Download Energy PPT

Energy
Copyright © 2008 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
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.
2
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.
3
Storage Mechanisms …
are ways in which energy is stored in objects.
Storage mechanisms fall into one of three
broad categories:
¢ Potential Energy
¢ Kinetic Energy, Ek
¢ Thermal Energy, Eth
4
Potential Energy …
is energy stored in the structural composition
of an object or the position of an object.
Potential energy mechanisms:
¢ Gravitational Potential Energy, Eg
¢ Elastic Potential Energy, Eel
¢ Chemical Energy, Ech
¢ Interaction Energy, Ei
5
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 at the bottom of a dam
6
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
7
Chemical Energy …
is stored in the chemical bonds of particles.
Examples:
¢ the chemical bonds in a peanut butter and
jelly sandwich
¢ the chemical bonds in coal
¢ the chemical bonds in the
anodes and cathodes of a
battery
8
Interaction Energy …
is stored in the energy that keeps particles
separated from each other.
Example:
Steam has a higher interaction energy than either
water or ice because the water particles are kept
farther apart when they are in the gas phase and
closer together when they are in the liquid and
solid phases.
9
Learning Check
What are the four mechanisms for storing
potential energy?
10
Learning Check
What are the four mechanisms for storing
potential energy?
¢  Gravitational potential energy
¢  Elastic potential energy
¢  Chemical energy
¢  Interaction energy
11
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
12
Thermal Energy …
is energy stored in the random motion of
particles (aka atoms and molecules).
Examples:
¢ the random motion of the hot gas particles
in the chimney of a fireplace
¢ random motion of particles that make up
hot water
¢ the random vibration of particles in ice
(even cold objects have thermal energy!)
13
Internal Energy …
is the sum of all the energy stored in an object:
gravitational potential energy
+ elastic potential energy
+ chemical energy
+ interaction energy
+ kinetic energy
+ thermal energy
= internal energy
Internal energy is represented by the symbol “U”
U = Ek + Eth + Eg+ Eel + Ech + Ei
14
Learning Check
What are the three broad categories of energy
storage mechanisms?
What is the name for the total energy stored in
an object?
15
Learning Check
What are the three broad categories of energy
storage mechanisms?
¢  Potential energy
¢  Kinetic energy
¢  Thermal energy
What is the name for the total energy stored in
an object? Internal energy
16
Energy Transfer Mechanisms …
transfer energy
¢  from one type of storage mechanism to
another type of storage mechanism within the
same object, or
¢  from one object to another object
The three transfer mechanisms are …
¢  work, W
¢  radiation, R
¢  heat, Q
17
Work …
transfers energy by pushing, pulling, lifting, falling,
expanding or contracting.
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 and decreases the
chemical potential energy of the person.
¢  Hot gasses perform work to expand the piston in
a car’s engine. This decreases the chemical
potential energy and thermal energy of the gas
and increases the kinetic energy of the car.
18
Radiation …
transfers energy stored in one object to energy stored
in another object through waves known as
electromagnetic radiation.
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 particles 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 particles of your skin.
19
Heat …
transfers thermal energy, interaction energy and/or
chemical potential energy stored in one object to
thermal energy, interaction energy and/or chemical
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 interaction 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 energy.
20
Heat can transfer energy between
objects by …
¢  Conduction when two objects
are in direct contact with each
other. Example, touching a pot
of boiling water.
¢  Convection when the hotter
object warms the air around it
which in turn warms the colder
object. Example, warming your
hands above a fire.
¢  Thermal energy always flows
from the hotter object to the
colder object.
21
Learning Check
What are the 3 transfer mechanisms that transfer
energy between objects?
What transfer mechanism transfers energy by
pulling, pushing, lifting, falling, expanding or
contracting?
What transfer mechanism transfers energy
through waves of energy?
What transfer mechanism transfers energy by
convection and conduction?
