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Unformatted text preview: UCSD Physics 12 Forms of Energy II
Wind, Chemical, Food, Mass-energy, Light UCSD Physics 12 The Physics 12 Formula List Lots of forms of energy coming fast and furious, but to put it in perspective, here's a list of formulas that you'll need to use:
Relation Type Work as force times distance Kinetic Energy (Grav.) Potential Energy Heat Content Power Mass-energy Radiative Flux Spring 2010 Formula W = Fd K.E. = mv2 E = mgh E = cpm T
P = E/ t E = mc2 F = T 4 2 UCSD Physics 12 Wind Energy Wind can be used as a source of energy (windmills, sailing ships, etc.) Really just kinetic energy Example: wind passing through a square meter at 8 meters per second Each second we have 8 cubic meters Air has density of 1.3 kg/m3, so (8 m3) (1.3 kg/m3) = 10.4 kg of air each second mv2 = (10.4 kg) (8 m/s)2 = 333 J 333 J every second 333 Watts of available power per square meter (but to get all of it, you'd have to stop the wind) Stronger winds more power (like v3) Spring 2010 Q 3 UCSD Physics 12 Chemical Energy Electrostatic energy (associated with charged particles, like electrons) is stored in the chemical bonds of substances. Rearranging these bonds can release energy (some reactions require energy to be put in) Typical numbers are 100200 kJ per mole a mole is 6.022 1023 molecules/particles typical molecules are tens of grams per moleworks out to typical numbers like several thousand Joules per gram, or a few Calories per gram (remember, 1 Cal = 1 kcal = 4184 J) Spring 2010 4 UCSD Physics 12 Chemical Energy Examples Burning a wooden match releases about one Btu, or 1055 Joules (a match is about 0.3 grams), so this is >3,000 J/g, nearly 1 Cal/g Burning coal releases about 20 kJ per gram of chemical energy, or roughly 5 Cal/g Burning gasoline yields about 39 kJ per gram, or just over 9 Cal/g Very few substances over about 11 Cal/g
Spring 2010 5 UCSD Physics 12 Energy from Food We get the energy to do the things we do out of food (stored solar energy in the form of chemical energy). Energy sources recognized by our digestive systems: Carbohydrates: 4 Calories per gram Proteins: 4 Calories per gram Fats: 9 Calories per gram (like gasoline) Spring 2010 6 UCSD Physics 12 Our Human Energy Budget A 2000 Calorie per day diet means 2000 4184 J = 8,368,000 J per day 8.37 MJ in (24 hr/day) (60 min/hr) (60 sec/min) = 86,400 sec corresponds to 97 Watts of power Even a couch-potato at 1500 Cal/day burns 75 W More active lifestyles require greater Caloric intake (more energy) Spring 2010 7 UCSD Physics 12 Nutrition Labels Nutrition labels tell you about the energy content of food Note they use Calories with capitol C Conversions: Fat: 9 Cal/g Carbs: 4 Cal/g Protein: 4 Cal/g This product has 72 Cal from fat, 48 Cal from carbohydrates, and 32 Cal from protein sum is 152 Calories: compare to label 152 Cal = 636 kJ: enough to climb about 1000 meters (64 kg person) Spring 2010
Q 8 UCSD Physics 12 Mass-energy Einstein's famous relation: E = mc2 relates mass to energy In effect, they are the same thing one can be transformed into the other physicists speak generally of mass-energy Seldom experienced in daily life directly Happens at large scale in the center of the sun, and in nuclear bombs and reactors Actually does happen at barely detectable level in all energy transactions, but the effect is tiny! Spring 2010 9 UCSD Physics 12 E = mc2 Examples The energy equivalent of one gram of material (any composition!!) is (0.001 kg) (3.0 108 m/s)2 = 9.0 1013 J = 90,000,000,000,000 J = 90 TJ Man, that's big! The U.S. energy budget is equivalent to 1000 kg/yr If one gram of material undergoes a chemical reaction, losing about 9,000 J of energy, how much mass does it lose?
