ISCOR 310

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ISCOR 310

• Basic Physical Laws Governing Energy Use
• Al Sweedler
• SDSU

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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
• 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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FORMS OF ENERGY

• 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
• 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

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Symbols and Units

• Property Symbol Units
• energy E joule
• velocity v meter/sec
• mass m kg
• relative position h meter
• temperature T Celsius
• acceleration a m/sec2
• power P joule/sec watt

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Energy content of a body 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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• 3. Temperature Energy associated with objects
  heat content. Called thermal energy Heat content
  proportional to absolute temperature (degrees
  Kelvin 0 oKelvin -273 oC)
• TE ? T
• 4. Mass Energy associated with objects mass
  (note mass is different from weight)
• Mass Energy mc2
• Total energy possessed by an object is thus
• Total Energy kinetic energy potential energy
  thermal energy mass energy
• In most situations where energy is converted from
  one form to another, we do not need to consider
  the mass energy of an object or system, with the
  important exception of nuclear energy.

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In most situations where energy is converted from
one form to another, we do not need to consider
the mass energy of an object or system, with the
important exception of nuclear energy. Total
Energy (excluding mass energy) of a system is
then TOTAL ENERGY OF A SYSTEM KE PE TE
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• Mechanical Work defined as applying a force which
  results in moving an object a certain distance
• Work force x distance
• Once can think of energy as the ability to do
  work
• Work and energy have the same units (joules).
  Unit of force is the Newton
• Power is the rate of doing work
• P E/t joule/sec watt

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Work and Energy

• Mechanical work done on a system equals the
  change in the mechanical energy of the system
• Won ?(KE PE)
• One can also change energy of a system by adding
  heat Heat is also a form of energy.
• Thus, the total amount of energy input to a
  system consists of the mechanical work done on
  the system plus the thermal heat put into the
  system. Combining these we get

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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 CAPACITY

• Heat capacity, or specific heat, is an objects
  ability to hold and store heat.
• Specific heat is the amount of heat (in calories)
  needed to raise the temperature of one gram a
  substance one degree Celsius.
• Q mc?T
• or
• ?T Q/mc
• Where Q amount heat added or removed, m mass
  of object, c specific heat, ?T change in
  temperature.

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PHASE CHANGES

• Heat added to a substance can result in a phase
  change a change of state, usually from a solid
  to a liquid to a gas.
• Heat needed to change from a solid to a liquid
  called heat of fusion
• heat of fusion - the amount of heat that must
  be added to a substance, per unit mass, at its
  melting point to change the substance from a
  solid to a liquid at the same temperature.

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PHASE CHANGES

• Heat needed to change from a liquid to a gas
  called heat of vaporization.
• heat of vaporization - the amount of heat that
  must be added to a substance, per unit mass, at
  its boiling point to change the substance from a
  liquid to a gas at the same temperature.

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HEAT TRANSFER

• Heat can be transferred between objects if there
  is a temperature difference between the objects.
• Heat is transferred by conduction, convection, or
  radiation.

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HEAT TRANSFER

• Heat transferred via conduction is given by
• Q/t (k)( A)(?T)
• ?
• Q/t amount of heat transferred per unit time. A
  surface area, ?T temperature difference, ?
  thickness, k thermal conductivity.

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HEAT TRANSFER

• Convection in a gas or liquid, heat is
  transferred by convection.
• Density of a fluid is less when it is warm than
  when it is cold, so it will rise carrying heat
  with it. Colder fluid, being more dense, will
  sink.
• This sets up convection currents.

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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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HEAT ENGINES

• A heat engine transforms heat into work
• W QH - QC (see page 108 in text)
• For a power plant (see page 77)
• QH W - QC where
• QH is the heat into power plant (fuel
  combustion)
• W is the net work out (to generate electricity)
• QC is the net heat out (of condenser)

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SECOND LAW OF THERMODYNAMICS

• Deals with direction of energy flow and physical
  processes
• e.g., heat always flows from hot to cold of its
  own accord, never the other way around.
• Drop in ink in water disperses, but dispersed ink
  will not move back to concentrated state by
  itself.

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SECOND LAW OF THERMODYNAMICS (Continued)

• Various statements of second law
• Heat can flow spontaneously (by itself) only
  from a hot source to a cold sink.
• No heat engine can be constructed in which hear
  from a hot source is converted entirely to work.
  Some heat must be discharged to a sink at a lower
  temperature. (see page 108)

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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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Maximum or Carnot Efficiency

• Maximum possible efficiency for a heat engine
  operating between a high source with absolute
  temperature TH and a cold sink with absolute
  temperature TC is

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Newtons Laws of Motion

• Speed Distance traveled/time (miles/hour or
  km/sec) s d/t
• Velocity gives speed and direction of motion
• Acceleration change of velocity in a given
  period of time a ?v/t (miles/hour/hour or
  meter/sec/sec m/sec2)

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Newtons Laws of Motion

• 1. A body at rest, or travelling with a constant
  speed in a straight line, will continue in that
  state unless acted upon by an outside force.
• 2. The acceleration of an object is directly
  proportional to the net force acting upon it and
  inversely proportional to its mass.
• a F/m
• Unit of force is the Newton.
• F ma (kg-m/sec2)
• 3. For every action force there is an equal and
  opposite reaction force.

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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