Energy and the Environment Lecture 7 PHYS/ENVS 3070 Professor Dmitri Uzdensky Sept. 10, 2014 PHYS/ENVS 3070 1Announcements: • Finish reading Chapter 1 of E&E textbook. • CAPA Homework #2 due this Friday, Sept. 12, 11pm. • Comments about this homework problems: • Problem 5 (water heater): typo in textbook’s inside cover correct value: 1 gallon = 3.785 liters. • Problem 7 (Boulder County population growth): this problem assumes exponential growth. • If you’re still puzzled by exponentials, read Appendix A.1 in the E&E book. If this doesn’t help, talk to a 3070 TA in the Help Room or come to my office hours. Sept. 10, 2014 PHYS/ENVS 3070 2Reminder about Physics Help Room: • Physics Help Room (Duane Physics Bldg, Room G2B90): sign in with your name and write the time of your request on the chalk board for 3070. Sept. 10, 2014 PHYS/ENVS 3070 3 • "Help"Room"TA"hours"(link"on"the"course"website):""h:p://www.colorado.edu/physics/helproom"Today’s(Lecture:(• Power"• AddiAonal"energy"units:"calories"and"BTUs"• …"Other"forms"of"energy:""– Thermal"– E"="mc2"– Nuclear"Energy"– ElectromagneAc"RadiaAon"Energy"Sept."10,"2014" PHYS/ENVS"3070" 4"POWER Power is the rate of doing work or expending energy. It is the energy given to or taken away from an object per unit time. timeEnergyPower =Power is a rate. It is not the same as energy! Analog: You might get paid a salary as a rate (e.g. $10 / hour). If you work 40 hours in a week, you have earned $400. The $400 is like energy and the $10/hour is like power. timePower Energy ×=Sept."10,"2014" PHYS/ENVS"3070" 5"Units"of"power:"1"Wa:"="1"W"="1"Joule/sec"1"kiloXWa:"="1"kW"="1000"W.""Clicker Question 1 2 3 4 5 6 7 8 9 108070605040302010Total (cumulative) Energy consumedEnergy(Joules)Time in minutesAt what time is the instantaneous Power consumption the maximum? A) time = 2 minutes B) time = 5 minutes C) time = 9 minutes D) It is the same at all times shown. Power = Change in Energy / Change in Time = slope on graph. Sept."10,"2014" PHYS/ENVS"3070" 6"Room"Frequency"DC"Electric Bill I buy electricity from the “Power Company” Here in Colorado My bill charged me for approximately 1000 kiloWatt-hours. Note that a kiloWatt-hour is (Power [kWatt]) x (Time [hours]) and is thus a unit of energy. So Xcel does not charge me for power, they charge for my total electrical energy usage. For example, they do not care if I used 1000 kWatts for one hour or 100 kWatts for 10 hours, etc. They only care about the total energy. Sept."10,"2014" PHYS/ENVS"3070" 7"Note that 1 kWatt-hour (kWh) can be converted into Joules. E = (1kW − hour) =1000 Jsec× (1hour)×60 min1hour×60 sec1minE =1000 J × 3, 600 = 3.6 ×106J = 3.6 MegaJoulesXcel charges approximately $0.09 (9 cents) per kWatt-hour. Thus, since I used ~ 1000 kWatt-hours, my electric bill was for approximately $90 for one month. Sept."10,"2014" PHYS/ENVS"3070" 8"KiloWatt-hours and JoulesDoes that energy usage make sense? If I had ten 100 Watt light bulbs on all the time for one month, how much energy is that? E = 10 ×[(100Watts) × 1month( )]However, we want kWatt-hours, not Watt-months. So we need to use the fact that there are 720 hours per month. ×720 hours1monthE = 720 kiloWatt-hoursClose to my usage. Sept."10,"2014" PHYS/ENVS"3070" 9"Where"is"the"energy"given"on"this"label?""New"Energy"Unit"="Food"Calorie"or"Calorie""1"Food"Calorie"="4184"Joules"Energy(Units:(Calories(Sept."10,"2014" PHYS/ENVS"3070" 10"Energy(Units:(Calorie(and(calorie !(calorie:""""1(cal(=(4.184(J(≈(4.2(J(((Energy"needed"to"change"the"temperature"of"1"gram"of"water"by""1°C""Calorie(or(Food(Calorie((or(kiloCcalorie):((Energy"to"change"the"temperature"of"1"kiloCgram"of"water"by"1°C"*"Thus"a"Food"Calorie"is"actually"the"same"as"1000"calories"(which"is"the"physics"or"chemistry"term)."Sept."10,"2014" PHYS/ENVS"3070" 11"So"really"that"is"1000"calories!"Sept."10,"2014" PHYS/ENVS"3070" 12"BriGsh(Thermal( Unit((BTU)("Energy"required"to"change"the"temperature"of"1"poundXmass"(454"g)"of"water"by"1°F.""1"BTU""="1,055"Joules."Sept."10,"2014" PHYS/ENVS"3070" 13"Table 2.6 says that the best coal (Anthracite) which is 88% carbon, has an energy content of 13,300 Btu / lb. By how many degrees can you raise the temperature of 260 pounds of water with the energy from one pound of coal? A) 5 degrees Fahrenheit B) 50 degrees Fahrenheit C) 250 degrees Fahrenheit D) 3000 degrees Fahrenheit E) 40 degrees Celsius"Clicker(QuesGon(13,300 Btu raises the temperature of 1 lbs of water by 13,300 0F OR 13,300 Btu raises the temperature of 260 lbs of water by 13,300/260 = 50 0F Sept."10,"2014" PHYS/ENVS"3070" 14"Room"Frequency"DC"1"Jelly"donut""(250"Calories)"CO2""+"H2O"+"Energy"How"many"Joules"of"energy"is"this?"⎟⎠⎞⎜⎝⎛×⎟⎠⎞⎜⎝⎛×calorie 1Joules 184.4Calorie 1calories 1000 Calories 250≈ 106 Joules !Sept."10,"2014" PHYS/ENVS"3070" 15"Biological"mechanism"needed"for"efficient"conversion."What"if"we"compare"this"1"million"Joules"from"the"donut,"to"the"PotenAal"Energy"of"a"60"kg"person"at"a"given"height"(H)."What"is"that"height"H?"mgHPE =)/8.960()10(26smkgJoulesmgPEH×==)/8.960()10(26smkgJoulesmgPEH×==feet 5000or meters 1700=HSept."10,"2014" PHYS/ENVS"3070" 16"DemonstraAon"rubbing"your"hands""1. Work"is"done"by"the"person"to"give"KineAc"Energy"to"the"hands."2. When"the"hands"stop"moving,"where"did"the"energy"go?"Other(forms(of(Energy?(Sept."10,"2014" PHYS/ENVS"3070" 17"Heat"was"not"considered"a"form"of"energy"unAl"the"midX1800’s.""Prior"to"that"it"was"thought"of"as"some"type"of"fluid"that"made"things"hot.""In"fact"heat"energy"(also"called"thermal(energy)"is"really"just"a"microscopic"form"of"kineAc"energy. "Another(Form(of(Energy(=(Heat(!(Sept."10,"2014" PHYS/ENVS"3070"
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