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Electricity
Table of Contents
Electric Charge and Static Electricity
Electric Current
Batteries
Electric Circuits
Electric Power
Electrical Safety
Electricity - Electric Charge and Static Electricity
Electric Charge
• Remember…
• In the last chapter, when two magnetic poles
interacted the result was magnetism.
– Charges that are the same repel each other.
– Charges that are different attract each other.
• An atom consists of three parts:
– Proton = positively charged (+)
– Neutron = no charge (0)
– Electron = negatively charged (-)
• The interaction between electric charges is
called electricity.
Electricity - Electric Charge and Static Electricity
Electric Charge
Electricity - Electric Charge and Static Electricity
Electric Charge
• Unlike magnetism, however, one charge
can exist without the other charge.
In magnets, a North
pole must be
accompanied by a
South pole. The poles
cannot exist without
each other.
In atoms, one
charge may exist
with out an opposite
charge present to
balance it out.
Electricity - Electric Charge and Static Electricity
Electric Force
• Electric force – the attraction
or repulsion between electric
charges.
• Electric field – extends
around a charged object.
– An electric field is a region
around a charged object where
the object’s electric force is
exerted on other charged
objects.
Electricity - Electric Charge and Static Electricity
Electric Force
• An electric field is a region around a charged
object where the object’s electric force is
exerted on other charged objects.
Electricity - Electric Charge and Static Electricity
Electric Force
• The strength of an electric field is related
to the distance from the charged object.
– Greater the distance = weaker electric field
• The strength of an electric field is
represented by how close the electric field
lines are to each other.
– The electric field is strongest where the lines
are closest together.
Strongest
Weakest
Electricity - Electric Charge and Static Electricity
Electric Force
Electricity - Electric Charge and Static Electricity
Static Electricity
• Static electricity – buildup of charges on an
object
– Static means “not moving or changing.”
– Charges build up on an object, but they do not
flow continuously.
Electricity - Electric Charge and Static Electricity
Transferring Charge
• Conservation of charge – law
that states that charges are
neither created nor destroyed
but only transferred from one
material to another.
• There are three ways that
charges can be transferred to
build up static electricity:
– Friction
– Conduction
– Induction
Electricity - Electric Charge and Static Electricity
Transferring Charge
• Friction – method of charging
an object by rubbing it
against another object
– The force that one object
exerts on another when the
two rub against each other.
– One becomes positively
charged; the other
becomes negatively
charged.
Electricity - Electric Charge and Static Electricity
Transferring Charge
• Conduction – method of
charging an object by
allowing electrons to flow
by direct contact from one
object to another object.
– Occurs when a charged
object touches another
object
– Electrons are transferred
from the more negative
object to the more positive
Electricity - Electric Charge and Static Electricity
Transferring Charge
• Induction - method of
charging an object by
means of the electric field
of another object
– The two objects have no
direct contact.
– The electric field around
the charged object
attracts or repels
electrons in the second
object
Electricity - Electric Charge and Static Electricity
Transferring Charge
• Electroscope – device that can be used
to detect the presence of a charge.
Electricity - Electric Charge and Static Electricity
Transferring Static Electricity
Q. What are three ways that static electricity can be
transferred?
A. Charging by friction, charging by conduction, and
charging by induction
Q. Why does an object become charged?
A. An object becomes charged when electrons are
transferred from one location to another.
Electricity
Static Discharge
• Static discharge – loss of
static electricity as electric
charges transfer from one
object to another.
– When a negatively charged
object and a positively
charged object are brought
together, electrons transfer
until both objects have the
same charge.
• Ex: Lightning
Electricity - Electric Charge and Static Electricity
Lightning
• Click the Video button to watch a movie
about lightning.
Electricity - Electric Charge and Static Electricity
Links on Static Electricity
• Click the SciLinks button for links on
static electricity.
Electricity - Electric Current
Flow of Electric Charges
• Electric current – continuous flow of electric
charges through a material.
Electricity - Electric Current
Flow of Electric Charges
• Rate of electric current – amount of
charge that passes through the wire in a
unit of time
– The unit for the rate of current is measured in
amperes, or amps (A) for short
– The number of amps describes the amount of
charge flowing past a given point each second.
Electricity - Electric Current
An Electric Circuit
• ADD TO NOTES – in margin:
• Electric circuit – a complete, unbroken
path through which electric charges can
flow.
Electricity - Electric Current
An Electric Circuit
• To produce electric current, charges must flow
continuously from one place to another.
– All electrical devices, from toasters to radios to
electric guitars and televisions, contain electric
circuits.
Electricity - Electric Current
Conductors and Insulators
• Electric charges tend to flow better through
certain materials
• A conductor transfers
electric charge well.
• An insulator does not
transfer electric charge well
Electricity - Electric Current
Conductors
• In a conductor, atoms
contain electrons that are
bound loosely.
• These conduction electrons
move throughout the
conductor easily, creating
an electric current.
– Metals, such as silver,
copper, aluminum, and
iron, are good
conductors
Electricity - Electric Current
Insulators
• The electrons in an
insulator are bound tightly
to their atoms and do not
move easily.
