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 = F x d

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 = m x g• Energy is force x distance.• E = F x d• For a falling object d=h (h=height)• E = F x h• PE= m x g x h

• 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

Potential Energy and Weight

• Weight = mass x acceleration due to gravity

w = m x g• Work = weight x

height

W = w x h

PE = w x h

PE = m x g x h

Fig. 3.3

Units for Energy

W = F x d

W = Kg x m / s2 x m

= N x m

= 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

JKE

NxmKE

mkgm

KE

kgKE

s

mkgKE

mKE

s

sm

v

))((

))((

2

1

2

2

2

2

2

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.

Fig. 3.11

Chemical Energy

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/photosynthesis/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 absorbed.

• Microwave radiation is used in cooking.

Increases

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 Mechanical Electrical or Nuclear

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% Natural gas ~23% Coal ~21% Biomass ~3% (Material from Photosynthesis)

This equates to ~89% of all energy consumed.About 1/3 of this energy was burned for heating

and the rest was burned to drive engines or generators.

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• 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.

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, 12New 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.

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.