## 20170424

Physics 205B, spring semester 2017
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.

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.

"Honestly, not much. This whole concept is very foreign to me."

"Half-life is the time for one-half of a statistically large sample to decay. Half-life is proportional to the time constant and the decay constant."

"I totally understand how half lives work because I have learned it before in other classes like chemistry and geology. Those classes really helped with this lecture!"

"I understand half-life and radioactive decay as I have had it in math."

"When there are more daughter atoms the sample is older. The daughter are released when the sample that is being melted, which will then determine the age of the material/sample."

"After a molten sample solidifies, it will start anew with having radioactive atoms with no daughter atoms. Melting a sample 'resets' its solidification age."

"Haven't got to it yet."

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 would like more practice in obtaining the half-life given the time constant. Also, I want to know how equations look for radioactive dating or if you just count up the ratio of daughter atoms to radioactive atoms."

"What solidification age is and how was supposed to know which had an older solidification age."

"The concepts seem pretty easy to understand. Doing some problems to see how the different constants work would be great!"

"Very confused as to what is going on and am having trouble understanding activity (half-life decay) and half life equations."

"It all is pretty confusing, but it should clear up after lecture."

"Not sure yet."

"I'm good."

State the SI units for activity (radioactive decays per time).
"Becquerel."

"Bq."

"I believe that it is Bq = 1 decay/second."

"A becquerel, Bq, is the unit for one disintegration per second."

For a radioactive decay process, the time constant τ ("average lifetime") is __________ half-life T1/2.
 less than. *** [3] equal to. ************ [12] greater than. ******** [8] (Unsure/guessing/lost/help!) * [1]

Two samples are each comprised of 800 unstable atoms that will undergo radioactive decay. The remainder of one sample is 200 inert, stable atoms not involved in a radioactive decay process. The remainder of the other sample is 200 daughter atoms of the radioactive decay process.
(Only correct responses shown.)
Sample with more activity (decays/time): (there is a tie). [21%]
Sample with older solidification age: adioactive sample with daughter atoms. [75%]

Describe what changes in a sample when it melted and then solidified that resets its solidification age as determined by radioactive dating.
"Gaseous daughter atoms are released. This tells us how long ago the sample started with radioactive atoms with no daughter atoms."

"After a molten sample solidifies, it will start over with having radioactive atoms with no daughter atoms. Melting a sample restarts its solidification age."

"Not sure what is meant by 'solidification age.'"

"Uhh...what?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Not sure what the relationship between half-life and the time constant is." (As long as you realize they're not the same thing, but can be converted from one to the other (by a factor of ln(2)).)

"Is exponential decay just an overview and half-life is more specific?" (They're related to each other mathematically, as are the exponential decay and half-life decay equations. Both either will give you the same results computationally.)"

"I have always associated half-life with chemistry. It is interesting to explore this concept from a physics standpoint."

"Do you prefer Rockstar or Monster energy drinks?" (RockStar sugar-free tastes much better than Monster sugar-free.)