Activity 4 Solutions: Transfer of Thermal Energy

Transcription

1 Aciviy 4 Soluions: Transfer of Thermal nergy 4.1 How Does Temperaure Differ from Thermal nergy? a) Temperaure Your insrucor will demonsrae molecular moion a differen emperaures. 1) Wha happens o molecular moion a higher emperaures? Molecules move faser a higher emperaures 2) Define emperaure in erms of molecular moion. Temperaure in Kelvin is a measure of he average kineic energy of he molecules of a subsance b) Thermal nergy 1) Wha is hermal energy? How does hermal energy differ from emperaure? Thermal energy is he TOTAL inernal energy of he aoms and molecules of a subsance. Temperaure is he AVRAG kineic energy of he molecules of a subsance. 2) Which has more hermal energy a cup of ho coffee or a bahub full of warm waer? The coffee is a a higher emperaure, bu he bahub has more hermal energy because he bahub conains many more molecules. c) Thermomeers xamine he four ypes of hermomeers and explain wha changing propery each ype of hermomeer relies upon. 1) alcohol hermomeers expansion of liquid alcohol when heaed 2) bimeallic srip hermomeers expansion of meal srips a varying raes when heaed 3) liquid crysal hermomeers crysals change color when heaed or cooled 4) infrared hermomeer frequency of he radiaion d) Temperaure Scales Your insrucor will discuss Fahrenhei, Celsius and Kelvin emperaure scales. 1) xamine a hermomeer wih boh Fahrenhei and Celsius scales. On he Celsius scale, how many degrees are beween he freezing poin and he boiling poin of waer? 100 degrees C 2) On he Fahrenhei scale, how many degrees are beween he freezing poin and he boiling poin of waer? _180 degrees F_ 3) Use he number of degrees beween he freezing and boiling poins of waer o make a raio of he number of Celsius degrees per Fahrenhei degrees. 100 Celsius degrees 5 Celsius degrees 180 Fahrenhei degrees 9 Fahrenhei degrees 1

2 2 1/11/05 4) Wrie an equaion o conver degrees Fahrenhei ino degrees Celsius. Use your raio from par 3 plus he fac ha he freezing emperaure of waer in he Celsius scale is 32 degrees lower han in he Fahrenhei scale. T C 5/9 (T F 32) 5) Use your equaion o conver 70 degrees Fahrenhei ino Celsius degrees. T C 5/9 (70 32) 5/9 (38) 21 0 C 6) In he Kelvin scale, waer boils a 373 Kelvin and freezes a 273 Kelvin. A change in how many degrees Celsius equals how much of a change in Kelvin? 1 Celsius degree 1 Kelvin degree 7) Wrie an equaion o conver degrees Celsius o Kelvin. T K T C e) Group Discussion Quesion: Which emperaure scale gives he greaes disincion beween emperaure degrees Fahrenhei, Celsius, or Kelvin? 4.2 How Is Thermal nergy Transferred? a) Transferring Thermal nergy Wha is he one essenial condiion for he sponaneous ransfer of hermal energy beween wo objecs? The objecs mus be a differen emperaures. b) Conducion 1) Before waching he demonsraion, predic he order in which he seel balls will fall off of a meal rod when i is heaed. Predicion: Answer: The ball closes o he hea source falls off firs. The ball farhes from he hea source falls off las. 2) Wha are he necessary condiions for hea ransfer via conducion beween wo objecs? The objecs mus be a differen emperaures and mus be ouching. c) Thermal Conduciviy Your insrucor will discuss hermal conduciviy 1) Before waching he demonsraion, predic he order in which he seel balls will fall off of rods made of differen meals. Predicion: Answer: _The ball aached o he rod wih he highes hermal conduciviy (copper) falls off firs. The ball aached o he rod wih he lowes hermal conduciviy falls off las._ 2) Touch he glass, meal, and cork squares. a) Do he squares feel as if hey are all a he same emperaure? No, cork feels warmes and meal feels cooles.

