Chapter 3
Energy
Work
• Work (W) is concerned with the application
of force (F) to an object and the distance
(d) the object moves as a result of the
force.
• W=Fxd
What is Energy?
• Energy is the ability to do work.
• One way of classifying energy is as
potential energy (PE) and kinetic energy
(KE).
Potential Energy
• The energy that an object has because of its position.
• Types of potential energy:
 Gravitational Potential Energy-Due to the attraction of
object to the earth.
 When a person raises a book the work that the person
does on the book is stored on the book as potential
energy. The book now has the potential of doing work on
something else.
 When a spring is stretched the work done to stretch the
spring is now stored as potential energy. The spring now
has the potential of doing work on something else.
Potential Energy
Work done on = Increase = Increase in
an object to
in PE
work the object
change its
can do
position
Work on book = PE of book = Work by book
Fig. 3.2
The Joule
• The joule is a measure of work accomplished on an
object.
• It is also a measure of potential energy or how much
work an object can do.
• In the English system the unit of work and energy is the
ft x lb.
• F = m x a For a falling object a = g, so
F=mxg
• Energy is force x distance.
• E=Fxd
• For a falling object d=h (h=height)
• E=Fxh
• PE= m x g x h
Potential Energy
• The potential energy of an object can be
calculated from the work done on the object to
change its position.
• You can exert a force equal to its weight as you
lift it some distance above the floor.
• Weight is the force of gravity acting on a mass.
• You can exert a force equal to its weight as you
lift it some height above the floor, and the work
you do is a product of its weight and height.
Potential Energy and Weight
• Weight = mass x
acceleration due to
gravity
w=mxg
• Work = weight x
height
W=wxh
PE = w x h
PE = m x g x h
Fig. 3.3
Units for Energy
W=Fxd
W = Kg x m / s2 x m
=Nxm
= Joules (J)
Calculation of Potential Energy
• How much potential energy does a backpack
have if it has a mass of 6.7 kg and is sitting on a
shelf 1.8 m above the floor?
m = 6.7 kg
PE = m x g x h
g = 9.8 m/s2
PE = 6.7 kg x 9.8 m/s2 x 1.8 m
h = 1.8 m
PE = 118 kg x m x m
PE = ?
s2
PE = 118 N x m or 118 J
Calculation of Work
• How much work is needed to raise a box
to a shelf which is .56 m above the ground
if the box has a mass of .75 kg?
m = .75 kg
PE = m x g x h
h= .56 m
PE = .75 kg x 9.8 m/s2 x .56 m
g = 9.8 m/s2 PE = 4.1 kg x m2
PE = ?
s2
W = PE = 4.1 N x m = 4.1 J
Kinetic Energy
• Moving objects have the ability to do work
on other objects because of their motion.
• The energy of motion is kinetic energy.
• It can be measured in terms of:
1. Work done to put the object in motion
or
2. Work the moving object will do in
coming to rest.
Kinetic Energy
• If you throw a football you exert a force on it as you
accelerate it through a distance before it leaves your
hand.
• The kinetic energy the ball now has is equal to the work,
or force times distance, that you did on the ball.
• The ball exerts a force on the hand of the person
catching the ball and moves it through a distance.
• The net work on the hand is the kinetic energy that the
ball had.
• Work done to = Increase = Increase in
put object in
in KE
work the
motion
object can do
Kinetic Energy
1
2
KE  m v
2
m 2
KE  (kg)( )
s
KE 
kg m
KE  (
s
2
kgm
s
2
KE  Nxm
KE  J
2
)( m)
Kinetic Energy
• If a bowling ball with a mass of 5.25 kg is
thrown with a velocity of 7.3 m/s, what is
the KE of the ball?
m=5.25 kg KE=1/2 mv2
v=7.3 m/s
KE=1/2 (5.25 kg)(7.3 m/s)2
KE = ?
KE= 140 kg x m2/s2
KE= 140 J
Kinetic Energy
• A football player with a mass of 115 kg
moving with a velocity of 8.5 m/s tackles a
stationary quarterback. How much work
was done on the quarterback?
m=115 kg
W=KE= ½ mv2
v=8.5 m/s
W = ½ (115 kg)(8.5 m/s)2
W=?
W=4154 J
Kinetic and Potential Energy
Conversion
• A roller coaster is a good example of
kinetic and potential energy conversion.
• When a roller coaster is going up work is
done on it. When it is at the top the work
that was done on it is stored as potential
energy.
