Online reading assignment: radioactive decay rates

Physics 205B, spring semester 2018
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 presentations on radioactive decay rates.

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.
"Exponential and half life decay are both covered well in general chemistry so it's somewhat of a review. But the M&Ms® example is still helpful."

"I understood the introduction to radioactive decay and half-lives. I feel like most of this is from previous biology and chemistry classes."

"Thankfully, this presentation was a bit of a refresher from my other physical science classes. Radioactive dating helps us determine how long the radioactive substance has been around, assuming that there are no daughter atoms to begin with."

"I understand that older substances have more daughter atoms in them. Different elements have different half-lives."

"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."

"Radioactive decay can be used to determine the age of an unknown material. This is done using half-lives to measure the time these radioactive materials decay in the unknown material."

"Gaseous daughter atoms can be compared to the amount of radioactive atoms left in a sample to determine how long ago the sample started. After a molten sample solidifies, it will start anew with having radioactive atoms with no daughter atoms."

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 actually feel okay with this stuff thanks to chemistry and other classes I've taken that have dealt with radioactive decay. Going through the slides one more time will help me understand things more."

"There wasn't anything too confusing from the presentation preview. I have learned about radioactive decay in other classes before."

"I don't understand the equation for half-life decay, or radioactive dating."

"I would like to go over problems to get more familiar with the equations."

"I'm confused with the section on radioactive dating, or carbon dating. How exactly is the age of something determined?"

"I don't understand the process to calculate the half life of a material or why melting an object affects its half-life."

"How is the clock reset when matter is solidified after melting if gaseous daughter particles escape?"

"I am struggling with the melting and solidification ages."

"At the moment I am not sure what I find confusing. Most of the lecture made sense to me as far as I am concerned."

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


"Bq, or becquerel."



"Not sure."

For a radioactive decay process, the time constant τ ("average lifetime") is __________ half-life T1/2.
less than.  ** [2]
equal to.  ***** [6]
greater than.  ******************* [19]
(Unsure/guessing/lost/help!)  ** [2]

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: radioactive sample with daughter atoms. [45%]

Describe what changes in a sample when it melted and then solidified that resets its solidification age as determined by radioactive dating.
"Daughter atoms are released and are compared to the radioactive atoms that are left in the sample. Melting it resets its solidification age."

"Since daughter atoms (which are used to date the substance) are gaseous, when the substance is melted they are released. Therefore, they are no longer in the substance effectively resetting the age of the substance."

"There are technically no radioactive atoms or daughter atoms in the sample anymore, so it can reset its solidification age."

"Gaseous daughter atoms are released. This tells us how long ago the sample started with radioactive atoms with no daughter atoms."

"When a substance is liquified the once solidified daughter atoms are released into the air. When the substance is allowed to cool back into a solid the daughter atoms to not solidify back into the original state. This makes its a less reliable in radioactive dating."

"Not sure."


Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I really still think I need help with Lenz's law and Faraday's law."

"The semester after spring break is harder than I thought. But I think I will be able to catch up for the final."

"I need to play Half-Life again."

"I'm excited to eat M&Ms®!!!" (Wait for it--we'll get to do this in the last lab.)

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