Transcript Energy

Chapter 9
Energy
Section 1: Objectives
•
Explain the relationship between
energy and work.
•
Compare kinetic energy and potential
energy.
•
Describe the different forms of
energy.
Energy and Work
•
Energy is the ability to do work.
•
Work is done when a force causes an
object to move in the direction of the
force.
• Work is a transfer of energy.
•
Energy and work are expressed in
units of joules (J).
Energy and Work
•
Kinetic energy is the energy of
motion.
• All moving objects have kinetic energy.
•
Kinetic Energy Depends on Mass
and Speed If you know an object’s
mass (m) and its speed (v), you can
calculate the object’s kinetic energy
with the following equation:
Energy and Work

The formula used to calculate kinetic
energy:
KE = (mass)(volume)^2
2
Energy is also expressed in Joules.
2
Calculating Kinetic Energy
Example # 1

What is the KE of a car that has a mass of
2,400 kg and is moving at 20 m/s?
Remember: KE = (mass)(volume)^2
2
Energy is expressed in Joules
Example # 2

A) What is the KE of a 4,000 kg elephant
that is running at 2 m/s?

B) What about the same elephant running
at 4 m/s?
Example # 3

A)What is the KE of a 2,000 kg bus that is
moving at 30 m/s?

B) What about the same bus moving at 40
m/s?
Energy and Work
•
Potential energy is the energy an
object has because of its position.
•
Gravitational Potential Energy The
amount of GPE that an object has
depends on its weight and its height.
Energy and Work
•
The equation to find gravitational
potential energy is:

•
GPE weight  height
Gravitational potential energy is equal
to the amount of work done on an
object to lift it a certain height.
Calculating GPE
Example # 1

What is the GPE of a cat that weighs 40
N and is standing on a table 0.8 meters
off the ground?
Remember: GPE weight  height
 Energy is expressed in Joules.

Example # 2

What is the GPE of a diver who weighs
500 N and is standing on a platform 10
meters off the ground?
Remember: GPE weight  height
 Energy is expressed in Joules.

Example # 3

What is the GPE of a diver who weighs
600 N and is standing on a platform that
is 8 meters off the ground?
Remember: GPE weight  height
 Energy is expressed in Joules.

Energy and Work

When you find out an object’s
gravitational potential energy, the
“ground” that you measure the object’s
height from depends on where it is.

The height you use in calculating
gravitational potential energy is a
measure of how far an object has to
fall.
Energy and Work
•
Mechanical energy is the total energy of
motion and position of an object.
• Both kinetic energy and potential energy are
kinds of mechanical energy.
•
The equation to find mechanical energy
is:

mechanical energy  potential energy 
kinetic energy
Energy and Work
•
The mechanical energy of an object
remains the same unless it transfers
some energy to another object.
•
But even if the mechanical energy of
an object stays the same, the potential
energy or kinetic energy can increase
or decrease.
Energy and Work
•
Thermal Energy is all of the kinetic
energy due to random motion of the
particles that make up an object.
•
All matter is made up of particles that
are always in random motion.
• So, all matter has thermal energy.
•
Thermal energy increases as
temperature increases and increases as
the number of particles increases.
Thermal Energy
Energy and Work
•
Chemical Energy is the energy of a
chemical compound that changes as its
atoms are rearranged.
•
Chemical energy is a form of potential
energy because it depends on the
position and arrangement of the atoms in
a compound.
•
The energy in food is chemical energy.
Energy and Work
•
Electrical Energy is the energy of
moving electrons.
• Electrical energy can be thought of as
potential energy that is used when you plug
in an electrical appliance and use it.
•
Sound Energy is caused by an object’s
vibrations.
• The object’s vibrations transmit some kinetic
energy to the air particles, which also vibrate.
These vibrations transmit sound energy.
Energy and Work
•
Light Energy is produced by the
vibrations of electrically charged
particles.
•
Nuclear Energy is energy that comes
from changes in the nucleus of an atom.
• Nuclear energy can be produced when nuclei
are joined in a fusion reaction or when a
nucleus is split apart in a fission reaction.
Chapter 9 Sec. 1 Pop Quiz
1) What 2 things must happen for work
to be done on an object?
 2) T/F All moving objects have kinetic
energy.
 3) What is the formula used to calculate
KE?
 4) What 2 things does GPE depend on?
 5) What is the formula used to calculate
GPE?

Chapter 9 Sec. 1 Pop Quiz
6) List 2 kinds of mechanical energy.
 7) What is the formula used to calculate
mechanical energy?
 8) Describe the particles that make up
matter.
 9) Why is chemical energy a form of
potential energy?
 10) List 2 ways nuclear energy can be
produced.

Section 2: Objectives
•
Describe an energy conversion.
•
Give examples of energy conversions
for the different forms of energy.
•
Explain how energy conversions make
energy useful.
•
Explain the role of machines in energy
conversions.
Energy Conversions
•
An energy conversion is a change from
one form of energy to another.
•
Any form of energy can change into any
other form of energy.
•
As the skateboarder on the next slide
travels up and down the half-pipe, his
energy changes back and forth between
kinetic energy and potential energy.
Energy Conversions
Energy Conversions
•
Elastic Potential Energy Stretching
a rubber band stores elastic potential
energy in the rubber band.
•
When you let the rubber band go, it
goes back to its original shape.
•
It releases its stored-up potential
energy as it does so.
Energy Conversions
•
Chemical energy is stored in the food you
eat.
•
Your body uses this chemical energy to
function.
•
Energy Conversion in Plants The
chemical energy in the food you eat comes
from the sun’s energy.
•
Plants use photosynthesis to convert light
energy into chemical energy.
Energy Conversions
Energy Conversions
•
Plants change light energy into chemical
energy.
•
The chemical energy in the food you eat
is changed into another kind of chemical
energy that your body can use.
•
Your body then uses that energy to give
you kinetic energy that you use in
everything you do.
Energy Conversions
•
Energy conversions are needed for
everything we do.
•
Heating our homes, getting energy
from a meal, and many other things
use energy conversions.
•
Machines help harness energy and
make that energy work for you.
Energy Conversions

Conversions Involving Electrical Energy
Some common energy conversions that involve
electrical energy are shown in the table below.

Alarm clock
electrical energy  light and sound energy
Battery
chemical energy  electrical energy
Light bulb
electrical energy  light and thermal energy
Blender
electrical energy  kinetic and sound energy







Energy Conversions
•
A machine can make work easier by
changing the size or direction (or both) of
the force needed to do the work.
•
Some machines allow you to use less
force over a greater distance to do the
same amount of work.
•
Machines as Energy Converters
Some machines help you use energy by
converting it into the form of energy that
you need.
Energy Conversions