• When the roller coaster starts going down
the potential energy is converted to kinetic
energy.
Forms of Energy
• Another way to classify energy is as follows:
• Sources of Energy common today. The first three are currently much
more widely used globally:
1. Chemical
2. Radiant
3. Nuclear
4. Hydropower
5. Wind Power
6. Biomass
7. Geothermal Energy
• Manifestations of energy ( The above energies can be converted to
the following):
1. Mechanical
2. Electrical
Mechanical Energy
• Energy of familiar objects and machines.
e.g. a.) car moving is kinetic mechanical
energy.
b.) water behind a dam is potential
mechanical energy.
c.) spinning blades of a steam turbine
is kinetic mechanical energy.
Chemical Energy
• Form of energy involved in chemical reactions.
e.g. 1.) oxidation reduction reactions such
as burning wood. (rapid oxidation)
release the chemical energy
stored in wood.
2.) foods you eat are oxidized in your
body and the energy is later
released as you move, etc.
3.) Batteries release energy stored in chemical
compounds through oxidation reduction reactions
which is then converted to mechanical or
electrical energy and used to power
miscellaneous devices.
Chemical Energy
Fig. 3.11
Mechanical Energy
Photosynthesis
Photosynthesis, which occurs in green plants, is a
process through which plants use the energy of
the sun to rearrange carbon dioxide (CO2) and
water (H2O) into glucose and oxygen:
Energy + Carbon Dioxide + Water = Glucose + Oxygen
Glucose is used to make Cellulose (Wood) and starch (potatoes, etc..)
http://earthguide.ucsd.edu/earthguide/diagrams/ph
otosynthesis/photosynthesis.html
Burning of Wood
• Wood + Oxygen = Carbon Dioxide +
Water + Energy
This is the reverse of photosynthesis.
• Chemical energy is potential energy which
is stored in molecules and later released in
a chemical reaction.
Radiant Energy
• Energy that travels through space. This is
light or sunlight (visible light)
Radiant Energy
• Visible light occupies a
small portion of the
electromagnetic spectrum
which makes up radiant
energy.
• Infrared radiation is heat.
Objects heat up when this
type of radiation is
Increases
absorbed.
• Microwave radiation is
used in cooking.
Increases
Electrical Energy
• Another form of
energy from
electromagnetic
interactions. It can
travel through wires to
your home from a
power plant.
Nuclear Energy
• Energy found in the nucleus of the atom.
Power Plants
Electrical Turbine-Converts chemical
or nuclear energy to electrical
energy
• Steam Turbines:
In a power plant, chemical or
nuclear energy is used to heat
water to steam, which is
directed against the turbine
blades.
• The mechanical energy of the
turbine turns an electrical
generator.
•
Chemical
or Nuclear
Mechanical
Electrical
Interconversion of Energy
• Any form of energy can be converted to another form. Most
technological devices are energy form converters.
Inter conversion of Energy
• A light bulb coverts electrical energy to radiant energy.
• A car converts chemical energy from gasoline to
mechanical energy.
• A solar cell converts radiant energy to electrical energy.
• An electrical motor converts electrical energy to
mechanical energy.
• Each technological device converts some form of
energy, usually chemical (from batteries) or electrical to
another form that you desire, usually mechanical (fan) or
radiant (light bulb).
Flow of Energy
• Plants are at the bottom of the food chain. They get their
energy by converting radiant energy from the sun to
chemical energy.
• You get the energy from plants and animals, who in turn
got their energy from plants.
• When you ride a bicycle the bicycle has KE as it moves
along. The bicycle got its KE from you.
• The bicycle converts its KE to heat (infrared radiation)
when you apply brakes or through friction with the road
surface.
• The infrared radiation is then released onto space.
• The radiant energy from the sun comes from nuclear
reactions that take place in the core of the sun.
Energy Conservation
• Total energy content in the universe is
constant.
• The ultimate source of all energy is the
sun.
• Einstein’s equation, E=mc2, where c is the
speed of light, relates mass and energy.
So ultimately all energy comes from the
mass of the sun.
The Law of Conservation of Energy
• Energy can neither be created nor
destroyed. It can only be converted from
one form to another, but the total amount
of energy remains constant.
Energy Sources Today:
Chemical Energy
• Fuels are things that can be burned to produce energy.
(Chemical sources of energy)
• The first fuel that was used was wood.
• Coal started to be used in the industrial revolution.
• In the twentieth century petroleum is the main fuel.
• The fuels that we use today correspond to:
• Petroleum ~40%
This equates to ~89% of all energy consumed.
