Energy and the Environment

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PowerPoint 5 Basic Physical Laws Governing
Energy Use
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Basic Physical Laws Governing Energy Use

• Forms of energy
• Energy units
• Energy, work and power
• Conservation of energy
• Energy conversion and efficiency
• Newtons laws of motion

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FORMS OF ENERGY

• Chemical
• Energy released when chemical bonds are broken.
  Energy is usually in the form of heat.
• Example combustion - CH4 2O2 ? CO2 2H2O
  heat

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Nuclear

• Energy released when nuclear bonds are broken.
  Energy is in form of heat and radiation.
• E mc2 m mass (kilograms) c speed of
  light (3 x 108 meters/sec, E joules)
• Nuclear energy used to generate heat to produce
  steam to generate electricity.

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Solar

• Solar energy results from thermonuclear reactions
  within the sun. Reaches earth in form of
  electromagnetic radiation (heat and light)
• Used to heat water and space (solar thermal) or
  to produce electricity (Photovoltaics)
• Wind and hydro are forms of solar energy

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Gravitational

• Gravitational energy is energy of relative
  POSITION.
• E mgh m mass (kg), g gravitational
  constant (9.8 m/sec2), h height (meters), E
  joules
• Energy derived from a dam depends on amount of
  water (mass) and height water falls
• y depends on its
• Velocity, relative position, temperature and mass
• 1. Velocity Energy associated with objects
  motion is called kinetic energy (KE)
• KE (1/2)mv2
• Units E(joule) m(kg)v2(m/sec)2 kg
  meter2/sec2
• 2. Relative position Energy associated with
  location of object relative to some reference
  point, such as surface of earth, or center of
  earth. Called potential energy (PE)
• PE mgh
• g acceleration due to gravity (meter/sec2)

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FIRST LAW OF THERMODYNAMICS

• Work done on a system plus heat added to a
  system the change in total energy of the
  system
• Won Qto ? (KEPETE)
• Won work done on the system
• Qto heat added to the system
• Implication of First law 1) energy cannot be
  created or destroyed 2) total amount of energy
  in universe is constant.

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Energy Conversion Efficiencies

• Energy must be converted from one form to another
  to do useful work.
• Amount of useful work one obtains from a given
  amount of energy depends on the efficiency of the
  conversion process.
• Efficiency () useful energy out x 100
  total energy in

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HEAT TRANSFER

• Radiation heat transfer in a vacuum.
• Heat is transferred by energy carried in
  electromagnetic waves.
• Energy of wave depends on frequency higher the
  frequency, greater the energy.
• V(m/s) ? (m) x f(cycles/second)
• In a vacuum, V c, the speed of light (3x108
  m/s)

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SECOND LAW OF THERMODYNAMICS (Continued)

• Efficiency and second law recall
• Efficiency ( 1- heat out/heat in) x 100 from
  first law. But, second law says that heat out is
  always less than heat in, if useful work is to be
  extracted from the process.
• Therefore, the efficiency for any physical
  process in which work is extracted can never be
  100.
• This means it is impossible, in principle, to
  build a perpetual motion machine.

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Some useful websites

• http//zebu.uoregon.edu/1996/phys161.html
• http//www.eia.doe.gov/
• http//www.energy.ca.gov/
• http//www.eren.doe.gov/
• http//www.census.gov/
• http//www.hubbertpeak.com/index.html