Chapter 20
Electricity
20.1 Electric Charge and Static Electricity
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20.1 Electric Charge and Static
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20.1 Electric Charge and Static Electricity
Electricity
Electric Charge
 Electric energy is the energy associated
with electric charges.
 Electric charge is a property that causes
subatomic articles to attract or repel each
other.
 The atom is neutral because it has an
equal number of protons and electrons.
20.1 Electric Charge and Static Electricity
Electricity
 An excess or shortage of electrons
produces a net electric charge.
 The SI unit of electric charge is the
coulomb (C) which equivalent to 6.24×1018
electrons.
20.1 Electric Charge and Static Electricity
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Electric Forces
 Like charges repel and opposite charges
attract.
 The force of attraction or repulsion
between electrically charged objects is
electric force.
20.1 Electric Charge and Static Electricity
Electricity
 Coulomb discovered that electric forces
obey a law similar to the law of universal
gravitation.
 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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Electric Fields
 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.
20.1 Electric Charge and Static Electricity
Electricity
 In figure 4, the lines representing the field
are closer together near the charge, where
the field is stronger.
 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.
20.1 Electric Charge and Static Electricity
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Fields of Positive and Negative
Charges
20.1 Electric Charge and Static Electricity
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Static Electricity and Charging
 Static electricity is the study of the
behavior of the electric charges, including
how charge is transferred between
objects.
 Charge can be transferred by friction, by
contact, and by induction.
20.1 Electric Charge and Static Electricity
Electricity
 Whenever there is a charge transfer, the
total charge is the same before and after
the transfer occurs.
 The law of conservation of charge says
the total charge in an isolated system is
constant.
20.1 Electric Charge and Static Electricity
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Charging by Friction
 Rubbing a balloon through hair is an
example of charging by friction.
 Electrons move from the hair to the
balloon because atoms in rubber have a
greater attraction for electrons than atoms
in hair.
20.1 Electric Charge and Static Electricity
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Induction
 Induction occurs when charge is
transferred without contact between
materials.
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20.1 Electric Charge and Static Electricity
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Static Discharge
 Static discharge occurs when a pathway
through which charges can move forms
suddenly.
 Lightning is an example of static
discharge.
20.2 Electric Current and Ohm’s Law
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20.2 Electric Current and Ohm’s
Law
20.2 Electric Current and Ohm’s Law
Electricity
Electric Current
 The continuous flow of electric charge is
an electric current.
 The SI unit of electric current is the
ampere (A), or amp, which is equal to 1
coulomb per second.
 In direct current charge flows only in one
direction.
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20.2 Electric Current and Ohm’s Law
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 In alternating current the flow of electric
charge regularly reverses its direction.
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20.2 Electric Current and Ohm’s Law
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 In direct current, electrons flow from the
negative terminal of a battery to the
positive terminal.
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20.2 Electric Current and Ohm’s Law
Electricity
 However, scientists define current as the
direction in which positive charges would
flow.
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20.2 Electric Current and Ohm’s Law
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Conductor and Insulators
 An electrical conductor is a material
through which charge can flow easily.
 Material through which charge cannot flow
easily is called an electrical insulator.
20.2 Electric Current and Ohm’s Law
Electricity
Resistance
 Resistance is opposition to the flow of
charges in a material.
 The SI unit of resistance is the ohm (Ω).
 A material’s thickness, length, and
temperature affect its resistance.
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20.2 Electric Current and Ohm’s Law
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 Resistance is lowered in a thicker wire
because more electrons can flow.
 Resistance is more in a longer wire
because charges travel farther.
 As temperature increases, resistance
increases because electrons collide more
often.
20.2 Electric Current and Ohm’s Law
Electricity
 A superconductor is a material that has
almost zero resistance when it is cooled to
low temperatures.
 The best superconductor yet found must
be cooled to about 138 K.
20.2 Electric Current and Ohm’s Law
Electricity
Voltage
 In order for charge to flow in a conducting
wire, the wire must be connected in a
complete loop that includes a source of
electrical energy.
20.2 Electric Current and Ohm’s Law
Electricity
Potential Difference
 Charges flow spontaneously from a higher
to a lower potential energy.
 The potential energy of a charge depends
on its position in an electric field.
20.2 Electric Current and Ohm’s Law
Electricity
 Potential difference is the difference in
electrical potential energy between two
places in an electric field.
 Potential difference is measured in joules
per coulomb, or volts.
 Potential difference is also called voltage.
20.2 Electric Current and Ohm’s Law
Electricity
Voltage Sources
 A source of voltage such as a battery does
work to increase the potential energy of
electric charges.
 Three common voltage sources are
batteries, solar cells, and generators.
 A battery is a device that converts
chemical energy to electrical energy.
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20.2 Electric Current and Ohm’s Law
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Ohm’s Law
 The unit of resistance is the ohm.
 Ohm discovered that voltage is not the
same everywhere in a circuit and
hypothesized that resistance reduces the
voltage.
