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MIT OpenCourseWare http://ocw.mit.edu 8.21 The Physics of Energy Fall 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. The Basics: SI Units A tour of the energy landscape Units and Energy, power, force, pressure CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation 8.21 Lecture 2 Units and the Scales of Energy Use September 11, 2009 8.21 Lecture 2: Units and the scales of energy use 1 The Basics: SI Units A tour of the energy landscape Units and Energy, power, force, pressure CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation Outline • The basics: SI units • The principal players:gy ener , power, force, pressure • The many forms of energy • A tour of the energy landscape: From the macroworld to our world • CO2 and other greenhouse gases: measurements, units, energy connection • Perspectives on energy issues --- common sense and conversion factors 8.21 Lecture 2: Units and the scales of energy use 2 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation SI ≡ International System MKSA = MeterKilogram, , Second,mpereA Unit s Not cgs“English” or units! Electromagnetic units Deriud v n e its ⇒ Char⇒ geCoulombs EnerJ gy oul es ⇒ Current ⇒ Amperes Po werW a tts ⇒ Electrostatic potentialV⇒ olts Pr e ssuP a r s e cals ⇒ Resistance ⇒ Ohms Fo rNe c e wto n s T h erma l un i ts More about these next... TemperatureK⇒ elvinK) ( 8.21 Lecture 2: Units and the scales of energy use 3 The Basics: SI Units A tour of the energy landscape Units and Energy, power, force, pressure CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation Outline • The principal players:gy ener , power, force, pressure 8.21 Lecture 2: Units and the scales of energy use 4 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation F or ce, Ener gy , P o wer and Pr essur e [ X ] means “TheW units e can of X” get the units of an y physical ⇔ ⇔ Basics: m kilograms, l metersquantity by using a definition or t ⇔ seconds, andQ ⇔ coulombsphysical la w that relates it to wesomething already kno w ... F or e xample: [l/ speed= t meters/second ] = Ne wton’• s secondF= orce la mass× w acceleration [ forc e[mass][acceleration] ] = ml= 2 /= kilogram t meter/second2kg= m/s2 1k gm2Ne=1 / s wton The force of gra vity on you: 2 graF vit==8mg y 0k9.8=× g 784 m/s Ne wtons • 1 × Kinetic enerEner gy= gy2 mass velocity-squared [ ener g y ] =elocity [mass][v2]=ml2/t2 = kilogram meter2/second2 = kg-m2/s2 1k gm2/s2 =1Joule= 1 Nwton-meter e Y our kinetic ener gy w alking at 3 miles per hour: = 1 2 1 3 miles × 1609 meters × 1 hours 2 Ekinetic 2 mv = 802 kg hour mile 3600 second= 7 2 Joules 8.21 Lecture 2: Units and the scales of energy use 5 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation Po•= werPo ener wer gy perdE unit /dt time [ p o wer ] =gy][time [ener−1]= ml2/t2 (1/t) = kilogram meter2/second3 = kg-m2/s3 1k gm2/s3 =1W= att 1Joule/second Po wer youstairs e x ert at 0.5 climbing meters per second ΔE = mgΔh=8kg 0× .5 m × 9.8 m/s=2 390Joules climP bing=ΔE/Δ=7t Joules 8 /1 second=W 390 atts • Pressure=Pressure force perdF unit /dA area [ pressur[force][area e ] = −1]=ml2 /t 1/l2 = kilogram/meter second2 =kg− m1s−2 = 11k P ascal gm−1s−2 = 1 N e2 wton/meter Y ou, standing on the ground: F orce Ne784 wtons 784 grap vit= y = = =∼ 34P, 000 ascals Area ∼ 36 in2 0.023 8.21 Lecture 2: Units and the scales of energy use 6 The Basics: SI Units A tour of the energy landscape Units and Energy, power, force, pressure CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation Outline • The many forms of energy 8.21 Lecture 2: Units and the scales of energy use 7 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation F orms of Ener gy A long time• to disco v e