9,000 J = mc2, so m = 9,000/c2 = 9 103/9 1016 = 10-13 kg (would we ever notice?) Spring 2010 10 UCSD Physics 12 Solar Energy is Nuclear, Using E = mc2 Thermonuclear fusion reactions in the sun's center Sun is 16 million degrees Celsius in its center Enough energy to ram protons together (despite mutual repulsion) and make deuterium, then helium Reaction per atom 20 million times more energetic than chemical reactions, in general
4 protons: mass = 4.029 He nucleus: mass = 4.0015
4 2 neutrinos, photons (light) Spring 2010 11 UCSD Physics 12 E = mc2 in Sun Helium nucleus is lighter than the four protons! Mass difference is 4.029 - 4.0015 = 0.0276 a.m.u. 1 a.m.u. (atomic mass unit) is 1.6605 10-27 kg difference of 4.58 10-29 kg multiply by c2 to get 4.12 10-12 J 1 mole (6.022 1023 particles) of protons 2.5 1012 J typical chemical reactions are 100-200 kJ/mole nuclear fusion is ~20 million times more potent stuff!
Q Spring 2010 12 UCSD Physics 12 Energy from Light The tremendous energy from the sun is released as light. So light carries energy. How much?? Best way to get at this is through the process of "blackbody" radiation, or thermal radiation... All objects emit "light" Though almost all the light we see is reflected light The color and intensity of the emitted radiation depend on the object's temperature
Spring 2010 13 UCSD Physics 12 Emitted Radiation's Color and Intensity depend on Temperature
Object You Heat Lamp Candle Flame Bulb Filament Sun's Surface Temperature ~ 30 C ~ 500 C ~ 1700 C ~ 2700 C ~ 5500 C Color Infrared (invisible) Dull red Dim orange Yellow Brilliant white The hotter it gets, the "bluer" the emitted light The hotter it gets, the more intense the radiation (more energy) Spring 2010 14 UCSD Physics 12 "Blackbody", or Planck Spectrum Spring 2010 15 UCSD Physics 12 Same thing, on logarithmic scale: Sun peaks in visible band (0.5 microns), light bulbs at 1 m, we at 10 m. (note: 0C = 273K; 300K = 27C = 81F) Spring 2010 16 UCSD Physics 12 Okay, but how much energy? The power given off of a surface in the form of light is proportional to the fourth power of temperature! F = T4 in Watts per square meter the constant, , is numerically 5.67 10-8 W/K4/m2 easy to remember constant: 5678 temperature must be in Kelvin: K = C + 273 C = (5/9) (F 32) Example: radiation from your body:
(5.67 10-8) (310)4 = 523 Watts per square meter (if naked in the cold of space: don't let this happen to you!) Spring 2010 17 UCSD Physics 12 Radiant Energy, continued Example: The sun is 5800K on its surface, so:
F = T4 = (5.67 10-8) (5800)4 = 6.4 107 W/m2 Summing over entire surface area of sun gives 3.9 1026 W Compare to total capacity of energy production on earth: 3.3 1012 W Single power plant typically 0.51.0 GW (109 W) In earthly situations, radiated power out partially balanced by radiated power in from other sources Not 523 W/m2 in 70F room, more like 100 W/m2 goes like Th4 Tc4 Spring 2010 Q 18 UCSD Physics 12 And those are the major players... We've now seen all the major energy players we'll be discussing in this class: work as force times distance kinetic energy (wind, ocean currents) gravitational potential energy (hydroelectric, tidal) chemical energy (fossil fuels, batteries, food, biomass) heat energy (power plants, space heating) mass-energy (nuclear sources, sun's energy) radiant energy (solar energy) Spring 2010 19 UCSD Physics 12 Assignments Read Chapter 1 and Appendix in textbook Homework #1 due April 9 in class: Chapter 1 problems: see assignments page for online problems don't forget to show your work on the multiple choice! Quiz #1 due Friday, April 9, by midnight available starting Thursday afternoon 3 attempts permitted all numerical/quantitative this week Spring 2010 20 ...
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