• Insulators are used to stop
the flow of charges.
– Rubber, glass, sand,
plastic, and wood are
good insulators.
Electricity - Electric Current
Voltage
• Potential energy – amount of energy
stored in an object due to the build-up of
charges.
• Voltage – difference in electrical potential
energy between two places in a circuit.
• Voltage source - device that
creates an electrical potential
energy difference in an
electric circuit
• Ex: batteries and generators
Electricity - Electric Current
Resistance
• Resistance – the
measure of how difficult it
is for charges to flow
through a material
• The greater the
resistance, the less
current there is for a
given voltage.
• The unit of measure of
resistance is the ohm (Ω)
Electricity - Electric Current
Factors That Determine Resistance
• There are four factors that determine resistance
1) The material from which the wire is made –
insulators have a higher resistance than
conductors
2) Length – long wires have more resistance
than short wires
Electricity - Electric Current
Factors That Determine Resistance
3) Diameter – thin wires have more resistance
than thick wires
4) Temperature of the wire – the hotter the
wire the more resistance it has.
Electricity
Path of Least Resistance
• If two paths present
themselves, the electric
flow will travel the “path of
least resistance”
• The bird that perches on an
electric fence does not get
hurt because it’s body
contains more resistance
than the wire
Electricity - Electric Current
More on Electric Current
• Click the PHSchool.com button for an
activity about
electric current.
Electricity - Batteries
The First Battery
• Energy can be transformed from one form
into another.
– Ex: Chemical energy (energy stored in
chemical compounds) can be transformed
into electrical energy
– Chemical reaction – process in which
substances change into new substances with
different properties.
– Alessandro Volta used these ideas to build
the first battery in 1800
Electricity - Batteries
The First Battery
• Volta built the
first electric
battery by
layering zinc,
paper soaked
in salt water,
and silver.
Electricity - Batteries
Electrochemical Cells
• Electrochemical cell – a device that
transforms chemical energy into electrical
energy.
Electricity - Batteries
Electrochemical Cells
• Electrochemical cell consists
of:
– Electrodes - two different
metals immersed in a
substance called an
electrolyte.
– Electrolyte – substance that
conducts electric current.
• Volta used silver and zinc as
electrodes and salt water as
his electrolyte.
Electricity - Batteries
Simple Cell
• In a simple cell a terminal is used to
connect the cell to a circuit
• Chemical reactions occur between the
electrolyte and the electrodes in an
electrochemical cell.
• These reactions cause one electrode to
become negatively charged and the other
electrode to become positively charged
creating a voltage between them.
Electricity - Batteries
Batteries
• A true battery is a combination of two or more
electrochemical cells in a series.
• Today, single cells are often referred to as
“batteries.” So the “batteries” you use in your
flashlight are technically cells rather than
batteries.
Electricity - Batteries
Electrochemical Cells
• An electrochemical cell in which the
electrolyte is a liquid is a wet cell.
Example:
Car Battery
Electricity - Batteries
Electrochemical Cells
• Dry cell – an electrochemical cell in which
the electrolyte is a paste.
Example:
AA Batteries
Electricity
Ohm’s Law
• The relationship between resistance, voltage,
and current is summed up in Ohm’s law.
• Ohm’s law – states that the resistance is equal
to the voltage divided by the current.
• The equation can also be rearranged to read:
Electricity - Electric Circuits
Decimals
• When calculating voltage, you often use
decimals.
• If a circuit has a resistance of 30.5 ohms and
a current of 0.05 amps, what is its voltage?
Voltage = current x resistance
Voltage = 0.05 amps x 30.5 ohms
Voltage = 1.53 volts
Electricity - Electric Circuits
Decimals
Practice Problem
• Use Ohm’s law to calculate the voltage
of a circuit with a resistance of 15.2
ohms and a current of 0.10 amps.
• 1.52 volts
Electricity - Electric Circuits
Calculating Resistance
• The brake light on an automobile is
connected to a 12-volt battery. If the resulting
current is 0.40 amps, what is the resistance of
the brake light?
Read and Understand
• What information are you given?
Battery Voltage = 12 V
Current = 0.40 A
Electricity - Electric Circuits
Calculating Resistance
• The brake light on an automobile is connected to
a 12 volt battery. If the resulting current is 0.40
amps, what is the resistance of the break light?
Plan and Solve
• What quantity are you trying to calculate?
The resistance of the brake light
• What formula contains the given quantities and
the unknown quantity?
Resistance = Voltage/Current
• Perform the calculation.
Resistance = 12 V/0.40 A = 30 Ω
Electricity - Electric Circuits
Calculating Resistance
• The brake light on an automobile is
connected to a 12-volt battery. If the resulting
current is 0.40 amps, what is the resistance of
the brake light?
Look Back and Check
• Does your answer make sense?
• The answer makes sense because you are
dividing the voltage by a decimal. The answer
should be greater than either number in the
fraction, which it is.
Electricity - Electric Circuits
Calculating Resistance
• Practice Problem
• In a circuit, there is a 0.5-A current in
the bulb. The voltage across the bulb
is 4.0 V. What is the bulb’s
resistance?