 He found a mathematical relationship
between voltage, current, and resistance.
20.2 Electric Current and Ohm’s Law
Electricity
 Ohm’s law states the voltage in a circuit
equals the product of the current and the
resistance, or V=IR.
 When the current is in amps and resistance
is in ohms, the voltage is in volts.
 Increasing the voltage increases the current.
 Keeping the same voltage and increasing the
resistance decreases the current.
20.3 Electric Circuits
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20.3 Electric Circuits
20.3 Electric Circuits
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Circuit Diagrams
 An electric circuit is a complete path
through which a charge can flow.
 Circuit diagrams use symbols to represent
parts of a circuit, including a source of
electrical energy and devices that are run
by electrical energy.
 See figure 12.
20.3 Electric Circuits
Electricity
Series Circuits
 In a series circuit, the charge has only one
path through which it can flow.
 If one element stops functioning in a series
circuit, none of the elements can operate.
 Adding bulbs to a series circuit increases
the resistance, decreases the current, and
each bulb shines less brightly.
20.3 Electric Circuits
Series Circuit
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20.3 Electric Circuits
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Parallel Circuits
 A parallel circuit is an electric circuit with
two or more paths through which charges
can flow.
 If one element stops functioning in a
parallel circuit, the rest of the elements still
operate.
20.3 Electric Circuits
Parallel Circuit
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20.3 Electric Circuits
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Power and Energy Calculation
 Power is the rate of doing work.
 The rate at which electrical energy is
converted to another form of energy is
electric power.
 The unit of electric power is the joule per
second, or watt.
20.3 Electric Circuits
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 Power is often measured in kilowatts (kW).
 Electric power can be calculated by
multiplying voltage by current (P=IV).
 To find the electrical energy used by an
appliance, multiply power by time (E=Pt).
 Unit is kilowatt hours (kWh).
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20.3 Electric Circuits
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Electrical Safety
 Correct wiring, fuses, circuit breakers,
insulation, and grounded plugs help make
electrical energy safe to use.
 A fuse prevents current overload in a
circuit.
 A circuit breaker is a switch that opens
when current in a circuit is too high.
 The circuit breaker must be reset before
the circuit can be used again.
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20.4 Electronic Devices
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20.4 Electronic Devices
20.4 Electronic Devices
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Electronic Signals
 The science of using electric current to
process or transmit information is electronics.
 An electronic signal is information sent as
patterns in the controlled flow of electrons
through a circuit.
 Electronics conveys information with
electrical patterns called analog and digital
signals.
20.4 Electronic Devices
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Analog Signals
 An analog signal is a smoothly varying
signal produced by continuously changing
the voltage or current in a circuit.
20.4 Electronic Devices
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 Analog signal
 changes continuously,
 has many voltage values,
 resembles the pattern of the original
signal,
 and is easily distorted.
20.4 Electronic Devices
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Digital Signals
 A digital signal encodes information as s
string of 1’s and 0’s.
 Digital signals are more reliable than
analog signals.
20.4 Electronic Devices
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 Digital signal
 changes abruptly,
 has only two voltage values,
 does not resemble the original signal,
 and is not easily distorted.
 A DVD encodes digital signals as a series
of pits on the DVD surface.
20.4 Electronic Devices
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20.4 Electronic Devices
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Vacuum Tubes
 To create an electronic signal, the flow of
electrons must be controlled.
 A vacuum tube was used to control
electron flow in early electronic devices.
20.4 Electronic Devices
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 Vacuum tubes can
 change alternating current into direct
current,
 increase the strength of a signal, or
 turn a current on or off.
20.4 Electronic Devices
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20.4 Electronic Devices
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Semiconductors
 A semiconductor is a crystalline solid that
conducts current only under certain
conditions.
 Most semiconductors are made with
silicon or germanium.
20.4 Electronic Devices
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 There are two types of semiconductors
 In n-type semiconductors, the current is
a flow of electrons.
 In p-type semiconductors, it appears as
though positive charge flows.
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20.4 Electronic Devices
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Solid-State Components
 Most modern electronic devices are
controlled by solid-state components.
20.4 Electronic Devices
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Diodes
 A diode is a solid-state component that
combines an n-type and p-type
semiconductors.
 Because the current can be in only one
direction, a diode can change alternating
current to direct current.
20.4 Electronic Devices
Electricity
Transistors
 A transistor is a solid-state component with
three layers of semiconductors.
 A transistor can be used as a switch
because the small current can turn current
on or off.
 Transistors can also be used as amplifiers.
 Small voltage on one side produces
large voltage on the other side.
20.4 Electronic Devices
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Integrated Circuits
 An integrated circuit is a thin slice of silicon
that contains many solid-state components.
 Integrated circuits are also called chips or
microchips.
 Integrated circuits are fast compared to
vacuum tubes.
 The current does not have to travel far to
get from point to point in the circuit.
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