Energy Conservation and Efficiency

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Energy Conservation and Efficiency

• Where Does the Wasted Energy Go?

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Heat and Thermal Energy

• All matter is made of atoms/molecules, which move
  around randomly
• Thermal Energy The energy stored in the
  individual motion of atoms or molecules
• Heat The thermal energy transferred from one
  place to another

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E mc2

• Albert Einstein showed that mass can be converted
  into energy and visa versa
• The total amount of energy (in Joules) contained
  in m kg of mass is
• E m x c2
• Where c is the speed of light (3 x 108 m/s)

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The Law of Conservation

• The total mass and energy of the universe never
  changes (I.e. it is conserved)
• Mass Energy Constant
• We extract energy by converting a tiny amount of
  mass into energy
• In practice, the amount of mass converted to
  energy is always small!

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Example

• A gallon of gasoline (about 3.3 kg) can produce
  1.3 x 108 Joules of energy
• How much mass is converted to energy?
• Answer m E/c2 1.3 x 108/9 x 1016
• 1 x 10-9 kg
• Where does the rest of the mass go?

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The Approximate Law of Conservation

• In most situations, the amount of mass converted
  to energy is tiny
• In practice, then
• 1.) The total amount of energy is conserved
• 2.) The total amount of mass is conserved
• Energy and mass are conserved separately!

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Example Conservation of Mass
Smoke, gas, soot
Fuel, Air
Total Mass In Total Mass Out
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Example Conservation of Energy
Chemical Energy in Fuel
Heat, Electricity
Total Energy In Total Energy Out
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Stored Energy

• Energy can be stored in a system if it enters but
  doesnt leave
• Energy In Energy Out Energy Stored
• (English) What goes in must either stay in or
  come out
• Energy can never be created or destroyed, only
  wasted!

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Example Solar House

• Energy in sun heats the house through windows
• Energy out heat leaks out through walls and
  windows
• Energy stored heat is stored in walls and floors

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Energy Efficiency

• Not all energy in a system can be converted into
  useful work (2nd law of thermodynamics)

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Example

• A fossil fuel plant releases 6 x 108 J of energy
  from burning oil in one day. It produces 2 x 108
  J of electricity. What is the efficiency?
• Answer efficiency
• (2 x 108)/(6 x 108)x100
• 30

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Power and Efficiency

• Power input and output can also be used to
  calculate efficiency in exactly the same way

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Where Does it Go?
Answer The wasted energy goes into heating the
lake and combustion gases
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Some Typical Efficiencies
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Overall Efficiency

• The overall efficiency of a multiple-step energy
  conversion is the product of the individual
  efficiencies

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Friction and Heat

• Friction is the principal means of energy loss in
  many machines
• Friction energy losses produce heat

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Automobile Efficiency

• Thermal Efficiency percentage of chemical energy
  in gasoline converted into work (moving the
  pistons)--typically 38
• Mechanical Efficiency percentage of engine work
  used to move vehicle--typically 50
• Overall Efficiency (0.38)(0.5)x100
• 19

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Where Does it Go?

• Most of the engine work creates heat (engine
  friction)
• Most of the remaining energy is used to overcome
  air and rolling resistance
• Maximum fuel efficiency at 40 mph!

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Automobiles Then and Now