Transcript Topic 7/8 Powerpoint Slides

Topic 7
ELECTRICITY IN THE HOME
Electricity in the Home
• Transformers are used to “step up” the
voltage for efficient transmission over
long distances.
• Other transformers “step down” the voltage to
the 240/120 V used in homes and factories.
• A circuit breaker acts as a switch and safety
device that can cut all power coming into the
home.
• If current exceeds a safe level, a bimetallic strip
in the breaker heats up, bends, and opens the
circuit.
Electricity in the Home
• A fuse contains a metallic conductor that melts
when excessive current heats it up. This opens
the circuit until the fuse is replaced.
• A branch circuit supplies power to one or more
wall plugs or lights connected in parallel by
cables in the house walls.
• As additional loads are plugged into a branch
circuit, the current flowing through the
connecting cables increases. The small
resistance of the cable wires causes them to
heat up when enough current flows
Electricity in the Home
• The breakers, plugs, lights, and switches
in each branch circuit are connected by
two “live” wires – a white insulated wire
(usually called the neutral wire) and a black
insulated wire (usually called the hot wire).
• The third wire, is the ground wire. It reduces
hazards by safely channeling back into the
ground any current that has “leaked” onto
metal components in the electric circuit.
Digital Devices
• Digital electronic technology – machines that
process numerically coded information.
• A Binary code is two states (on or off) that represent
numbers and letter.
• Logic circuits containing many switches can process
binary information.
• The electronic switches in modern digital devices are
transistors, that can be turned on and off by electric
signals.
• Every digital device you use, is designed around
components that contain enormous numbers of
transistors.
Measuring Electric Power
• Power is energy per unit
of time.
• Electric power describes
the amount of electric
energy that is converted
into other forms of
energy every second.
• Electric power can also
describe the amount of
electric energy that is
transferred from one
place to another in a
certain amount of time.
energy (joules)
power (watts) =
time (seconds)
-or𝐸
𝑃=
𝑡
Measuring Electric Power
• One joule per second is
also called a watt (W)
• Electric power is notpower watts
usually calculated = current amps × voltage (volts)
directly by measuring
energy and time.
-or• It is much more common
to measure the voltage
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and current in a circuit
and use these quantities
to calculate power
Example
A current of 13.6 A passes through an
electric baseboard heater when it is
connected to a 110 V wall outlet. What is the
power of the heater?
On Your Own
• Please work on the next four examples
in your notes. If you get stuck, raise
your hand.
Paying for Electric Energy
• Most power companies measure electric
energy in a customary unit – the kilowatt
hour (kWh).
• One kilowatt hour is the total energy
supplied to a 1000 W load during 1 h of
operation.
Reading a Power Meter
Solving Cost of Power
Problems
• Determine the # of kW of power used.
• Multiply the # of kW by the # of hours to get
kWh.
• Multiply the # of kWh by the cost/kWh
Example
A family uses 3000 kWh of electric
energy in a two-month period. If the energy
costs 11.0 cents per kilowatt hour, what is the
electric bill for the period?
On Your Own
• Please work on the next three
examples in your notes. If you get stuck,
raise your hand.
Electric Devices and Efficiency
• If an electric light bulb were perfect,
all of the electric energy it took in
(input energy) would be converted into
light (useful output energy).
• No real device, however, is a perfectly
efficient energy converter. Some input
energy is always converted into waste heat.
Calculating Efficiency
• You can express efficiency as a percentage using the
following mathematical relationship.
useful energy output
efficiency =
× 100
total energy input
• To find the efficiency of electric devices, it is often
necessary to calculate energy inputs or outputs. To
determine the electric energy input of a device, the
E
formula for power P = can be manipulated to
t
solve for energy.
Efficiency of Common Light
Bulbs
• Incandescent bulbs are about 5% efficient,
which means about 5% of the input electrical
energy is converted to waste heat.
• Halogen bulbs are filled with high-pressure gas
containing traces of iodine
– Halogen bulbs are about 15% efficient,
– Last two to six times longer than incandescent bulbs
• Fluorescent tubes convert about 20% of the total
input energy into useful light energy
– Last 10 to 13 times longer than incandescent bulbs, and
are much more expensive
Example
A 1000 W electric kettle takes 4.00 min to boil
some water. If it takes 1.96 × 105 J (196 000 J)
of energy to heat the water, what is the
efficiency of the kettle?
On Your Own
• Please work on the next three examples
in your notes. If you get stuck, raise
your hand.
Home Electric Safety
• Do not overload an outlet or a circuit.
• Never work on or clean appliances that
are still plugged in.
• Replace frayed or worn out electric cords.
• If the bare wires touch directly (a short
circuit), a high current flows between
them, causing sparks and often starting a fire.
• Use receptacle cover on easily accessible outlet.
