Some Basic Concepts of Energy

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Some Basic Conceptsof Energy
Prepared for BIO/EES 105 Energy in our World

• Kenneth M. Klemow, Ph.D.
• Wilkes University

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Overview of topics

• Overview
• Energy defined
• Forms of energy
• The physical nature of energy
• Energy and Newtonian Laws of Motion
• Units of measure
• Conversions
• Terminology pertaining to energy

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What is energy?

• Ability to do work
• Physicists distinguish between kinetic and
  potential energy
• Energy comes in different forms
• Radiation
• Mechanical energy
• Chemical energy
• Atomic energy
• Electromagnetic energy
• Electrical energy
• Heat energy

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Energy relates to Newtonian motion
Sir Isaac Newton 1642 - 1727
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What is energy? Physics definition

• Energy Force x distance
• Force Acceleration x mass
• Acceleration Speed / time
• Speed Distance / time

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Speed - Velocity

• Speed distance / time
• Ways of expressing
• Miles / hour
• Km / hour
• Feet / second
• Meters / second
• Other relationships
• Distance Speed x time
• Time Distance / speed
• Velocity is a vector implies speed and direction

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Speed Conversions

• 1 ft/s 0.305 m/s
• 1 mi/h 0.447 m/s
• 1 km/hr 0.28 m/s

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Some quick problems

• 1. A car drives 72 miles in 120 minutes. What
  is its velocity in miles per hour?
• 2. A person runs at 6 miles per hour. How far
  can that person run in 10 minutes?
• Expressed in miles
• Expressed in feet
• 3. How long does it take for that person to run
  528 feet?

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Another problem

• A car is traveling 60 miles per hour. How many
  feet can it travel in one second?

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Acceleration

• Acceleration Change in velocity / time
• Expressed as distance / time X time
• Or distance / time2
• Occurs when an object is speeding up or slowing
  down
• Units include
• Miles / hour2
• Km / hour2
• Feet / second2
• Meters / second2

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Helpful conversions

• 1 ft/s2 0.305 m/s2
• 1 m/s2 3.28 ft/s2

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A quick problem

• A Kia Rio can accelerate to 30 km / hour in 6
  seconds. What is its acceleration?
• Express in terms of m / second2
• (see Example 2.2 on p. 40)

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Can calculate velocity by knowing acceleration
and time

• Velocity Acceleration X Time
• Problem
• Return to the Kia
• What is velocity after 1 second?
• After 3 seconds?
• After 6 seconds?
• After 9 seconds?
• After 12 seconds?

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Gravity

• Gravity has an acceleration (Agrav)
• Metric 9.8 m/s2
• English 32 ft/s2

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Can calculate distance moved as a function of
acceleration and time

• X (1/2) x A x T2 (see p. 62 of text for
  derivation)
• Problem Imagine you drop a stone from a cliff,
  and it takes three seconds to hit the water
  below.
• How high was the cliff above the water?
• How fast was the stone moving when it hit the
  water?

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Momentum and Force

• Momentum mass x velocity
• Force mass x acceleration
• Common unit of measure for force
• Newton (N kg x m / s²)
• Other relationships
• Mass Force / acceleration (kgF/a)
• Acceleration Force / mass (AF/kg)

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Sample problem

• A rock having a mass of 2 kg falls into the water
  from a cliff. What is the force that it exerts?
• Does that force vary if the cliff is 50 high, as
  opposed to being 100 high?

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Mass and Weight are sometimes confused.

• Mass is a property of a body (measure of
  inertia).
• Irrespective of its position relative to gravity.
  
• Often expressed as Kg.
• Weight depends on gravity. An object will weigh
  more on earth than on moon because gravitational
  force greater on earth.
• Weight often considered to be unit of force,
  expressed as Kg x Agrav
• Where Kg is mass and Agrav is acceleration due to
  gravity.

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Newtons Laws of Motion

• 1. A body will continue to remain at rest or in
  motion with a constant velocity unless it is
  acted upon by an outside force.
• 2. The acceleration of an object is directly
  proportional to the net force acting on it, and
  is inversely proportional to its mass (A F/Kg).
• 3. For every action force, there is an equal and
  opposite reaction force.

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Energy (work)

• Energy Force x Distance
• Joule (J) Newton x meter
• Energy of an apple 1 m from the floor
• Some additional measures of energy
• Foot pound 1.4 J
• 1 calorie 4.187 J
• 1 BTU 1054 J

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Energy can be potential or kinetic

• Potential energy
• Stored energy, able to do work if released.
  Examples include
• Objects placed at an elevation
• Water behind dam
• Release energy if they fall
• Objects placed at mechanical tension
• Wound up spring
• Release energy if tension is relieved
• Chemical bond energy
• Organic molecules
• Energy released if combusted
• Potential energy due to elevation
• PEG weight x height Kg x Agrav x h

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Energy can be potential or kinetic

• Kinetic energy
• Energy of motionExamples include
• Moving water
• Moving catapult
• Can be expressed mathematically as
• 1/2 Kg x v2

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Power

• Rate at which energy is produced, used, or
  transferred.
• Expressed as energy per time
• Common units include
• Watt (J / s)
• Ft-lb / sec
• Horsepower
• 1 hp 550 ft-lbs / sec
• 1 hp 746 Watts

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Common unit is kilowatt hour

• Question A kilowatt hour is a measure of
• Power
• Energy
• Force
• Acceleration
• None of the above

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Energy and power

• Power energy / time
• Energy power x time

www.belmont.k12.ca.us
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Work is one way of transferring energy to an
object

• W D (KE PE)

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Conversion and conservation two important
concepts

• Both have two meanings
• Conversion
• Translating between different units of measure
• Joule lt-gt Calorie lt-gt BTU
• Changing from one form to another
• Chemical energy -gt Thermal energy
• Conservation
• First law of thermodynamics
• Energy cannot be created or destroyed, only
  converted
• Reduce wasteful energy consumption
• Switch from incandescent to light-emitting diode
  (LED)

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Some conversion factors(See Table 3.4 on pp. 86
87 for more complete list)

• 1 kilowatt hour 3.60 x 106 J
• 1 barrel oil equivalent 6.119 x 109 J
• 1 ton wood equivalent 9.83 x 109 J
• 1 ton coal equivalent 29.31 x 109 J
• 1 ton oil equivalent 41.87 x 109 J
• 1 quad (PBtu) 1.055 x 1018 J

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Energy conversions(From Table 2.2 in text)
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Laws of Thermodyamics

• First law Energy cannot be created nor
  destroyed, can only be converted (conservation of
  energy)
• In an isolated system, total energy will always
  remain constant
• Second law No energy conversion is perfect
  always get some loss as heat.
• Gives direction to a reaction
• Get increase in disorder (entropy).

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Consequences of Second Law

• In system involving movement, always get loss as
  friction
• Thus perpetual motion machines are impossible
  (yet people still try to invent them)
• Waste heat given off to environment
• Ultimately go off to space

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Efficiency an important measure
Efficiency

• Efficiencies can vary from 5 - 95
• In multistep processes, efficiency is the product
  of efficiency of each step.
• Comparative assessments of energy processes /
  devices typically take great pains to accurately
  measure efficiency

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Efficiencies of energy conversion devices and
systems

• Refer to Table 3.1 on p. 78 of text

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Example of multistep efficiency