Online reading assignment: internal energy conservation

Physics 205A, fall semester 2015
Cuesta College, San Luis Obispo, CA

Students have a bi-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.

"Internal thermal energy is not concerned with the macroscopic movement or rotation of an object, rather, the microscopic motion of the individual atoms and molecules within an object. Looks like we will be able to understand the physics of how objects heat up internally due to separate and internal forces of energy."

"Energy cannot be created or destroyed, thus the energy lost by a system (-Q) is equal to the energy gained by its surroundings (+Q), and vice-versa. When that energy is being transferred between objects of different temperatures it is called 'heat.' Heat comes from the internal energy of a substance, which is the sum of molecular energies, including kinetic and potential molecular energies. When bonds are broken within a substance its internal energy increases, and when bonds are formed its internal energy decreases. So for a particular substance, the gas form has more energy than the liquid form, which has more energy than the solid form."

"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. Also, if the thermal internal energy of a system increases, its temperature increases, and thus external heat from the environment is positive, being added into the system. In contrast, if the thermal internal energy of a system decreases, its temperature decreases, and thus external heat from the environment is negative, being removed from the system."

"Thermal internal energy is the measurement of movement of a systems individual molecules. This measurement relies on the objects mass, specific heat capacity and changes in temperature. The lower thermal internal energy the lower the temperature and vice versa. We are more concerned with changes in thermal internal energy rather than measuring amounts. Bond internal energy is much like gravitational potential energy in that it can store energy. Heat is the transfer of internal energy on a microscopic scale. Objects cool when hear is lost. Thermal equilibrium is reached when two objects become the same temperature and flow stops."

"The internal energy of a system is the total energy of all of the molecules in the system except for the macroscopic kinetic energy."

"I am familiar with using E_therm equation from chemistry."

"The idea that "heat: only refers to the energy actually in transit form hot to cold. Something cannot contain heat just internal energy the heat is the energy moving form hot to cold."

"There does not exist 'cold,' but only 'lack of heat' helps with understanding this topic."

"During vaporization, bonds are broken and the bond internal energy decreases. Similarly, during the reverse processes bonds are formed and bond internal energy decreases."

"We should be careful to not confuse this topic with the thermodynamics we learn in chemistry. Also, the total thermal internal energy of an object's atomic and molecular motion depends on its mass (m) and its temperature T in Kelvins, and has units of joules (J), not calories or kilocalories like in chemistry class. We are going to look at the initial-to-final changes in the internal energies of systems."

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.

"We covered thermodynamics extensively in my chemistry class earlier this semester, so most of the material in these sections was familiar. I'd like to see some applications of the transfer/balance equation, though."

"I got confused while I was reading about bond internal energy when you mentioned about atoms and molecules need to get closer in order to connect."

"The only thing that I found a little confusing was the the transfer balance equation and exactly what it meant. Everything else made sense to me, though."

"Almost everything."

"I could use more practice with the transfer balance equation."

"I did not have anything confusing with the reading assignment. But I did have trouble applying those concepts to the examples."

Two objects that are brought into contact with each other will reach thermal equilibrium when they have the same:

internal energy.	************* [13]
temperature.	*********************** [23]
(Both of the above choices.)	************ [12]
(Neither of the above choices.)	[0]
(Unsure/lost/guessing/help!)	** [2]

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.	********************************************* [45]
decreases; increases.	[0]
does not change; does not change.	* [1]
(Unsure/lost/guessing/help!)	**** [4]

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.	******** [13]
salt block.	*** [3]
(There is a tie.)	****************************** [30]
(Unsure/lost/guessing/help!)	**** [4]

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.	******************************** [32]
decreases; increases.	******* [7]
does not change; does not change.	****** [5]
(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.	***** [5]
water bath.	******** [8]
(There is a tie.)	******************************* [31]
(Unsure/lost/guessing/help!)	****** [6]

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.	*** [3]
decreases; increases.	** [2]
does not change; does not change.	**************************************** [40]
(Unsure/lost/guessing/help!)	***** [5]

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.	** [2]
pint of beer.	** [2]
(There is a tie.)	*************************************** [39]
(Unsure/lost/guessing/help!)	******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Can we conduct an experiment with boilermakers in class?" (Already did that, after finishing grading your midterms.)

"How is internal thermal energy related to temperature?" (The same way translational kinetic energy is related to speed. And gravitational potential energy is related to height. These are all types of energies, each of which changes when a certain observable parameter (temperature, speed, height) changes.)

"How do we know what the specific heat is for stuff? Is there an equation for 'c' that we are supposed to know? Or would that value be given/have to solve for in most equations regarding internal thermal energy." (Generally specific heat values for different materials are given to you, unless you need to solve for it. Treat it like any other property of a material that you can look up, or have to solve for.)

"The only part I find confusing, is when you asked which object experienced the greatest amount of change (increase or decrease) in thermal energy, I said that they were all tied because I believe that whatever one object loses, the other one should pick it up--but I believe I am getting confused about this part." (Assuming that the objects that are interacting are isolated from the environment, then you are correct.)

"Do you enjoy teaching?" (Yes. Enough to teach sixty more semesters of physics.)

"What were we supposed to read?" (#smh.)