Transcript Pearson Prentice Hall Physical Science: Concepts in Action

Pearson Prentice Hall
Physical Science: Concepts in Action
Chapter 15
Energy
15.1 Energy and Its Forms
• Objectives:
• 1. Describe and compare how energy
and work are related
• 2. Explain what factors kinetic energy
of an object depends on
• 3. Discuss how gravitational potential
energy is determined
• 4. Summarize the major forms of
energy
How Energy & Work are Related +
Kinetic Energy
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Def: energy is the ability to do work
Work is a transfer of energy
Def: kinetic energy is the energy of motion
The kinetic energy of any moving object depends
on its mass and speed
• The formula is: KE = ½ mv2 where m = mass and v is
the velocity (which must be squared)
• the units for m are kg & v = (m/s)2 or m2/s2
• the units for PE are kg*m2/s2 which is also called
joules, J
Potential Energy
• Def: potential energy is stored energy as a
result of position or shape
• PE is energy with the potential to do work
• Two forms of PE are gravitational PE and
elastic PE
• Def: gravitational PE is PE that depends
upon an object’s height
• Gravitational PE increases when an object is
at a higher height
• An object’s gravitational PE depends on its
mass, height & acceleration due to gravity
• The formula for gravitational PE = mgh where
m= mass (kg), g= 9.8 m/s2 (the free fall
acceleration of gravity) & h = height in meters,
m
• When you multiply all the units together you
get kg*9.8m/s2*m or kg*m2/s2 which is J
• Def: elastic PE is the PE of an object that is
stretched or compressed
• An object is elastic if it springs back after being
stretched
Practice Problems
• Calculate the KE of a 1500kg car moving
at 29m/s.
• A bowling ball traveling at 2.0m/s has 16J
of KE. What is the mass of the bowling
ball in kg?
Practice Problems
• Calculate the PE of a car with a mass of
1200kg at the top of a 42m hill.
• Calculate the PE of a 55g egg held out of
a 2nd story window, 6m off the ground.
Forms of Energy
• The major forms of energy are mechanical
energy, thermal energy, chemical energy,
electrical energy, electromagnetic energy
and nuclear energy
• Def: mechanical energy is the energy
associated with the motion and position
of everyday objects
• Def: thermal energy is the total PE and KE
of all the microscopic particles in an object
• Def: chemical energy is the energy
stored in chemical bonds
• Def: electrical energy is the energy
associated with electric charges
• Def: electromagnetic energy is a form
of energy that travels through space in
the form of waves
• Def: nuclear energy is the energy
stored in atomic nuclei
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15.2 Energy Conversion &
Conservation
Objectives:
1. Describe how energy can be converted
from one form to another
2. Explain the law of conservation of
energy
3. Discuss the energy conversion that
takes place as an object falls toward
Earth
4. Discuss how energy and mass are
related
Energy Can be Converted from One
Form to Another
• Def: energy conversion is the process of changing
energy from one form to another
• Sometimes energy is converted to other forms in
a series of steps
• Ex: striking a match uses chemical energy from
your muscles, then friction between match and
box converts KE to thermal energy, thermal
energy triggers a chemical reaction releasing
more chemical energy
• Often energy converts directly from one form to
another
• a wind up toy, for example, is PE to KE
Conservation of Energy + Energy
Conversions and Gravity
• The Law of Conservation of Energy states that
energy cannot be created or destroyed
• The gravitational PE of an object is converted to
the KE of motion as an object falls
• Pendulums constantly convert PE to KE and KE to
PE as the pendulum swings
• At the bottom of the swing, the pendulum has
maximum KE and zero PE
• On either side the pendulum will have a
combination of PE + KE
• Q: Where is the PE the greatest and KE zero?
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Energy and Mass
Mechanical energy = KE + PE
Mechanical energy is also conserved
(KE + PE)beginning = (KE + PE) end
Einstein has an equation: E = mc2 where E is energy
(J), m is mass (kg) & c2 is the speed of light squared
(3 x 108 m/s)2
• This equation says that energy and mass are
equivalent and can be converted into each other
• It also means that a tiny amount of matter can
produce enormous amounts of energy
• Mass and energy together are always conserved
15.3 Energy Resources
• Objectives:
• 1. Give examples of the major
nonrenewable and renewable
energy sources
• 2. Explain how energy
resources can be conserved
Nonrenewable and Renewable
• Nonrenewable energy resources
include oil, natural gas, coal and
uranium
• Oil, natural gas and coal are fossil fuels
• Fossil fuels were formed underground
from once living organisms
• Most nonrenewable resources are
considered inexpensive, available and
are known to cause pollution
• Renewable energy resources include
hydroelectric, solar, geothermal, wind, and
biomass
• Def: hydroelectric energy is energy obtained from
flowing water
• Hydroelectric energy production usually involves
the building of a dam
• Hydroelectric is available, used today and
generally nonpolluting
• Def: solar energy is sunlight converted to usable
energy
• Solar energy is expensive and its use is limited
until technology improves
• Def: geothermal energy is thermal energy
beneath the earth’s surface
• It is nonpolluting and available in this area due to
naturally occurring hot springs
• most places are not near a volcano or hot springs
• Def: biomass energy is the energy stored in living
things
• Biomass can be converted directly to thermal
energy
• Agricultural waste such as turning corn into
ethanol for auto fuel is an example
• This technology is moderately expensive
• Wind energy requires a lot of land and a
place that has a lot of wind
• It is expensive and not practical at this time
although research continues
• Hydrogen fuel cells are being used in some
places to generate electricity by reacting
hydrogen with oxygen
• The main source of hydrogen is water
• This technology is expensive and considered
to be a research and development project
for future energy sources
Conservation of Resources
• Energy resources can be conserved by
reducing energy needs and by
increasing the efficiency of energy use
• Def: energy conservation is finding
ways to use less energy or use energy
more efficiently
• Q: Can you think of some ways we can
conserve energy resources?