• 8.0 Ω (4.0 V ÷ 0.5 A)
Electricity - Electric Circuits
Calculating Resistance
• Practice Problem
• A waffle iron has a 12-A current. If the
resistance of the coils is 10 Ω, what
must the voltage be?
• 120 V (12 A X 10 Ω)
Electricity
Features of a Circuit
• All electric circuits have the same basic
features.
1) Circuits have devices that are run by
electrical energy.
2) A circuit has a source of electrical
energy.
3) Electric circuits are connected by
conducting wires.
Electricity - Electric Circuits
Features of a Circuit
• Simple symbols are used to diagram a
circuit.
Electricity - Electric Circuits
Series Circuits
• In a series circuit, there is only one path
for the current to take.
Electricity - Electric Circuits
Parallel Circuits
• In a parallel circuit, there are several
paths for the current to take.
Electricity - Electric Circuits
Measuring Currents and Voltage
• An ammeter is a device used to measure
current.
– The ammeter needs to be connected in series
with that device.
• A voltmeter is a device used to measure
voltage, or electrical potential energy
difference.
– The voltmeter needs to be wired as a parallel
circuit.
Electricity - Electric Circuits
Series and Parallel Circuits Activity
• Click the Active Art button to open a
browser window and access Active Art
about series and parallel circuits.
Electricity - Electric Circuits
Comparing and Contrasting
Series Circuit
Only one
path for
current to
take
Parallel Circuit
Unbroken
path that
has a
current
Several
paths for
current to
take
Electricity - Electric Power
Electric Power
• Power – rate at which energy is
transformed from one form to another
– Measured in watts (W).
Electricity - Electric Power
Calculating Power
• A household light bulb has about 0.5 amps of
current in it. Since the standard household
voltage is 120 volts, what is the power rating
for this bulb?
Read and Understand
• What information have you been given?
Current = 0.5 A
Voltage = 120 V
Electricity - Electric Power
Calculating Power
• A household light bulb has about 0.5 amps of
current in it. Since the standard household
voltage is 120 volts, what is the power rating
for this bulb?
Plan and Solve
• What quantity are you trying to calculate?
The power of the light bulb
Electricity - Electric Power
Calculating Power
• What formula contains the given quantities
and the unknown quantity?
Power = Voltage X Current
• Perform the calculation.
Power = 120 V X 0.5 A
Power = 60 W
Electricity - Electric Power
Calculating Power
• A household light bulb has about 0.5 amps of
current in it. Since the standard household
voltage is 120 volts, what is the power rating
for this bulb?
Look Back and Check
• Does your answer make sense?
• The answer is reasonable, because 60 W is a
common rating for household light bulbs.
Electricity - Electric Power
Calculating Power
• Practice Problem
• A flashlight bulb uses two 1.5-V
batteries in series to create a current
of 0.5 A. What is the power rating of
the bulb?
• 1.5 W (3.0 V X 0.5 A)
Electricity - Electric Power
Calculating Power
• Practice Problem
• A hair dryer has a power rating of
1,200 W and uses a standard voltage
of 120 V. What is the current through
the hair dryer?
• 10 A (1,200 W ÷ 120 V)
Electricity - Electric Power
Paying for Electrical Energy
• The total amount of energy used by an
appliance is equal to the power of the
appliance multiplied by the amount of time the
appliance is used.
Electricity - Electric Power
Paying for Electrical Energy
• The amount of electrical
energy used in your
home is measured by a
meter.
• The electric company
uses the meter to keep
track of the number of
kilowatt-hours used.
You pay a few cents for
each kilowatt-hour.
Electricity - Electric Power
Links on Electric Power
• Click the SciLinks button for links on
electric power.
Electricity - Electrical Safety
Electrical Equipment and Fires
• If electrical equipment
is not properly used
and maintained, it can
cause fires. The circle
graph shows the
percentage of fires
caused by different
types of electrical
equipment.
Electricity - Electrical Safety
Electrical Equipment and Fires
• Reading Graphs:
What determines the
size of each wedge
in the graph?
The percentage of
fires caused by a
certain type of
electrical equipment
Electricity - Electrical Safety
Electrical Equipment and Fires
• Reading Graphs:
What percentage of
fires are caused by
appliances?
15%
Electricity - Electrical Safety
Electrical Equipment and Fires
• Interpreting Data:
Which category is
responsible for the most
fires? Which category is
responsible for the
fewest fires?
Cooking equipment most fires.
Heating and cooling
equipment - the fewest
fires.
Electricity - Electrical Safety
Grounding
• One way to protect
people from electric
shock and other
electrical danger is to
provide an alternate
path for electric
current.
Electricity - Electrical Safety
Breaking a Circuit
• In order to prevent
circuits from
overheating, devices
called fuses and
circuit breakers are
added to circuits.
Electricity - Electrical Safety
Links on Electrical Safety
• Click the SciLinks button for links on
electrical safety.
Electricity - Electrical Safety
Graphic
Organizer
Devices to prevent
circuits from
overheating
include
Fuses
Circuit
breakers
work by
work by
Melting
Bending
away from
wires