Transcript Monday (A Day) November 26, 2012

ENERGY KEY IDEAS
〉What is the relationship between energy and
work?
〉Why is potential energy called energy of
position?
〉What factors does kinetic energy depend on?
〉What is nonmechanical energy?
ENERGY OVERVIEW
Whenever work is done,
energy is transformed
or is transferred from
one system to another
system.
 Energy: the capacity
to do work
 Energy is measured in
joules (J).
 Energy
 Potential Energy
 Kinetic Energy
 Other Forms of Energy





Mechanical
Chemical
Nuclear
Electrical
Light
FORMS OF ENERGY
〉Potential energy (PE)
is sometimes called
energy of position
because it results
from the relative
positions of objects in
a system.
 Potential Energy: the
energy that an object has
because of the position,
shape, or condition of the
object
FORMS OF ENERGY
〉Kinetic energy
depends on both the
mass and the speed
of an object.
 Kinetic Energy: the
energy of an object due
to the object’s motion
 KE = ½  mass  speed
squared, or KE= ½mv 2
FORMS OF ENERGY
 Mechanical Energy: the
amount of work an
object can do because
of the object’s kinetic
and potential energies
 In most cases,
nonmechanical forms
of energy are just
special forms of either
kinetic or potential
energy.
FORMS OF ENERGY
 Chemical reactions involve
potential energy.
 The amount of chemical
energy associated with a
substance depends in part
on the relative positions of
the atoms it contains.
 Living things get energy
from the sun.
 Plants use photosynthesis
to turn the energy in
sunlight into chemical
energy.
 The sun gets energy from
nuclear reactions.
 The sun is fueled by
nuclear fusion reactions in
its core.
FORMS OF ENERGY
 Energy can be stored in
fields.
 Electrical energy results
from the location of
charged particles in an
electric field.
 When electrons move
from an area of higher
electric potential to an
area of lower electric
potential, they gain
energy.
FORMS OF ENERGY
 Light can carry energy
across empty space.
 Light energy travels from
the sun to Earth across
empty space in the form
of electromagnetic
waves.
 Electromagnetic waves
are made of electric and
magnetic fields, so light
energy is another
example of energy
stored in a field.
PRACTICE
You should already have learned
that energy is always conser ved.
Instead of being created or
destroyed, energy just changes from
one form to another. For example,
sunlight is the ultimate source of
energy on Ear th. Look at the
illustration below, and identify the
types of energy involved.
 1 . H ow do e s s un l ight prov i de t h e
e n e rg y t h e g i rl n e e ds to s w i ng t h e
ba t ? ( H in t: Wh a t do yo u n e e d to
h ave e n e rg y ?)
 2 . Wh e n t h e g i rl h i t s t h e ba l l , s h e
exe r t s a fo rc e o n i t . D o e s s h e do
wo rk o n t h e ba l l i n t h e s c i e nt ific
s e n se o f t h e te rm ? E x pl a in yo ur
a n s we r.
 3 . Af te r t h e g i rl h i t s t h e ba l l, t h e
ba l l m oves ve r y fa s t a n d h a s
e n e rg y. W h e n t h e ba l l h i t s t h e
fi e l der’ s g l ove, i t s to ps m ov ing.
G i ven t h a t e n e rgy c a n n ever be
de s t roye d but m e re ly c h a n ges fo rm ,
w h a t h a ppe n s to t h e e n e rg y t h e
ba l l o n c e h a d ? ( H in t: If yo u a re t h e
fi e l der, w h a t do yo u h e a r a n d fe e l
a s yo u c a t c h t h e ba l l ?)
DISCUSS WITH YOUR PARTNER
1 . When does the sled have the
most potential energy? When
does it have the least
potential energy?
2. Where does the sled have the
most kinetic energy and the
least kinetic energy?
3. What happens to the relative
amounts of potential and
kinetic energy as the sled
slides down the hill? What
happens to the total energy?
4. Af ter the sled reaches the
bottom of the hill, it coasts
across level ground and
eventually stops. What
happened to the energy the
sled had?
CONSERVATION OF ENERGY
KEY IDEAS
〉How does energy change?
〉What is the law of conservation of energy?
〉How much of the work done by a machine is actually useful
work?
LAW OF CONSERVATION OF ENERGY
 Energy cannot be created
or destroyed. In other
words, the total amount of
energy in the universe
never changes, although
energy may change from
one form to another.
 Energy does not appear or
disappear.
 Whenever the total energy
in a system increases, it
must be due to energy that
enters the system from an
external source.
ENERGY IN SYSTEMS
 Systems may be open,
closed, or isolated.
•open system: energy and
matter are exchanged with
the surroundings
•closed system: energy but
not matter is exchanged
•isolated system: neither
energy nor matter is
exchanged
 Most real-world
systems are open.
EFFICIENCY OF MACHINES
〉How much of the work done
by a machine is actually
useful work?
〉Only a portion of the work
done by any machine is
useful work— that is, work
that the machine is
designed or intended to do .
 Not all of the work done by a
machine is useful work.
 because of friction, work
output < work input
 Efficiency is the ratio of useful
work out to work in.
PRACTICE
State the Law of Conservation of Energy
in your own words. Give an example of a
situation you have encountered in which
this law was demonstrated.