Section 6.1 of your text introduces some of the commonunits that are used when d

Question

Section 6.1 of your text introduces some of the commonunits that are used when discussing energy and power. The nextthree questions (a-c) give you practice in using these unitsand converting between them. a. Thenutrition label on a box of crackers states that there are 64.0Calories per serving. A serving consists of four crackers. If youwere to eat fourteen of these crackers, how many kilojoules ofenergy would you consume ? b. The watt is aunit that is used to measure electrical power. Using 6 watts for 10seconds gives 60 joules of energy. (You can now determine therelationship between watts, seconds, and joules.) If a 75 wattlight bulb was left burning until it consumed a total energy of3920.0 kJ, how long did it burn ? Give your answer in hours. c. A localutility company charges 12.0cents per kilowatthour (kWh). Note thatkilowatt hour is kilowatts times hours (not kilowatt/hour). Whatwould be the cost of the energy consumed by one hundred 75.0W lightbulbs, each of which burned for the same number of hours as the onein the above question ? Giveyour answer in cents (1/100th of a dollar) to the correct number ofsignificant figures. a. Thenutrition label on a box of crackers states that there are 64.0Calories per serving. A serving consists of four crackers. If youwere to eat fourteen of these crackers, how many kilojoules ofenergy would you consume ? b. The watt is aunit that is used to measure electrical power. Using 6 watts for 10seconds gives 60 joules of energy. (You can now determine therelationship between watts, seconds, and joules.) If a 75 wattlight bulb was left burning until it consumed a total energy of3920.0 kJ, how long did it burn ? Give your answer in hours. c. A localutility company charges 12.0cents per kilowatthour (kWh). Note thatkilowatt hour is kilowatts times hours (not kilowatt/hour). Whatwould be the cost of the energy consumed by one hundred 75.0W lightbulbs, each of which burned for the same number of hours as the onein the above question ? Giveyour answer in cents (1/100th of a dollar) to the correct number ofsignificant figures.

Explanation / Answer

a .   (64.0cal /4)x14 = 224 cal          224cal = 224cal (4.18J/1cal)                     =936.32 J                    =0.936kJ .............................. b . Power = Energy /Time      6watt = 60 J/10s         75watt = 3920 kJ /Time in seconds     75watt = 3920 x103J /Time inseconds     Time in seconds = 3920x103J / 75watt                               =52.27 x 103 Time in Hours   = (52.27 x 103s)/(360s/1Hr)                          = 145.19Hr                           .................................. c . The energy consumed by one hundred 75.0W lightbulbs, each of which burned for 145.19Hr is                              energy consumed = 75.0watt *145.19Hr *100                                                          =  75.0watt*145.19Hr(360s/1Hr) *100                                                         = 392013000watthour                                                          =392013 kWh      cost = 392013 kWh*12.0cents/kWh            = 4704156 cents Time in Hours   = (52.27 x 103s)/(360s/1Hr)                          = 145.19Hr                           .................................. c . The energy consumed by one hundred 75.0W lightbulbs, each of which burned for 145.19Hr is                              energy consumed = 75.0watt *145.19Hr *100                                                          =  75.0watt*145.19Hr(360s/1Hr) *100                                                         = 392013000watthour                                                          =392013 kWh      cost = 392013 kWh*12.0cents/kWh            = 4704156 cents

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