PowerPoint #5: Basic Physical Laws Governing Energy Use

1. PowerPoint #5: Basic Physical Laws Governing Energy Use

2. 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

3. 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

4. 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.

5. 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

6. 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)

7. 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.

8. 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

9. 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)

11. 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.

12. 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