## 20160425

Physics 205B, spring semester 2016
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.
"The 'activity' is the number of atoms that decay during a time interval. Half-life is the time it take for one-half of a sample to decay. The decay constant is the likelihood that a particular atom will decay in certain time interval. Finally, the time constant is the average lifespan of an atom in a sample."

"I understand quite a bit about half-life and radioactive decay. I've taken chemistry courses about the purposes behind radioactive dating and how to use the half life equations. That's essentially all it is, is understanding how to use the half life equations in certain situations to solve the problems. But i guess that's all what physics is. It is knowing which equation to use for each particular situation."

"Not much."

"I thought it was really interesting to find out how the older a substance is, the more daughter atoms it has. Also finding the age of the material was really interesting. It all has to do with the daughter atoms, something I didn't even know existed."

"Not sure what I do know."

"That radioactivity is a relation of atoms the are stablizing slowly or rapidly and give off energy as a result. I get that different elements will have different half-lives."

"Decay is exponential."

"In order to determine the rate at which radioactive nuclei disintegrate within an object is like taking me back to stats class; you basically have to collect data for a given amount of time until one can come up with a relative decay/activity rate that he/she is confident in. After this collection of data has been made then one can come up with a relative decay/activity constant which they can later reference."

"Radioactive decay counts the number of disintegrations per half-life. Each half life theoretically activates one-half of an isotope."

"In this case, R0 is the initial rate of decays per second at t = 0. We can use a Wolfram Mathematica template with our M&M™ data to find an expression for the decay constant λ, which for a statistically large sample is the probability that a given single M&M™ will decay in a given time interval. "

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.
"The different types of decay such as exponential and activity? I have no idea what is going on with the equations or constants. Help!"

"Nothing really confuses me in this section."

"What does e stand for in the decay equation?"

"I'm so lost I don't know what to ask"

"I feel I just need to reread the chapter a couple of more times, and I will really get it. It wasn't that difficult when I really sat down to it. All I need to do is practice more."

"I actually understand this stuff because of chemistry. :) Yay!"

"I thought that everything with the half life seemed pretty straight forward. Im still really confused by all of the hand rules and fields and stuff so I'm really trying to wrap my mind around that first before the quiz/midterm."

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

"Curies?"

"Lambda?"

"It is decays/sec."

"The SI unit for radioactivity is the becquerel (Bq)."

"The becquerel (Bq), one of which is 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. ******** [8] greater than. ********************** [22] (Unsure/guessing/lost/help!) **** [4]

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). [24%]
Sample with older solidification age: adioactive sample with daughter atoms. [57%]

Describe what changes in a sample when it melted and then solidified that resets its solidification age as determined by radioactive dating.
"The radioactivity is what changes here."

"Can't say I know =/"

"Not sure sounds cool."

"Gaseous daughter atoms are trapped within a sample over time. When this sample is melted, then these gaseous daughter atoms are released and are compared to it's original non-daughter atom state. By melting the object, you are transforming the daughter atoms into a gaseous state and releasing them from the object they are trapped in. Once resolidified, there will not be any daughter atoms within the object."

"After a molten sample solidifies, it will start of again having radioactive atoms with no daughter atoms. So, melting a sample 'resets' its solidification age."

"When it is melted then solidified then the gaseous daughters are released. Because of this it resets the solidification age."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I assume it's no coincidence that this class and my chemistry class are almost always covering the same topic at the same time?" (But physics class will make it more fun.)

"Radioactive decay? I think I recall covering this in chemistry class. But that was ages ago."

"Honestly , I'm just ready for school to be over. I am so done with life!"

"When a sample is melted to restart the timer is that melting the daughter element/compound? Will the daughter atoms always be released in a gaseous form? If not, then would you not be able to date the material?" (Typically, the daughter elements are either helium or radon gas, which would bubble out and be released from the sample before it cools and hardens. These gases do not naturally occur inside of rocks unless they were produced by radioactive decays, so they can be used for radioactive dating. Other elements used for radioactive dating that aren't gaseous must specifically be daughter elements that would not naturally occur inside of a sample.)

"I understand we may get to eat some M&M™s in class? Other than that I am lost."

"I have a problem with eating M&M™s since I do not like chocolate." (#wut)

"I was really confused on what the solidification age of a substance tells us? I don't really get it." (It tells you how long it has been since that sample cooled and hardened. Geologists always want to know how long a sample has been solidified, as it reveals when a volcano last erupted, etc.)

"How did physicists/chemists before us determined the half-life of specific substances? I understand that they can collect data and perform a decay rate test, but how accurate are all of the tests done across the board?" (You don't even need to know how many radioactive atoms you have in your sample, you just need to record how the decay rate changes over time to find out the half-life of that atom. We'll do this with M&M™s, where we don't know their half-life, but we'll observe how they "decay" over time to do an exponential curve-fit to empirically find out the decay constant and their half-life. The larger your sample, the more accurate your half-life will be)

"Currently traveling at 36 m/s so I'm unable to complete this assignment at the moment." (80 mph? #wut)