22
Learning Check
What are the 3 transfer mechanisms that transfer
energy between objects? work, radiation & heat
What transfer mechanism transfers energy by
pulling, pushing, lifting, falling, expanding or
contracting? work
What transfer mechanism transfers energy
through waves of energy? radiation
What transfer mechanism transfers energy by
convection and conduction? heat
23
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.
24
The Law of Conservation of Energy
Since energy can neither be created nor destroyed,
Amt of energy
lost from
one object
Wood loses
energy stored
as chemical energy
25
=
heat
transfers
energy
Amt of energy
is gained by
another object(s)
Surroundings gain
energy stored
as thermal energy
The Law of Conservation of Energy
changes in the
kinetic energy
thermal energy
gravitational PE
elastic PE
chemical energy
interactive energy
of an object
26
equal
=
the sum of
work
+
radiation
+
heat
added or removed
from the object
The Law of Conservation of Energy
The conservation of energy equation
ΔU = ΔEk + ΔEth + ΔEg+ ΔEel + ΔEch + ΔEi = W + Q + R
Sign Convention
Negative numbers = Energy transferred out
Positive numbers = Energy transferred in
27
Systems
When investigating 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 the
beaker of water and the Bunsen burner
or it could be just the beaker of water.
28
Some Simplifications for Chemistry …
In chemistry we will focus on …
¢ chemical energy
Storage
¢ interaction energy
mechanisms
¢ thermal energy
¢ heat
Transfer mechanism
So …
The change in a system’s thermal, interactive and
chemical energies is equal to the energy heat
transfers in or out of the system.
ΔU = ΔEth + ΔEch + ΔEi = Q
29
Learning Check
What does the Law of Conservation of Energy
mean?
Why is it important to define the system when
investigating or analyzing energy?
30
Learning Check
What does the Law of Conservation of Energy
mean?
Energy is neither created nor destroyed so the
amount of energy lost from one object is gained
by another object(s).
Why is it important to define the system when
investigating or analyzing energy?
We must define the system so we can accurately
track energy changes into and out of the system.
31
Learning Check
What storage mechanisms do we focus on in
chemistry?
How are a system’s total stored energy and the
energy transferred into and out of the system
related?
32
Learning Check
What storage mechanisms do we focus on in
chemistry?
thermal energy, chemical energy and interactive
energy
How are a system’s total stored energy and the
energy transferred into and out of the system
related?
The change in a system’s total stored energy is
equal to the energy transferred into or out of the
system.
33
Tracking Energy Changes
We will use column graphs and arrows to track the flow of
energy between objects. Here are the key concepts to follow:
¢  Identify the storage mechanism(s) in each object that gain
and lose energy during the change.
¢  Identify the transfer mechanism(s) that transfer the energy
between the objects.
¢  Determine which object gains energy and which one looses
energy.
¢  Use column graphs with labels to show the units of energy
that are lost or gained by each storage mechanism.
¢  Use arrows with arrows to show the units of energy being
transferred.
¢  Let’s look at the example on the next slide.
34
Tracking Energy Changes
¢ 
Lets study how energy flows when an ice cube is put in a
glass of room temperature water.
¢ 
Energy flows from hot to cold, so energy stored as Eth is
transferred via the heat transfer mechanism, Q, to the ice
cube. The ice cube stores this energy as Eth because its
temperature increases and as Ei because it changes from a
solid to a liquid. Ei does not change for the water because it
started as a liquid and remains a liquid.
Energy is conserved because the energy the warm water
lost is equal to the energy the ice cube water gained.
¢ 
35
Tracking Energy Changes
¢ 
Lets study how energy flows when a student lifts their
backpack from the floor to the desk.
¢ 
The student uses his or her muscles to lift the backpack, so
they are using the energy stored in their muscles. Therefore,
the energy in the Ech mechanism decreases. The energy
flows from the student to the box via the Work transfer
mechanism. The position of the box is higher when it is
sitting on the desk, so the energy coming into the box is
stored in the Eg mechanism.
Energy is conserved because the amount of energy the
person lost is equal to the amount of energy the box gained.
¢ 
36