r eneration. gy conserv Ener• disappears? gy No! Changing form... kinetic to potential to chemical toEinstein’thermal to s rest... ener gy In• f actation itconserv is of, a ener single gynumber characterizing a system or e v en each part of a system,through thattime can and be traced as it flothat wschanges, and mak esimportant ener gy in so physics. All these dif ferent• Units: forms must ha v e the× same units of mass length/2 time 2 ) Re vie w a little• 8.01 & 8.02 and see what’ s t o come 8.21 Lecture 2: Units and the scales of energy use 8 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation 1 2 × 2 KINETIC ENERGY 2 mv mass [speed] l 2 l2 [mass] × [speed]2 = m × = m t t2 Ener gyestf in mani mn o tio ORKW A NDTENTIAL PO ENERGY Fd force × distance ml l2 [force] × [distance]= × l = m t2 t2 orceWh acts ef n o a n a n objectistand cod e voes e it r a thatwo can r k ski h o net w i cupor ener as be s t gy ored pot entas i a l e ner g y 8.21 Lecture 2: Units and the scales of energy use 9 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation × f= orce × ORKW BRESSURE Y P PV pressurev olume areav olume m l2 [pressurev] × [ olume]= × l3 = m lt2 t2 1 ≡ 1 THERMAL ENERGY NR2 T 2 nkBT T ener gyThermal per de gree oftemperature freedomAl. ternatve atly, i † • n —• N number of moles —numbermolecules of • k 1.381 × 10−23 −1 —• theR gas constantR =8.31447 Joules B — Boltzmann’s Constant JK mole K • T • T — temperaturein K elvins — temperaturein K elvins Joules l2 NR[ ]= T moles × × K = m † mole K t2 Note thatmole thedeparts from MKS units:gram Itmolecular is a in eticK enll erf th a g eyes o mlecul o mak es s i t weight,molecular the weight of a appearanceas heat compound e xpressed in grams. 8.21 Lecture 2: Units and the scales of energy use 10 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation ) THERMAL ENERGY NR≡ TnkBT Con v enient,colloquialunit of enerthe gy: BTU 1 BTU = ener gy to heat 1 pound of ◦ ◦ OH2 fromto 60 61F ) ELECTRICAL ENERGY Q × V Q + ++++++++++++++++++++++++++++++ • Q — CHARGE V • V — ELECTROST A TICTENTIAL PO – – – – – – – – – – – – – – – – – – – – – – – – – El ectrostat i cEl⇒ potent ectr i c iF afi⇒ l e orce ld and the force does w ork on a char ge that is mo v e d . Ho w t o measureelectrostaticpotential in SI Units? potentialWhatCoulomb do you haof v char e to ge mo v ethrough one to get (or lose) 1 Joule of ener gy: 1 Joule 1 Joule/sec W1 att V1 olt= = = 1 Coulomb 1 Coulomb/sec 1 Ampere 8.21 Lecture 2: Units and the scales of energy use 11 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation W O F O R M S O FT E NER G Y T HA T MAMILIAR A Y... NO T B E A S F U AQ NTUMENERGY E = hν h =6.626 × 10−34 Js is Planck’• h s constant is the• ν frequenc y o f light. Ener• gyquantumone of of(photon). light [ener gy]h] ×[frequenc = [ y] 2 1 ml =[h] × t2 t ] =[h [ener×[time] gy] Js = SRI NE S E TEIS’ T N E N: E = mc R2 G Y Ein sati tein vl ’ ity sth r esh atd o m w a e lf ss is itseormf a f o ener gy with the s peed of light as the c. on v e rs ion f actor .. E = mc2 ml2 ]=[E mc2]= [ t2 8.21 Lecture 2: Units and the scales of energy use 12 The Basics: SI Units A tour of the energy landscape Units and Energy, power, force, pressure CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation Outline • rT anslations and scales: units and energy subcultures 8.21 Lecture 2: Units and the scales of energy use 13 The Basics: SI Units tour of the energy landscapeA Units and , force, pressure, powerEnergy CO2 and other greenhouse gases conversion The many forms of energy Common sense and computation There’ sthat n oquantity is other described in moreUnits of Energy and Power dif ferent units than ener gy .
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