3 b) Measure he emperaure of he squares wih an infrared hermomeer. How do heir emperaures compare? All are a room emperaure c) Why do he squares feel as if hey are a differen emperaures? Maerials wih a high hermal conduciviy, such as meal, are beer a conducing hea away from your hand, so hey feel colder han maerials wih a lower hermal conduciviy, such as cork. 3) Your insrucor will place ice cubes on wo black squares on your able. Wha happens? Why? The ice cube on he aluminum block mels more quickly han he ice cube on he foam block because aluminum has a higher hermal conduciviy han foam. d) Convecion Wach he demonsraions of hermal energy ransfer via convecion 1) Wha are he necessary condiions for hermal energy ransfer via convecion? There mus be a difference in emperaure and difference in he densiy of he subsances he less dense subsance rises, producing a convecion curren. 2) Does convecion involve a ransfer of maer? yes 3) Does conducion involve a ransfer of maer? no e) Radiaion Place he flood ligh an equal disance from he wo cans fied wih balloons. 1) Which balloon inflaes firs? black can Why? The black can absorbs more of he radian energy from he floodligh. The shiny can reflecs more of he radian energy. 2) Why is he inside of a hermos silver-colored? So ha he walls of he hermos absorb less of he hermal energy conained in he conens of he hermos. 3) Does hermal energy ransfer via radiaion involve a ransfer of maer? _no_ 4) Does hermal energy ransfer via radiaion require objecs o be ouching? _no_ f) xamples of hermal energy ransfer Place a small paper cup of waer on he screen of he meal sand. Ligh he burner wih a mach and carefully move he burner under he paper cup. 1) Does he paper cup burn? _no very much_ Why or why no? Thermal energy from he flame is used o hea he waer o boiling. The waer emperaure does no rise above he 3

4 boiling poin. This emperaure (100 0 C or F) is lower han he combusion emperaure of paper. 2) Wha do you hink would happen if he paper cup were full of pennies insead of waer? The cup would burn since he meling poin of pennies is higher han he combusion poin of paper. 3) Wha forms of energy ransfer are involved? The flame gives off radian energy, some of which reaches he boom of he cup. Hea from he flame rises via convecion o he screen and he cup. Conducion from he heaed screen heas he waer and he cup. Convecion currens in he waer also ranspor hea o he surface of he waer in he cup. 4.3 How Can Thermal nergy Transfer Be Minimized? a) Hea flow hrough a surface 1) Wha facors deermine how much hea flows hrough a surface, such as a glass window? he hickness of he window (L) he hermal conduciviy of he glass (K) he area of he window (A) he difference in emperaure beween he wo sides of he glass (T ho T cold ) 2) Wrie an equaion for hea flow hrough a surface. K A ( T T L 3) How much hea flows hrough a glass window ha is 2 meers by 2 meers in area and 1.5 cm hick if he ouside emperaure is 10 0 C and he inside emperaure is 25 0 C? (The hermal conduciviy of glass is 0.84 J/s m 0 C ) K A ( T T L (0.84 J/s m 0 C) x (4 m m 2 ) x ( 15 0 C) 3,360 J/s b) R-value of insulaion xamine a piece of home insulaion. The R-value of a maerial is a raio of wo variables: he hermal conduciviy of he maerial K and is hickness L. 1) Use raio reasoning o wrie an equaion for R so ha good insulaing maerial has a larger R-value han poor insulaing maerial. 4

5 The hicker he maerial, he less hea flow hrough i. Therefore, R and L are direcly proporional. The larger he hermal conduciviy, he greaer he hea flow. Therefore, R and K are inversely proporional. R L / K 2) Rewrie your equaion for hea flow from par a.3, using R insead of L and K. A ( T T R 3) Wha would happen o he hea flow hrough a wall if you increased he hickness of he insulaion from 2 inches o 6 inches? Assuming he oher variables are no changed, he hea flow would be 1/3 of wha i was wih 2 inches of insulaion. c) Changing emperaures and properies of maer 1) Predic some properies of maer ha you hink change wih changing emperaure. 2) Wach he demonsraions of maerials cooled wih liquid nirogen. Lis changes you see in he properies of maer cooled o low emperaures. Maerials become hard or brile and conrac 5