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PowerPoint #5: Basic Physical Laws Governing Energy Use

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PowerPoint #5: Basic Physical Laws Governing Energy Use. Basic Physical Laws Governing Energy Use. Forms of energy Energy units Energy, work and power Conservation of energy Energy conversion and efficiency Newton’s laws of motion. FORMS OF ENERGY. Chemical

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PowerPoint #5:

Basic Physical Laws Governing Energy Use

basic physical laws governing energy use
Basic Physical Laws Governing Energy Use
  • Forms of energy
  • Energy units
  • Energy, work and power
  • Conservation of energy
  • Energy conversion and efficiency
  • Newton’s laws of motion
forms of energy
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
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.
solar
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
    • 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 object’s 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)

first law of thermodynamics
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 =  (KE+PE+TE)

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.

energy conversion efficiencies
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

heat transfer
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)
second law of thermodynamics continued
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.
some useful websites
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
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