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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
- 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.

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
- 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

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 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

- 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)

- 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

- 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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