Transcript (potential) energy

Energy and Work
0 Energy can be defined as: the ability to
do work
0 Most of the time we can’t see energy
but it is everywhere around us.
0 Energy can never be created or
destroyed…. It can only be STORED or
TRANSFERRED
0 SI unit for energy is: Joules
Energy and Work
0 All energy is either Potential Energy or
Kinetic Energy
All Energy
Potential
Energy
Kinetic
Energy
Potential Energy
0 PE is the energy stored in an object
0 “Potential” means the energy has the
ability to do something useful later
on.
Examples of Potential
Energy
0 A stretched rubber band.
0 Water at the top of a waterfall.
0 A battery.
0 A drawn bow and arrow.
Potential Energy
0 Elastic Potential Energy: the energy
stored in any type of stretched or
compressed elastic material.
Potential Energy
0 This is an example
of Potential
Energy: there is
stored energy in
the elastic bands
which will convert
to kinetic energy
when it is released.
How are these pictures
examples of potential energy?
Gravitational Potential
Energy
0
Gravitational Potential Energy:
energy stored in objects separated
by a distance; results because of
gravitational attraction between
objects.
0 Depends on 2 factors:
0 mass
0 height
Gravitational Potential
Energy
0 The higher an object and the more
massive, the more gravitational PE it has.
0 Gravitational Potential Energy Equation:
PE = mgh
0 grav. PE = mass x free fall acceleration x height
Gravitational Potential
Energy
0 A flower pot with a mass of 15 kg is
sitting on a window sill 10 m above the
ground. How much potential energy
does the flower pot contain?
0 PEgrav = mgh
0 PEgrav = (15 kg)·(10 m)·(9.8 m/s2)
0 PEgrav= 1500 J
Potential Energy
Potential Energy
Potential Energy
& Chemical
Reactions
0 Chemical Energy: energy stored
within atoms and molecules that can
be released when a substance reacts.
Potential Energy
& Chemical
Reactions
0 Chemical reactions involve PE. Why?
When a chemical reaction takes place, bonds
between atoms break apart and a new
substance is formed, which involves changes
in energy from the relative positions of
atoms in the substance.
0 If a reaction releases energy, there is a
decrease in PE. Example: when a match is
struck.
Kinetic Energy
0
KE is the energy of moving objects
due to their motion
0 Depends on 2 factors:
0 mass
0 Speed
0 “Kinetic” means movement
Kinetic Energy
0Kinetic Energy Equation:
KE = ½ mv2
0KE = ½ x mass x speed squared
Kinetic Energy
0 Example: How much kinetic energy does
a bicycle with a mass of 14 kg traveling
at a velocity of 3.0 m/s east have?
0 KE = ½ mv2
0 KE = ½ (14 kg)·(3.0 m/s)2
0 KE = ½ (14 kg)·(9.0 m2/s2)
0 KE = 63 J
Examples of Kinetic Energy
Kinetic Energy
0 Scientific proof for
why car crashes are
more dangerous at
high speeds: KE
depends on speed
more than mass
because speed is
squared; so increase
in speed = large
increase in KE
0 SI unit for KE: Joules
Kinetic Energy
0 A water bottle is knocked off of a desk.
When does it have the MOST KE?
0 At the top of the fall.
0 In the middle of the fall.
0 At the bottom right before it hits the
ground.
Kinetic Energy
0 A water bottle is knocked off of a desk.
When does it have the MOST KE?
0 At the top of the fall.
0 In the middle of the fall.
0 At the bottom right before it hits the
ground.
Kinetic Energy
0 Atoms and molecules have KE. Why?
Because atoms and molecules are
constantly in motion, and KE is energy of
motion.
0 KE increases as objects get hotter and
decreases as objects get cooler.
Why Physics is Important!
0 Someone failed
their high school
physics, loading
the cart so much
that it tipped
backwards. Poor
Donkey!
Mechanical Energy
0 Mechanical energy: the sum of PE and
KE in a system
0 Example: An apple falling from a tree
has both PE and KE
Mechanical Energy
Nonmechanical energy
0 In almost every system, there are hidden
forms of energy that are related to the
arrangement of atoms that make up the
objects in the system. For example, when you
eat an apple, it gives you energy.
0 Nonmechanical energy: energy that lies at
the level of atoms and doesn’t affect
motion on a large scale.
0 In most cases, nonmechanical forms of
energy are just special forms of either
kinetic or potential energy.
Living Things & Energy
0
Where do we get the energy needed to live?
0 The energy comes from food. When we
eat a meal, we eat plants, animals, or
both. Animals also eat plants, other
animals or both.
Living Things
& Energy
0 Plants and algae do not need to eat because
they get their energy directly from
sunlight. Plants use photosynthesis to
turn the energy in sunlight into chemical
energy. This energy is stored in sugars
and other organic molecules that make up
cells in living tissue. Thus, when you eat a
meal, you are really eating stored
(potential) energy.
Nuclear Reactions
0 How does the sun
get its energy?
From nuclear
fusion; (fusion is
the process in
which light nuclei
join to form
heavy nuclei with
a large energy
release).
Nuclear Reactions
0 Nuclear power plants use nuclear
fission to release energy. In fission, a
single large nucleus is split into two or
more smaller nuclei with a large
release of energy.
Electricity
0 Electricity: a form of energy that results
from the flow of charged particles in an
electric field.
0 This is how the lights and appliances in
our homes get their energy.
Lightning, a form of
electrical energy, results
from moving electrons
between the ground
and a cloud.
Light Energy
0 Consider a bright sunny day at the
beach. Where is it hotter: in the sand or
under the umbrella?
Light Energy
0 The reason its hotter in the sand is that
light carries energy.
0 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 is
another example of energy stored in a
field.