Download Heat and Energy

Energy
Copyright © 2008 by Pearson Education, Inc.
Publishing as Benjamin Cummings
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Energy
is a substance like quantity that can cause
change.
• Makes objects move.
• Makes things stop.
• Is needed to “do work”.
• It does NOT have mass or volume.
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Energy …
 is stored in objects using one or more
storage mechanisms.
 flows from one storage mechanism to
another storage mechanism within the
same object.
 flows from one object to another object.
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Storage Mechanisms …
are ways in which energy is stored in objects.
Storage mechanisms fall into one of three
broad categories:
 Potential Energy
 Kinetic Energy
 Thermal Energy
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Potential Energy …
is energy stored in the structural composition
of an object or the position of an object.
Potential energy mechanisms:
 Gravitational Potential Energy
 Elastic Potential Energy
 Chemical Energy
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Gravitational Potential Energy …
is stored in an object that is positioned above
another object that exerts a gravitational pull
on it.
Examples:
 a boulder positioned above the earth
 a roller coaster at the top of a hill
 water stored in a reservoir above a
turbine
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Elastic Potential Energy …
is stored in an object that is compressed or
stretched.
Examples:
 the twisted rubber band in a toy airplane
 an archer’s bow
 a stretched bungee cord
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Chemical Potential Energy …
is stored in the chemical bonds of molecules
and the attractive forces between molecules.
Examples:
 coal burned in a power generating plant
 steam created in a power generating plant
 the chemicals that make up
the anode and cathode in a
battery
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Learning Check
What are the three mechanisms for storing
potential energy?
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Kinetic Energy …
is energy stored in the purposeful movement
of an object.
Examples:
 a bolder falling off a cliff to the earth below
 a roller coaster going down its tracks
 water moving past a turbine blade
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Thermal Energy …
is energy stored in the random motion of particles
(aka atoms and molecules).
Examples:
 the random motion of hot gas particles going up
a fireplace chimney.
 the vibrating motion of the particles that make
up the brakes that are slowing down a car.
 the vibrating motion the particles that make up
ice (even cold objects have thermal energy!)
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Learning Check
What are the three broad categories of energy
storage mechanisms?
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Energy Transfer Mechanisms …
transfer energy from
 one type of storage mechanism to another
type of storage mechanism within the same
object, or
 one object to another object
The three transfer mechanisms are …
 work
 heat
 radiation
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Work …
transfers energy stored in one object as kinetic
energy to energy stored in another object as kinetic,
gravitational potential and/or elastic potential
energy.
Examples
 A person performs work to lift a box from the floor
to the top shelf. This increases the gravitational
potential energy of the box.
 A person performs work to throw a ball in the air.
This transfers the kinetic energy stored in the
person’s hand to kinetic and gravitational
potential energies stored in the ball.
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Radiation …
transfers thermal energy stored in one object to
thermal energy stored in another object.
Examples
 When you feel the sun’s warmth on your face.
Radiation is transferring thermal energy stored in
the sun to thermal energy stored in the molecules
of your skin.
 When you put your hand a few inches away from a
light bulb. Radiation is transferring thermal energy
stored in the light bulb to thermal energy stored in
the molecules of your skin.
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Heat …
transfers thermal energy or chemical potential energy
stored in one object to thermal energy or potential
energy stored in another object.
Examples
 When you use the stove to boil a pot of water.
Heat is transferring thermal energy stored in the
hot burner to energy that is stored in the water as
thermal energy and chemical potential energy.
 When you use a chemical ice pack to stop the
swelling of a sports injury. Heat is transferring the
thermal energy stored in your body to energy that
is stored in the chemicals as chemical potential
energy.
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Learning Check
What are the 3 transfer mechanisms that transfer
energy between objects?
What transfer mechanism transfers energy stored
as kinetic energy in one object to kinetic,
gravitational potential and/or elastic potential
energies?
What 2 transfer mechanisms transfer energy
stored as thermal energy?
What transfer mechanism transfers energy stored
as thermal energy and chemical potential energy?
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The Law of Conservation of Energy
Energy can neither be created nor destroyed
during physical and chemical changes.
Nuclear changes are the exception to this
law and the law of conservation of matter.
During nuclear changes mass is changed
into energy.
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The Law of Conservation of Energy
When investigating and analyzing energy changes it
is essential to clearly define the system you are
investigating.
A system is simply the environment you are
studying. It could be a single object or a group of
objects.
In this picture the system could
be just the coiled spring or the
spring and the car. You need
to define your system before you
can analyze energy’s effects.
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The Law of Conservation of Energy
Since energy can neither be created nor destroyed, the
change in the system’s total stored energy is equal to
the energy transferred into or out of the system.
changes in
kinetic energy
thermal energy
gravitational PE
elastic PE
chemical PE
equal
the sum of
work
heat
radiation
added or removed
or ΔEk + ΔEth + ΔEg+ ΔEel + ΔEch = W + Q + R
Energy transferred out of a system is “-” and energy
transferred into a system is “+”
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Mechanical Energy …
is the sum of an object’s energy that is
stored as …
 gravitational potential energy
 elastic potential energy
 kinetic energy
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Internal Energy …
is the sum of all an object’s energy:
 gravitational potential energy
 elastic potential energy
 kinetic energy
 thermal energy
 chemical potential energy
Internal energy is represented by the symbol “U”
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Learning Check
What does the Law of Conservation of Energy
mean?
Why is it important to define the system when
investigating or analyzing energy?
What storage mechanisms comprise a system’s
internal energy?
How are a system’s internal energy and the
energy transferred into and out of the system
related?
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Recording Energy Observations …
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Some Simplifications for Mechanics …
For most of the year we will be studying mechanics.
 Chemical potential energy is not a factor.
 Thermal energy is a factor in real world examples that
take into account friction.
 Thermal energy is zero in ideal frictionless conditions.
 Heat and Radiation are not a factor in energy transfers.
So …
Real World
U = Ek + Eg + Eel + Eth
Frictionless
U = Ek + Eg + Eel
Energy Changes ΔU = W
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Learning Check
If an object’s kinetic energy increases, was
energy transferred into or out of the object?
What was the energy transfer mechanism?
If a rubber band’s elastic potential energy
decreases, was energy transferred into or out of
the rubber band?
What was the energy transfer mechanism?
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