Natural gas ~23% About 1/3 of this energy was burned for heating
and the rest was burned
Coal
~21%
to drive engines or generators.
Biomass
~3%
(Material from
Photosynthesis)
History of Energy Sources
• The energy source mix has changed from past
years and it will change in the future.
• Wood supplied ~90% of the energy until the
1850’s when the use of coal was increased.
• By 1910 coal was supplying ~75% of the energy.
• Then petroleum began making increased
contributions to the energy supply.
• Now increased environmental and economic
constraints and decreasing supply of petroleum
are producing another supply shift.
Energy Sources Today
• Nuclear energy and hydropower are non
chemical sources of energy.
• They can be used to generate electrical
energy.
• Solar and geothermal energy are
alternative sources of energy as well and
they provide about .5% of all energy
consumed.
Energy Sources Today
•
In summary, the main sources of energy today are:
1. Fossil fuels (Chemical Energy):
Petroleum
Natural Gas
Coal
Biomass
2. Hydropower
3. Nuclear
4. Solar
5. Geothermal
6. Wind Power
Petroleum and Natural Gas
• Petra = Rock Oleum = Oil
• Petroleum is oil that comes from oil bearing rock.
• Natural Gas has a similar origin. Both come from organic
sediments, materials that have settled out of bodies of
water.
• Most organic material is from plankton, tiny free floating
animals and plants such as algae. They accumulate and
sometimes a local condition permits the accumulation of
sediments that are particularly rich in organic materials.
• Petroleum and Natural Gas formed from the remains of
tiny organisms that lived millions of years ago.
Petroleum and Natural Gas
• Bacteria, pressure, appropriate
temperature and time are all
important for petroleum
formation, but it is not well
understood.
• Natural gas is formed at higher
temperatures than petroleum.
• Petroleum forms a thin film
around the grains of the rock
where it formed. Pressure from
the overlying rock and water
move the petroleum and gas
through the rock until it
reaches a rock type structure
that stops it.
• If natural gas is present it will
occupy the space above the
accumulating petroleum.
Petroleum and Natural Gas
• One barrel of oil = 42 US
gallons.
• The supply of petroleum and
natural gas is limited. Most of
the continental drilling
prospects appear to be
exhausted and the reach for
new petroleum supplies is now
offshore. Over 25% of our
nation’s petroleum is estimated
to come from offshore wells.
• Imported petroleum accounts
for more than half of the oil
consumed, with most coming
from Mexico, Canada,
Venezuela, Nigeria, and Saudi
Arabia.
Uses of Petroleum
•
•
•
•
45% Gasoline
40% Diesel
15% Heating Oil
Other uses:
Making medicine
Clothing fabrics
Plastics
Ink
•
•
•
•
•
Coal formed from an accumulation
of plant materials that collected
under special conditions millions
of years ago.
Plants died and sank. Stagnant
swamp water protected the plants
and plant materials from
consumption by animals and
decomposition by microorganisms.
Over time chemically altered plant
materials collected at the bottom
of pools of water in the swamp.
This carbon rich material is peat. It
is used as fuel in many places.
Under pressure and high
temperatures peat will eventually
be converted to coal.
Coal contains impurities which
leave an ash when it is burned.
One of the impurities is sulfur,
which produces a pollutant, sulfur
dioxide, a contributor to acid rain.
Coal
Moving Water
• Used as a source of energy for thousands of years.
• Considered a renewable energy source, inexhaustible as
long as rain falls.
• Today hydroelectric plants generate ~3 % of the nation’s
total energy consumption at about 2,400 power
generating dams across the nation.
• In 1940 hydropower furnished ~40% of the US electric
power. Today ~9%. It is projected to drop to ~7% in the
future.
• Geography limits the number of sites that can be built.
• Water from reservoir is conducted through large pipes
called penstocks to a powerhouse, where it is directed
against turbine blades that turn the shaft on an electric
generator.
Nuclear
• Energy is released as the nucleus of
uranium and plutonium atoms split or
undergo fission. This takes place in a
reactor, a large steel vessel. Water is
pumped through the reactor to produce
steam, which is used to produce electrical
energy.
• Radioactivity is a danger associated with
nuclear power plants.
