ES105 2006 January 31 Energy: the ability to do work I. Forms of energy A. Mechanical B. Electrical C. Chemical D. Radiant (light) E. Heat F. Nuclear Unit of Energy is Joule Joule is (km x m2)/s2 Watt is a unit of power Watt = 1 Joule/s II. Power is energy over time: A. hiker reaches hilltop fast, climber slow B. hiker has more POWER III. Mechanical Energy A. Potential Energy stored because of position, or state of being B. Kinetic Energy possessed because object is moving IV. Energy can be converted from one form to another: A. Mechanical Æ electrical B. Chemical Æ mechanical C. Radiant Æ heat D. Radiant Æ electrical E. Heat Æ mechanical V. Ultimately, Earth energy comes from two sources: the sun, and heat released internally by radioactive decay A. Our power ultimately is nuclear: 1. Sun’s energy is from a fusion reaction. a. Hydrogen atoms are converted to helium by nuclear fusion. 600 million tons/s b. Heat of fusion of atoms is the radiant energy passed through space that we receive. 2. We receive about 1.73 x 1017 watts of energy from Sun—99+% a. Internal heat is about 3.2 x 1013 watts—0.02% b. Tides (generated by gravitational pull of Sun and Moon) are about 3 x 1012 watts—0.002% c. Nearly 30% of Sun’s energy is reflected back to space Page 1 of 6d. Energy of fuels, such as petroleum or coal, is the energy Sun converted to chemical energy by photosynthesis in plants VI. Photosynthesis A. Solar energy absorbed by green plants B. Chlorophyll is a catalyst that allows the conversion of radiant energy to glucose, a sugar C6H12O6: 6 CO2 + 6 H2O + Sun energy Æ C6H12O6 + O2C. This reaction actually has produced the levels of oxygen we have in the atmosphere today. Ancient algae in the seas created it by photosynthesis. D. Rare deep sea vent communities not supported by sunlight 1. energy source from the hot water 2. radioactive decay, and gravity. 3. heating converts seawater sulfates to hydrogen sulfide. a. Bacteria thrive on the hydrogen sulfide in the hot water, b. which are consumed by larger organisms, c. giving rise to an ecosystem (National Geographic, October 1977, pp.441-453). VII. Energy and Chemical Reactions A. Temperature affects rate of reactions 1. Reactions proceed more rapidly at higher temperatures a. Coal + oxygen Æ carbon dioxide + water faster in the presence of heat b. Molecules collide more frequently at higher temperatures c. More energy is present to break chemical bonds B. Concentration of reactants influences rate of reaction C. Presence of catalysts allows some reactions to proceed at much greater rates than without catalyst 1. Peroxide H2O2 degrades to water over time 2. Introducing platinum metal speeds the reaction to seconds instead of years 3. Heat is released in this reaction D. Most reactions either release heat or consume heat 1. Heat energy is commonly measured in calories a. Food calories are actually kilocalories (kcal) b. 1 Joule=0.24 Calorie 2. Exothermic reactions release heat 3. Endothermic reactions consume heat Page 2 of 64. The energy of the reaction can be noted in the chemical equation. The units must be included for it to be meaningful 5. Can calculate energy from the equation Examples C3H8 + 5 O2 Æ 3 CO2 + 4 H2O + 526 kcal a. How much energy is released when 2 moles of propane are burned? (526 kcal/mole of propane)(2 moles propane) = 1052 kcal released Of course you could calculate heat released when a certain gram amount of propane is burned…because you can convert grams to moles. Say 308 g propane x (1 mole/44 grams) = 7 moles 7 moles X 526 kcal/mole = 3682 kcal released b. Energy consumed is similarly noted, and can be calculated N2 + O2 + 4.32 kcal Æ 2 NO 5 moles of N2 reacting with 5 moles of O2 will consume about 21 ½ kcal in the production of NO gas Reactions that consume heat energy are called endothermic Is photosynthesis an exothermic or endothermic reaction? VIII. Thermodynamic laws A. Conservation of Energy—first law of thermodynamics 1. Energy is neither created or destroyed— 2. you have the same amount before and after any reaction or event B. Second law of thermodynamics —Heat flows from places with it to places without it C. Corollaries of these laws 1. Energy can change form, commonly to a less useful one 2. Warm places can be made cold, but with the input of energy 3. Some energy is wasted in any transfer Page 3 of 6a. Released as heat b. Released as heat of friction D. Entropy is the term to describe the increase in randomness of system 1. Systems naturally tend toward greater disorder 2. Order can be increased, with input of energy a. Easy to pollute, difficult to clean up: b. think about CFC in the atmosphere, or oil in the sea. IX. Energy sources and uses A. Humans first organism to systematically exploit energy reserves 1. First used chemical energy stored in wood—prevalent until 1850 2. Kinetic energy of water harnessed about 2000 years ago—grind grain in Egypt 3. Wind energy used to grind grain, pump water to grind grain perhaps as long ago as 1000 years B. ‘Fossil fuels’ include coal, petroleum and natural gas 1. ‘Fuel’ burns readily to release energy— a. what type is released? Chemical b. what is it converted to? Mechanical (usually) 2. fuels are ‘reduced’ forms of matter a. burning oxidizes the material— b. maximum number of atoms bonded to oxygen C. Recall first law of thermodynamics—conservation of energy 1. We convert energy from high-grade forms to lower-grade forms a. ‘Production’ of fuel means ‘make available for exploitation’ 1) Oil isn’t available until it has been pumped 2) Coal needs to be mined 3) Turbines in dams ‘produce’ electricity from the kinetic energy of falling water b. ‘Consumption’ of energy is our utilization of the energy source, where we convert it from one form to another, and reap the rewards of the conversion 1) We put gasoline in our car, and convert chemical energy to forward motion 2) We turn on the light switches, and convert electrical energy to light Page 4 of 6D. Fossil fuel reserves and consumption 1. Fossil fuels are still being created today…at a very slow rate 2. For our purposes, the supply of fossil fuels is limited a. ‘non-renewable’ b. We are depleting our reserves rapidly—perhaps half of petroleum has already been burned, in
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