ohms) offered by the
wires
Electricity Flow
 Electricity may flow in series
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or parallel circuits
Series circuits- When
electrons follow a single path
from their source to their
destination
The circuit provides only one
path for current to flow through
The electrons must pass
through every bulb
A break in any one bulb shuts
off the current for all other
bulbs
Electricity Flow
 Parallel Circuit- When more than
one path is provided for the
passage of electrons, the
electrons divide themselves
among the separate conductors,
and current flows in each of the
paths
 A break in the filament of one bulb
will shut off that bulb only
 All other bulbs will still receive
electrons and pass through them
 These bulbs will remain in
operation
Characteristics of Series and
Parallel Circuits
 Series Circuit
 One light goes out, all go out
 Bulbs becomes dimmer as
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lights are added
Current is reduced with
addition of bulbs
Total resistance is increased
with addition of bulbs
Current follows the same
path
Voltage of the power source
is divided among ALL bulbs
 Parallel Circuit
 One light goes out, the rest
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remain on
Bulbs remain same
brightness as more are added
Current is increased with
addition of bulbs
Total resistance is decreased
with addition of bulbs
Current is divided among
several paths
ALL bulbs have same voltage,
which is equal to the voltage
of the power source
Continuity of the Circuit
 An electric lamp (light bulb) is simply a wire called a
filament encased in a glass envelope containing very
little oxygen
 For a light bulb to burn, a current must pass through its
filament
 As the electrons pass through the filament of the bulb,
this special wire (usually made of tungsten metal) will
heat up and give off heat and light energy
 If the filament is broken, or if for any other reason the
current cannot pass through, the bulb will not light
Amount of Resistance
 When an electric current must pass through a
high-resistance material, such as the filament of
a light bulb, the number of electrons that pass
through is reduced
 In a series circuit, the current must pass
through ALL resistors
 Therefore, adding a resistor, such as an
additional light bulb, increases the total
resistance of the circuit
Amount of Resistance
 In a parallel circuit, just the reverse is true
 When an additional resistor is added, this provides a new
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path for electrons
The number of electrons that flow through the new resistor
adds to the number already passing through the established
resistors
The total current in the circuit is increased even though the
current through each of the established resistors remains the
same
Since the current was increased, according to Ohm’s law the
resistance must have been reduced
Adding resistors in a parallel circuit decreases the total
resistance to the flow of electricity
Brightness of Bulbs
 In the circuit, the brightness of the bulbs depends on the
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amount of current flowing through them
The total resistance of a series circuit increases with the
addition of resistors
A greater resistance means a reduced current flow and
dimmer light bulbs
In parallel circuits, the addition of a resistor should not affect
the brightness of the bulbs
An exception occurs when a parallel circuit is powered by a
limited source of power, such as a weak dry cell
The available electrons flowing out of the cell will be divided
among all of the bulbs and the bulbs may grow dimmer
Magnitude of Current
 The current in a series circuit is the same at
any point in the circuit
 The addition of a resistor any place in the circuit
adds to the total resistance of the circuit and
reduces the current that flows through it
 In parallel circuits the addition of a bulb
provides a new path for the flow of electricity and
adds to the total current of the circuit
Voltage
 The total voltage available to a circuit is determined by
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the source of power- a generator, a battery, etc.
The voltage is an expression of the energy that is
available from the source
In a series circuit, the voltage of the power source is
divided among the resistors
In a sample series circuit that has 3 bulbs of equal
wattage and a 6 volt battery as a power source, each
bulb would account for 2 volts of power or energy
If a 4th bulb is added, each bulb in the circuit will then
receive ¼ of the available voltage, or 1.5 volts
Voltage
 In parallel circuits, each bulb has available the
full voltage of the power source
 3 bulbs in a parallel circuit with a 6 volt power
source each have the full 6 volts of power
 If a 4th bulb were added, each bulb would still
have 6 volts of available power
Electricity Produces Heat and
Light
 Electrons in electrical conductors sometimes yield energy in
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forms of heat and light
Thus, 2 very practical applications of electrical energy are the
production of heat and light
Heat and light energy are given off from a conducting wire
when the motion of the electrons that are being forced through
the wire is opposed by the wire’s natural resistance
The opposition causes the electrons to give up energy to their
surroundings
When the production of heat or light is desired, as in heating
elements, stoves, and incandescent bulbs, high resistance iron,
tungsten, or nichrome wires are used in the circuit
Electricity Produces Motion
 Each moving electron produces a
magnetic field about itself
 Many electrons moving through a
conductor will produce a strong
magnetic field about the
conductor
 The magnetic field about the
conductor may repel and/ or
attract other magnetic
substances causing them to
move
 This is the basis for the
construction of electric motors
Electricity Produces Chemical
Activity
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Metal atoms often lose 1 or 2 electrons
to nonmetallic atoms in the formation
of salts
Metal atoms are then said to be metal
ions
Metal ions can become metal atoms
again when they replace their lost
electrons
Metal atoms may be deposited from a
salt solution upon the surface of a
substance that provides the needed
electrons
This is the basis for the electrolytic or
electroplating cell