Unit 9: Energy, Work, and Power

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Transcript Unit 9: Energy, Work, and Power

Unit 9: Energy, Work, and Power
Forms of Energy and Energy Transformations
PS-6.1: Explain how the law of conservation of energy
applies to the transformation of various forms of energy
(including mechanical energy, electrical energy, chemical
energy, light energy, sound energy, and thermal energy).
PS-6.2: Explain the factors that determine potential and
kinetic energy and the transformation of one to the other.
Differentiate between the different forms of energy:
mechanical, electrical, chemical, light, sound, and thermal
Use diagrams to explain energy transformations from
one form to another.
Use models to show transformations between PE and
KE and conservation of total mechanical energy.
What is energy?
◦ Ability or capacity to do work.
◦ Energy is never created or destroyed.
 Law of Conservation of Energy
◦ Energy can be changed from one form to another
◦ Total amount of energy in a system is CONSTANT—it stays the
How is energy being
used by the man riding
his bike?
What are the different forms of energy?
Energy can be present in different forms of energy or a
combination of forms.
◦ Potential
◦ Kinetic
◦ Sum of potential + kinetic energy
Mechanical Energy
Mechanical energy is energy due to the position of something or
the movement of something.
Mechanical energy can be potential kinetic or the sum of the two.
Potential energy (PE): stored energy
Kinetic energy (KE): energy of motion
How do these pictures
show mechanical
Potential Energy
Potential energy is stored energy
3 types of PE
◦ Elastic Potential Energy
 Stored energy when a material is stretched or compressed
 Rubber Band
◦ Chemical Potential Energy
 Energy stored in the bonds between atoms
 Energy stored in food
 Energy stored in batteries
◦ Gravitational Potential Energy
 Stored energy due to position of an object
 Height above the ground
 Weight Lifting
Total Mechanical Energy
PE can be converted to KE and back to PE
 Watch the animation and explain the
transformation of energy.
What is happening to TME?
Efficiency of Energy Transformations
When PE is converted to KE, some of the energy is
changed to heat/thermal energy due to friction.
Energy transformations are NOT 100% efficient.
Check for Understanding
The animation shows a dart being fired from a spring.
A. What forms of energy are present?
B. What happens to elastic potential energy PEs when the dart is fired?
C. What happens to KE at first when the dart is fired?
D. When does PEg increase?
E. Explain why total mechanical energy TME does not change.
Chemical Energy
Chemical energy is a type of energy that is associated with atoms, ions,
and molecules and the chemical bonds that these particles form.
When a chemical reaction happens, chemical energy is changed to another
◦ Batteries store chemical energy
◦ Your body stores energy from food as chemical energy
◦ Photosynthesis
Plants use energy from the sun to make sugar. Energy is stored in the bonds of the sugar
Electrical Energy
Electrical energy deals with the
movement of electrons from an area of
high potential to low potential
Other Forms of Energy
◦ Heat
◦ Associated with movement of particles
◦ Friction can cause KE to be converted to thermal energy
◦ Energy associated with electromagnetic waves
◦ Light energy does NOT need a medium to travel
◦ Light can be transmitted through space
 Sun gives us light energy
 Sun is also a source of nuclear energy—nuclear fusion
◦ Energy associated with mechanical longitudinal waves
◦ Sound needs a medium to travel
◦ It will not be transferred through space
Check for Understanding
Which of the following could be used to convert
light energy to electrical energy?
A. Windmill
B. Chemical storage battery
C. Solar Cell
D. Rotating coils in a magnetic field
The stored energy in a battery can best be
described as ____.
A. Thermal
B. Chemical
C. Nuclear
D. Kinetic
Check for Understanding
What kind of energy does a windmill
use to do work?
a. Electrical
b. Nuclear
c. Chemical
d. Mechanical
Law of Conservation of Energy
Energy cannot be created or destroyed.
 Energy can only be transformed or
converted from one form to another
Examples of Energy Transformation
Conservation of Energy in a Flashlight
◦ Step 1: Battery has chemical potential energy.
◦ Step 2: Chemical energy changes to electric energy.
◦ Step 3: Electric energy flows through the light bulb
and turns electric energy to light and heat.
The total of the energy from the chemical
reaction (chemical PE in the battery) is equal to
the total energy that it transforms into
(electrical, light, and heat/thermal)
Examples of Energy Transformation
Conservation of energy when baseball is thrown to another ball
◦ Step 1: A ballplayer converts chemical energy from the food she has
eaten to mechanical energy when she moves her arm to throw the ball.
◦ Step 2: The work done on the ball converts the energy of the arm
movement to kinetic mechanical energy of the moving ball.
◦ Step 3: When the second player catches the ball, the ball does work
on the player’s hand and glove giving them some mechanical energy.
The ball also moves the molecules in the glove heating them up.
The player that catches the ball absorbs the energy of the ball and
this energy turns to heat.
The total heat produced is equal to the energy used to throw the
Your Turn . . .
Explain the energy transformations below.
Gravitational Potential Energy
Gravitational Potential
Energy GPE
◦ Energy of position
◦ Stored energy due to height
◦ GPE = mass x g x height
Factors that affect GPE
◦ Weight (mass x g)
 Large weight = greater GPE
◦ Height above a defined surface
 An object at a greater height has
more GPE
The boulder has
more gravitational
potential energy when
measured from point A
compared to B.
PE = mgh
Kinetic Energy
Energy of motion
◦ Greater when the speed is greater
◦ Greater when the mass is greater
KE = ½ mv2
 This means KE = mass x velocity2
Transformation of PE to KE
At maximum height, the ball has ALL
PE and no KE
As the ball rolls down the hill, some PE
is converted to KE
At the bottom of the hill the ball has
no PE and ALL KE
When the unbalanced force of friction
acts on the ball, the KE is converted to
thermal energy.
Total mechanical energy is conserved
PE at the top of the hill = KE at the
bottom of the hill
Kinetic and potential energy conversions
Describe the energy
conversions in this picture:
 At the top: All PE, no KE
 ¼ of the way down: 3/4 PE, 1/4 KE
 ½ way down: 1/2 PE, 1/2 KE
 ¾ down 1/4 PE, 3/4 KE
 At the bottom: No PE, All KE
 Is the sum of KE + PE a
constant? Yes! It’s always
10,000 J in this case.
Energy Transformation in a Pendulum
Energy Transformations in a Pendulum
When a pendulum swings it has mechanical energy. At the top of the
swing all of its mechanical energy is potential energy that depends on its
height and weight.
The kinetic energy is greatest at the bottom of the swing because its speed
is greatest.
Potential energy is zero at the bottom of the swing because the height is
Between the top of the swing and the bottom of the swing it has both
potential and kinetic energy because it has both height and movement.
Eventually the pendulum will stop. It stops because of friction.
The friction transforms the energy that was originally mechanical energy in
the swinging pendulum into heat.
Where are PE and KE maximums in
this picture?
Check for Understanding
What factors affect GPE???
What factors affect KE???
If you are on a swing, where would your speed be the
greatest? WHY?