class07lecnotes

class07lecnotes - ~ J., rl1~~ I ~ 22.) lolo ~J~J...

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~ J., rl1~~ I ~ 22.)· lolo ~J~J " (6. l I /1. (": /rfu,rtuh? /1u'~ ) Ii. 1- Example 1. Calculate the thennal energy in one unit volume of air, making the assumption that it is composed entirely of nitrogen molecules. kr I'J l!!- ) z.. Efh:: lA;~ktlAc. .. = If. /m ~z.:: J:!. "J((z. ../tL;: /IRi;. f!.!31) z tit L Example 2. What is the thennal energy of one unit volume of the solar corona, assuming it only contains protons (which is not actually true)? n::: /0 ' / / 1??.3 J;.(. /J 61< ) j [it, ,. ~. /tI'Ie~ t:l. ~ > 41. .: t n.l T.t .;:. 10 IV. /'1f10}!:. NDX fo nXl ... "fki :: l~'IO-r)J A.' ""1 {.udl- , ,t~ f.L ~ I} 4 ~ rx 2 ~ Comparing the abOve results with each other it becomes obvious that the corona is a an extremely tenuous medium, or reinforce the importance of air! The Equivalence of Work and Heat A: How can you increase the temperature of your nose? This can be done either by rubbing it, or by holding it up above a flame (or any other source of heat), #13 o Paul C lIarrls 2000 Rubbing your nose means you do work on it. The other case involves heat transfer. Both have the same effect on your nose: They cause its temperature to increase. As a result the thennal energy of your nose increases. A: How much rubbing do you need to do in order to increase the temperature of your nose by 1°C? This is the mechanical equivalent of heat. In your lab you will calculate the work that needs to be done on a cylinder in order to raise its temperature by a given amount. By dividing your finding by the raise in temperature achieved, you can calculate the mechanical equivalent of heat for this object. Heat is transferred between two objects as a result of the difference between their temperatures. Heat requires no macroscopic motion of the system. On a macroscopic 1.1
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level, adding heat to a system increases the kinetic energy of its molecules (atoms). The temperature goes up. The thermal energy of the system also goes up. We can have the following three situations: When the temperature of the ambient medium, Tout, is the same as that of the object, Tin, the object is in thennal equilibrium with the ambient medium. No heat exchange takes place in this case. A: What other case of equilibrium did we study in this class thus far? ijydrostatic equilibrium (pressure balance between different points within a system). The SI units for heat are Joules. lJoule=lkg m 2 /s 2 An alternate unit is the calorie, which you will be using in your lab. 1 calorie is the amount of heat required to change ) the temperature of one kilogram of water by 1°C under STP. ! 1/;wr1(V1d. tJwp~~,-( J./ ~rl: lcal::4.186 J The food calorie, Cal, differs from the calorie defined above. lCal=I000cal. It represents the amount of energy made available to your body by eating a particular food. For example, eating one bar of Lindt bittersweet chocolate, gives your body 210 Cal, which is 21Oxl000x4.186J=8.73xldt This is horrifying, considering that the entire thermal energy in one m 3 of air is 1.93x1OSJ. so more than 4 times less. Every time you eat a
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class07lecnotes - ~ J., rl1~~ I ~ 22.) lolo ~J~J...

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