## 20141201

### Online reading assignment: internal energy conservation

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on internal energy conservation.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"The thermal internal energy of an object is based on the movement of the atoms of the object. The faster the atoms move, the higher the thermal internal energy of the object. As objects heat up the bonds that hold the object together can break apart, examples of this are ice melting, or water evaporating. Heat will travel from one object to another in an attempt to create an equilibrium."

"Thermal internal energy is dependent on temperature. If there is no energy transferred into or out of the thermal internal energy of a system, then it is effectively thermally isolated from the environment, and the heat exchanged between the system and the external environment is zero."

"Internal energy conservation, or where chemists and physicists throw down. Physicists are concerned with energy changes due to transfer of energy between systems."

"We went over Q = m·c·∆T. I also learned you could cook seafood on a large and heated salt block."

"Thanksgiving week took a toll."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I didn't get how °C and K could be used interchangeably for heat capacity and specific heat. I do not understand the reason of 'only temperature changes are involved.'"

"I am confused about how to use the equations. I understand the concepts of the heat and energy but am confused about how to relate them to their equations."

"I found the transfer/ balance equation to be a little confusing when trying to apply it to the examples in the presentation. The example about seafood, in particular, was confusing because I thought that if the seafood was getting hotter (from the block) then it is gaining internal thermal energy and then the block would be losing it. But why are we not concerned with the energy lost to the environment? And I don't understand how to determine which one loses more energy."

"I am confused on in which object would have the most amount of change when involving two objects at different temperatures. I am pretty sure that they would each have the same amount of internal energy change since they are transferring the energy to each other, but I am not sure if this is the same if two objects have the same temperature."

Two objects that are brought into contact with each other will reach thermal equilibrium when they have the same:
 internal energy. ******** [8] temperature. *********************** [23] (Both of the above choices.) ********************* [21] (Neither of the above choices.) [0] (Unsure/lost/guessing/help!) ***** [5]

Raw seafood is placed on a block of salt that has already been heated up. The energy contained in the high-temperature block of salt is then transferred to the seafood, cooking it. While it is being cooked, the internal thermal energy of the seafood __________, while the thermal internal energy of the salt block __________.
 increases; decreases. ************************************************** [50] decreases; increases. ** [2] does not change; does not change. [0] (Unsure/lost/guessing/help!) ***** [5]

For the seafood cooking on the salt block (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
 seafood. ********************* [21] salt block. *** [3] (There is a tie.) ************************** [26] (Unsure/lost/guessing/help!) ******* [7]

Frozen meat is placed in a water bath, in order to defrost it. At the very start of this defrosting process (where the frozen meat just begins to warm up from its below-freezing temperature, and the ice crystals inside have not yet reached the melting point), the internal thermal energy of the meat __________, while the thermal internal energy of the water __________.
 increases; decreases. **************************************** [40] decreases; increases. ********* [9] does not change; does not change. ** [2] (Unsure/lost/guessing/help!) ****** [6]

For the frozen meat in the water bath (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
 frozen meat. **************** [16] water bath. ************ [12] (There is a tie.) ********************** [22] (Unsure/lost/guessing/help!) ******* [7]

A shot of whiskey is mixed with a pint of beer to make a boilermaker. Assuming that the whiskey and beer have approximately the same temperature before they are mixed together, the internal thermal energy of the whiskey __________, while the thermal internal energy of the beer __________.
 increases; decreases. ** [2] decreases; increases. **** [4] does not change; does not change. ******************************************* [43] (Unsure/lost/guessing/help!) ******** [8]

For the shot of whiskey being mixed with the pint of beer (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
 shot of whiskey. ****** [6] pint of beer. ** [2] (There is a tie.) **************************************** [40] (Unsure/lost/guessing/help!) ********* [9]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why is it that the bond internal energy increases when the bonds are broken? And why does it decrease when the bonds are closer? Using the analogy, doesn't a spring have greater elastic potential energy when it's compressed?" (That's true for compressing springs shorter than their equilibrium distance, but starting with the bond distances between atoms at absolute zero, adding energy to these bonds (increasing their temperature) will increase their separation distances. Add enough energy, then these bonds will be stretched enough to break apart.)

"I'm not sure which objects lose or gain more thermal energy. If we ignore the environment will they each have an equal amount of change?" (Yes, if there are only two thermal internal energy terms in the transfer/balance equation.)

"Does thermal energy go down when temperature goes up?" (No.)

"In order to have a change in temperature, there must be a transfer of energy between systems. Considering that, is there a way to absolutely isolate a system to prevent zero energy transfer?" (It is not possible to completely, thermally isolate a system from its environment. But you can get pretty close.)

"How we are supposed to determine which object experiences the greatest amount of decrease in thermal internal energy if we are not given any set numbers?" (With physics. Which we'll do in class today.)

"Heating and cooling is obviously the only way that thermal internal energy can be added or removed right?" (Yes, whether exchanging heat with the environment, or with another object in contact with it.)

"Excited to see how physics is applied to what chemistry has taught me about thermal energy."

"This presentation made me hungry." (My job is done.)

"Is the final cumulative of everything we have learned so far? What concepts will be on the final?" (Yes, but there will only be seven questions, so there will only be seven concepts to study for. Wait until next week for the study guide to be posted.)

"Are we going to do an experiment involving boilermakers this week?" (I already did, last week. After I graded your midterms.)