• Never use appliances close to a sink of bathtub with water
in it or when your hands are wet.
• Ground fault circuit interrupter, a GFCI, monitors current
flowing into and out of a load. If any electricity is diverted
out of the circuit wires, the GFCI immediately cuts off the
current.
Electric Safety Outdoors
• Never allow your body or something you
are holding to come into contact with live
electric wires.
• Never use ungrounded or frayed two-prong
electric cords outdoors.
• Do not operate electric equipment outdoors
when it is raining.
• Before digging deeply in your yard, make sure
that there are no underground utility cables.
Locate Marks
Topic 8
ELECTRICITY PRODUCTION AND
THE ENVIRONMENT
Electric Energy from Burning
Fuels
• Fuel oil, natural gas, and coal, are burned in
large thermo-electric generating plants to
produce about one quarter of our country’s
electricity.
Fossil Fuels Affect Land and
Air
• Open pit mining of surface deposits disturbs
soil and vegetation.
• Underground mines produce waste materials
called “tailings,” which accumulate near the
mine.
• Water seeping through the tailings becomes
acidified and contaminated.
• When fossil fuels are burned, contaminants
such as visible particles and invisible gases
escape form smokestacks.
Fossil Fuels Affect Land and
Air
• Electrostatic precipitators can remove most
of the solid particles.
• Scrubbers can remove sulfur dioxide.
Scrubbers spray a water solution through
the waste gases. SO2 and other pollutants
react with chemicals in the water and are
removed.
• Complete burning of pure coal or natural gas
would produce carbon dioxide gas.
Fossil Fuels Affect Land and
Air
• Carbon dioxide is a greenhouse gas, which
means that it helps delay the escape of heat
from Earth’s atmosphere. This could lead to
a rise in the average temperatures on Earthglobal warming.
• Natural gas burns much cleaner than coal,
producing less SO2.
Electric Energy from Flowing
Rivers
• Hydro-electric plants use water pressure to generate
electric energy. Hydro-electric power plants appear
to be a very clean form of electric energy generation.
However, reservoirs, which store behind the dams,
flood many hectares of valuable land. Homes, small
villages even entire towns may need to be moved,
displacing people and industries.
• When submerged vegetation decays, microorganisms responsible for this process use up the
oxygen supplies in the water sometimes decaying
matter produces methane gas. Other forms of life,
such as algae, take over and change the ecosystem.
Energy From Atomic
Reactions
• Bombarding uranium atoms with tiny
particles called neutrons causes the uranium
to split into two smaller atoms.
• In the process called nuclear fission, a
tremendous amount of energy is released.
This is referred to as thermonuclear electric
generation.
Energy From Atomic
Reactions
• Fission reactors do not release soot or gases
that cause acid rain, nor do they release
greenhouse gases. However, used (spent)
reactor fuel is highly radioactive for
thousands of years.
• The reactors themselves also become
slightly radioactive so they are difficult and
expensive to decommission.
Energy From Atomic
Reactions
• In the Sun and other stars, nuclear fusion
joins very small atoms to form a larger atom.
Huge amounts of energy are released in this
process
• Developing technology to create and use
continuous controlled fusion as a power
source is an extremely difficult and
expensive project.
Heating the Environment
• All thermonuclear and thermo-electric-generating
plants release thermal energy into the environment.
• Thermal pollution occurs when this warm water is
returned directly to the lake or river from which it is
taken, increasing the water temperature. Even a
change of a few degrees can affect the plant and
animal life in the water.
• To reduce thermal pollution, large generating plants
have cooling ponds or towers where waste water
can return to the temperature of the surroundings
before it is released.
Cogeneration
• It makes environmental sense to design
electricity-generating stations as
cogeneration systems that produce
electricity and also supply thermal energy,
such as hot water or steam, for industrial or
commercial heating.
Distribution of Electricity
• Power grid transformers built before 1977
often contained insulating chemicals called
“polychlorinated biphenlys” or PCBs. PCBs
persist, bioconcentrate, and can cause
health problems.
• High voltage transmission lines themselves,
which produce radiation similar to radio
waves, may be linked to health problems in
people living nearby.
Alternative Energy Sources
• Energy from the Sun, wind, and tides, which
until now has been too expensive or too
difficult to harness, is rapidly becoming
competitive with conventional energy sources.
• Wind-driven electricity generation must be used
together with other electric energy sources or
storage devices.
• Solar-powered-generating systems often
include storage batteries to supply electricity at
night or in cloudy conditions.
Alternative Energy Sources
• Ocean tides are another source of energy
that can be used to generate electricity.
• The hot inner parts of the Earth contain a
great deal of thermal energy, called geothermal energy. Ground water flows down
from the surface, absorbs thermal energy,
and rises again as hot spring and geysers.
Steam produced by geothermal activity can
be used to rotate