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ENERGY
Energy – The Word vs. The Physics Concept
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The word “energy” is used loosely
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As a commodity – “I paid my energy bill today”
Does “energy” come through pipes like water?
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As a mood or behavior – “He played with a lot of energy”
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In physics, “energy” is a specific measurable quantity
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Units: Joules ( J ), calories ( cal ), kilowatt-hour ( kWh )
Exists in many forms:
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Chemical, electrical, gravitational, kinetic, etc.
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Energy and Force: “Work”
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The energy of a system is made up of two ingredients:
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How much force the system can exert on another system
How large a distance that force can be exerted for
To emphasize the importance of “distance of action”,
physicists invented a concept called “work”
Work = F | | d
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F|| = force exerted by system
d = distance force acts for
In physics, energy is the ability to do work
Work and Direction
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Only one “component” of force does work
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The component parallel to the distance traveled
F
θ
s
Work F = F || s =F horizontal s
Work gravity = 0
Work N = 0
Work friction = − F friction s
Different Forms of Energy
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Force can be thought of as a way of converting energy
from one form to another
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There are many different types of forces
This is why energy takes many different forms
Devices and machines use forces to convert energy
into a desired form
Kinetic → Light
Pressure → Kinetic
A Couple Common Forms of Energy
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“Kinetic Energy (KE)”
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A moving object will exert a force on anything it runs into
Therefore it has energy (faster speed → more energy)
1
2
KE = m v
2
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“Gravitational Potential Energy (PE
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mg
y
)”
grav
Drop an object from a height; it pulls on Earth as it falls
Therefore it has energy (greater height → more energy)
PE grav = m g h
Energy: Storage vs. Usage
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Energy storage
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A physical system contains energy which can be released
at a chosen time
Energy usage
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Energy is converted to a form where it can not easily be
stored and released again
Examples: Light from a TV set, Heated air molecules
Chemical Energy
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A form of energy which is stored inside molecules
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Actually, in the energy of electron orbits in the molecules
Chemical energy is the most common stored form
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Fuels (coal, oil, wood, etc.)
Release energy by burning
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Batteries
Release energy by pushing electric current
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Food
Release energy by digestion
Friction and Heat: “Used” Energy
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Heat energy (or thermal energy) can be viewed
at the microscopic level
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It's really the total KE of individual molecules and atoms
When friction and/or air resistance occur
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KE of large objects is converted to heat energy
Heat energy cannot be easily converted back to KE
The energy is said to be “used” or “lost”
Energy Example: Regenerative Braking
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“Traditional” Car Brakes
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Operate based on friction
KE of car → Heat energy
“Wastes” energy, which originally came from burning fuel
Regenerative Brakes
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Operate based on electromagnetism
KE of car → Stored energy in battery
Energy can be re-used later!
Car must have big enough battery to handle the energy
Hybrid car batteries store about 5 times more energy
than a traditional car battery
Quantifying and Measuring Energy
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How do we compare one form of energy to another?
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Example: a gallon of gasoline vs. a speeding car
( Chemical )
( Kinetic )
Units
1 Joule = 1 Newton 1 meter 
Work = F d
1 J = 1 N⋅m
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Burning one gallon of gasoline releases 125 MegaJoules!
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The KE of an average car on the highway is 1 MegaJoule
Power
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A system which does work
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Releases its energy by exerting a force
Power
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Power =
The rate at which a system does work
Measured in Watts (W) or horsepower (hp)
A strong force
A weak force
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Does work slowly
Puts out a
small power
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Does work quickly
Puts out a
large power
work
time
Conservation of Energy
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Systems do work by exerting a force
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If a system's energy is increasing:
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Newton's Third Law:
An equal and opposite force is exerted on the system
An equal and opposite work is done on the system
Another system's energy is decreasing an equal amount
Systems can exchange energy
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But never create or destroy it!
The total energy in the universe is conserved
Conservation of Energy Example
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Object of mass m falling from height h
At top
PE grav = m g h
KE = 0
h
At middle
1
PE grav = m g h
2
1
KE = m g h
2
At bottom
PE grav = 0
KE = m g h
So gravity is a force that converts PE into KE!
We can also calculate how fast the object
will be moving at the bottom:
KE = m g h =
1
m v2
2
v = 2 g h
Elastic PE (springs)
Whether spring is stretched or compressed, there is
a positive amount of PE stored in the spring
When block is released, PEspring is converted to KE
k
Simple Machines
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Work is the product of force and distance
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Conservation of Energy:
Work input = Work output
So we can exert large forces over short distances
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Converted from small forces over large distances
Requires the use of “machines”
Lever
Pulley
Efficiency and Energy “Loss”
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Force converts energy from one form to another
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Friction and air resistance cause energy “losses”
A “perpetual motion machine” is an unattainable ideal
Efficiency =
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useful energy output
total energy input
Example: Car
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Sources of energy “loss”:
Friction between engine parts
Air resistance on car body
Hot exhaust (not all energy in fuel used)
Overall efficiency: Only about 20% (!)
Energy on Earth
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Most forms of energy on Earth originate as sunlight and
end up as heat in the atmosphere
Sunlight
Photosynthesis
Food
Chain
Oil, coal,
wood, etc.
Ground Heat,
Water Cycle
and Weather
Solar Energy
Devices
Hydroelectric,
Wind Power,
Wave Power
“Used” Energy
(Heat)
Stored Chemical Energy is Scarce
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Plants and animals are converted into oil and coal
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Human oil and coal usage
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These are nicknamed “fossil fuels”
The geological processes take millions of years
At a rate thousands of times faster than production rate
This presents a problem for the future!
Biofuels
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Modern technology can produce oil-like fuels from plants
and algae in a relatively short period of time
Use of biofuels to replace fossil fuels increases each year
Non-Solar Energy
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These forms of energy do not originate as sunlight:
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Nuclear Energy – two different processes:
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“Fission” of heavy elements (uranium, plutonium, etc.)
“Fusion” of light elements (hyrdogen, helium, etc.)
Fission is much easier technologically than fusion
Geothermal Energy
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Heat energy trapped inside the Earth
Drives geological activity on Earth
Future Energy Strategy
Energy Source
Fossil Fuels
Pros
Cons
Infrastructure in place
Relatively little remaining
Relatively inexpensive in
the short-term
Release of CO – global
Abundant, renewable
Solar / Wind / Wave
Biofuels
Can often be produced onsite; No need to transport
Can use current oil
infrastructure
Doesn't add new CO
Nuclear
2
warming
Requires large initial
investment
Sun must be shining / wind
must be blowing
Difficult to mass produce at
this time
2
Converter kits required
Produces large power
Dangerous (meltdown)
No CO emissions
Produce radioactive waste
2
Electric Vehicles and Hybrids
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About 25% of energy consumption goes to transportation
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Electric vehicles and hybrids decrease consumption of oil
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Mostly combustion of oil-based fuels
Electricity: more efficient and cheaper energy source than oil
However, most electricity still comes from coal (fossil fuel!)
Using solar energy with an electric vehicle is emission-free
Electric Vehicle Limitations (due to large, costly batteries)
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Speed: Only very expensive electric cars can handle freeway
Range: Usually about 40-60 miles without recharging
Gas / electric hybrid vehicles have better energy efficiency
than traditional gas-only vehicles without the limitations
Summary
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Energy allows a system to exert a certain force
over a certain distance
Energy comes in many forms
Energy is the underlying concept for much of
physics; it will resurface often