Energy Usage
• To provide 1 MW(1 million watts or 1
thousand kilowatts), which supplies the
electrical needs of 1,000 people for 1 hour,
you would need:
• 1000 lbs of coal
• 80 gallons of oil
• 9000 cubic ft of gas
• .13 g uranium
Pollution
• Byproducts of burning petroleum products, coal, and
natural gas are pollutants such as carbon monoxide
(poisonous), excessive amounts of carbon dioxide and
water vapor (lead to global warming), and acid rain
caused by sulfur and nitrogen oxides and also by carbon
dioxide from exhausts from engines.
• Byproducts of nuclear power plants are the dangers of
nuclear radiation escaping and the disposal of the
nuclear waste.
• Hydropower, solar power, and wind power don’t produce
pollution but have the disadvantage of either not being
readily available, such as hydropower and wind power,
or not being very efficient (all three).
Energy Sources Tomorrow
• Solar Energy:
• Solar Cells-A thin crystal of silicon, gallium,or some
polycrystalline compound that generates electricity when
exposed to light. They have no moving parts and
produce electricity directly, without the need for hot fluids
or intermediate conversion states. Used in space
vehicles and satellites. On earth they are limited
because of the manufacturing cost. Used in watches and
calculators.
• Passive Application: Energy flows by natural means
without mechanical devices such as motors or pumps.
Solar energy is captured, stored, and distributed
throughout a house.
Energy Sources Tomorrow
• Solar Energy:
• Active application- Requires a solar collector in which
sunlight heats water, air, or some liquid. The liquid or air
is pumped through pipes in a house to generate
electricity or used directly for hot water.
• Power tower-Heliostats (special mirrors) surround a
tower and focus sunlight on a boiler at the top of the
tower. A mixture of salts, potassium nitrate and sodium
nitrate, will be heated to about 566oC and melted. It wil
then be pumped to a steam generator just like other
power plants. Water could be heated directly in the
power tower boiler. Molten salt is used because it can be
stored in an insulated storage tank for use when the sun
is not shining.
Energy Sources Tomorrow
• Wind Energy-Has been used for centuries to move
ships, grind grain into flour, and pump water. Wind
turbines are used to generate electrical or mechanical
energy. The problem is the inconsistency of wind.
• Biomass-Any material formed by photosynthesis,
including small plants, trees, and crops, and any
garbage, crop residue or animal waste. It can be burned
directly as a fuel, converted into a gas fuel (methane), or
converted into liquid fuels such as alcohol. The problems
include the energy expended in gathering the biomass
and to convert it to a gaseous or liquid fuel.
Energy Sources Tomorrow
• Geothermal energy-Energy from beneath the
Earth’s surface.
• Geysers, hot springs and venting steam such as
Yellowstone Park are clues that this form of
energy exists.
• The problem is getting to the geothermal energy
(getting it to the surface) and using it in a way
that is economically attractive.
• It is currently used to a certain extent and will
very likely be exploited much more in the future.
Exercises Chapter 3
• Applying Concepts p. 81-82
# 2, 3, 4, 12, 13, 14, 15, 16, 17, 18, 19, 20
• Parallel Concepts Group A p. 82-83
# 1, 2, 3, 4, 7, 8, 9, 10, 11, 12
New Book: p. 87-89 # 1, 2, 3, 7, 9, 10, 11,
12, 13, 14, 16, 18, 19, 20, 21, 22, 23, 24,
25, 26, 27, 29, 30, 32, 34, 36.
p. 89-90 Group A: #1, 2, 3, 4, 7, 8, 9, 10, 11,
12.
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Review for Chapter 3
Kinetic Energy and Potential
Energy-What they are and the
formulas.
Relationship between Potential
Energy and Work. Potential
energy of an object is equal to its
ability to do work.
Formulas for Work, Energy,
Potential Energy, Kinetic Energy
The joule-SI unit of energy and
work.
Forms of energy: mechanical,
chemical, radiant, electrical and
nuclear.
Photosynthesis (carbon dioxide +
water+ energy=glucose plus
oxygen.)
Burning (glucose +
oxygen=carbon dioxide + water +
energy).
•
•
•
•
•
•
•
•
Interconversion of Energy-Any
energy form can be converted to
any other energy form.
Flow of Energy (From the sun to
plants to animals and humans to
mechanical energy and back to
the atmosphere).
Conservation of Energy
History of Energy Sources: Initially
wood was used, then coal, then
petroleum.
Energy Sources Today: Chemical
(Petroleum>natural
gas>coal>biomass)>
Nuclear>Hydropower>Solar.
What is petroleum, natural gas
and coal and what are the uses of
petroleum.
What is hydropower, solar power
and nuclear power.
Pollution from chemical energy
and dangers from nuclear energy.