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Energy Notes (Chapter. 5)
Energy: the ability to do work or
to cause a change
Energy
• Work is done when a force causes something
to move.
• Any matter has energy if it can produce a
change in itself or in its surroundings
• When work is done and a change occurs,
energy moves from place to place or changes
from one form of energy to another.
An Energy Analogy
An Energy Analogy:
• Money can be used to help you understand
energy
• If you have $100, you could store it in a variety of
different forms:
1. Cash in your wallet 3. A Bank Account
2. Gold or Silver Coins 4. Traveler’s Check
An Energy Analogy
You could transfer money to different forms:
1. Deposit cash into a bank account
2. Trade the cash for gold
It’s still money, regardless of the form, and the
same is true for energy!
Kinetic Energy
• Kinetic Energy (KE): energy a moving object
has because of its motion
• Depends on Mass and Velocity
• Formula: KE = ½mv2
m=mass v=velocity
• Units: ½ (Kg)(m/s)2 = kgm2/s2= Joule  J
KE
0.5
m
v2
Kinetic Energy
• Example: How much kinetic energy does a
0.5 kg ball have when it’s moving at 2 m/s?
1. KE = ?
m = 0.5 kg
v = 2 m/s
2. KE = ½mv2
3. KE = ½ (0.5kg)(2m/s)2 = 1 Joule or 1J
Potential Energy
Potential Energy (PE): stored
energy associated with position
Potential Energy
Example: A hanging apple in a tree
• It has stored energy because of its position
above the surface
• If it stays in the tree, it will keep its stored
energy due to its height above the ground
• If it falls, the stored energy of position is
converted to energy of motion (Kinetic)
Potential Energy
Elastic Potential Energy: energy stored by
something that can stretch or compress.
Example: A stretched rubber band or a spring
• (When you let a rubber band go, it flies across
the room as kinetic energy)
Potential Energy
Chemical Potential Energy: potential
energy stored in the chemical bonds of
atoms and chemical compounds that is
released when the bonds break as in a
chemical reaction.
Chemical Potential Energy
Examples: Natural Gas, Gasoline, Food
• When natural gas is burned, those bonds are
broken, & energy is released
• Digestion of Food Releases Energy
• Plants Store Energy after Photosynthesis,
converting sunlight to sugars
• Note: One food calorie (C) is equal to one
kilocalorie (kC) which is equal to 4180 Joules.
Gravitational Potential Energy
Potential Energy is sometimes called:
Gravitational Potential Energy (GPE)
Formula: PE = mgh
= mass x gravity x height
PE = wh = weight x height
(Remember: weight = mass x gravity)
(gravity = 9.8 m/s2)
Gravitational Potential Energy
PE
m
g
h
The higher something is above the ground, the
more P.E. it has
PE and KE
POTENTIAL ENERGY
Water Behind a Dam
KINETIC ENERGY
Waterfall
Car Parked at Top of Hill Car Rolling Down a Hill
Gasoline
Engine Runs
Food
You Run a Mile
Mechanical Energy
Mechanical Energy (ME): the total amount of
potential and kinetic energy in a system.
ME = PE + KE
*An object with ME can do work on another
object. It puts things in motion. It moves
cars and lifts elevators. A machine uses ME to
do work, and so do our bodies.
Other Forms of Energy
Thermal Energy: caused by the constant motion
that the particles of the object are in.
*Combines the PE and KE of the particles of an
object
*Particles move faster when heated and slower
when cooled
Example: Ice Cream melting on a hot day
Heat Transfer
Heat Transfer: 3 ways heat can move:
Conduction
Convection Radiation
Heat Transfer
Conduction: heat is transferred from one
particle of matter to another without the
movement of matter through direct contact or
touching.
• Example: feet on hot sand or a heating pad
on your sore back
Heat Transfer
Convection: Heat is transferred by the
movement of currents within a fluid.
*less dense, heated particles move to the
top and cooler, denser particles flow down
to take their place, creating convection
currents.
Example: Warm air rises and cold air sinks
Heat Transfer
Radiation: Transfer of energy by
electromagnetic waves and does not require
direct matter to matter contact.
Example: Sunlight (solar energy) reaching
Earth’s surface
Example: Warming your hands by a fire
Electrical Energy
Electrical Energy: the flow of electric
charges in the form of moving electrons, which
are the negative charged particles in an atom.
Example: Stored in a battery (Potential)
Example: Lightning Strike (Kinetic)
Nuclear Energy
Nuclear Energy: potential energy that is stored
in the nucleus of an atom
Fission: Nuclear energy released when an
atom splits as in nuclear power plants
Fusion: energy released when the nuclei of
atoms fuse together as in solar energy when
the sun fuses Hydrogen atoms into Helium
Electromagnetic Energy
Electromagnetic Energy: energy that travels in
waves have both electric and magnetic
properties.
*can travel through the vacuum of space
Examples: sunlight, radio waves, x-rays,
microwaves, IR rays, UV rays, visible light,
sound waves
Energy Transformations
Energy Transformations: a change
from one form of energy into another
Energy Transformations
Single Transformation: one form of energy is
transformed to do work
• Example: a toaster changes electrical energy
to thermal energy to heat bread.
• Example: An apple accelerates to the ground
after it comes loose from the tree and its
potential energy becomes kinetic energy.
Energy Transformations
Example: A baseball’s curved path after the ball
is hit by the bat, where kinetic energy and
potential energy are converted into each other
as the ball rises and falls.
Energy Transformations
Low KE/High GPE
High KE/Low GPE
High KE/Low GPE
Energy Transformations
Multiple Transformation: a series of
transformations are needed to do work
• Example: mechanical energy is used to strike a
match to produce thermal energy, which
causes the particles of the match head to
release stored chemical energy, which is
transformed into thermal and electromagnetic
energy as heat and light are produced.
Energy Transformations
Energy Transformations in a Pendulum:
There is a continuous transformation between
kinetic and potential energy in a pendulum. At the
highest point of the swing, when the pendulum has
no movement, gravitational potential energy is the
greatest. As it swings down, the potential energy is
transformed into kinetic energy and reaches its
greatest kinetic energy as the bottom of the swing.
This repeats itself as the pendulum swings up to the
highest position on the other side.
Pendulums and Transformations
Law of Conservation
Just like the
Law of Conservation of Mass or Matter
which states that in a reaction, mass and
matter cannot be created nor destroyed,
it simply changes form…the
Law of Conservation
Law of Conservation of Energy: when one type
of energy transforms into another, no energy is
created nor destroyed. It simply changes form.
Total amount of energy is the same before and
after any transformation.
Conservation of Energy & Friction
But what about FRICTION? When energy is lost
due to friction, the energy has transformed from
kinetic to thermal energy (heat). The energy is not
destroyed, it has just changed form. Friction
transforms mechanical energy into thermal energy,
which is how you warm your hands when rubbing
them together, and why no machine is 100%
efficient. Mechanical energy is transformed into
thermal energy due to friction.