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I disagree with my biology teacher please help

  1. Jan 30, 2007 #1
    What is the meaning of "radioactive decay"?

    What is the proper use of the meaning of "radioactive decay"? My teacher states it is the amount of decay that has occured and I think it is a measurement of the "rate" of decay while it is being measured. I have re-phrased this question from an earlier post which gives the specifics of the disagreement. I know the definition of the term but what is its usage,,,, Who's right?

    To whom it may concern:
    I am having a debate with my biology teacher. I was asked the following multiple choice question:

    Question: Radioactive isotopes are used in dating materials from the distant past. Which of the following statements is accurate about radioactive dating techniques?

    1. It gives relative ages of rock strata.

    2. It gives exact ages of rock strata.

    3. It uses a technique in which the degree of radioactive decay is measured, the younger the rock the more radioactive decay.

    4. It uses a technique in which the degree of radioactive decay is measured, the older the rock the more radioactive decay.

    5. It does not work well with fossil remains that have not absorbed radiation because scientists cannot measure the radiation with any degree of certainty.

    This is the question exactly as it was posed on the test. I picked answer 3. and got it wrong. The teacher states the corect answer is 4. I think he is wrong. The accepted use of the term "radioactive decay" is not how much decay HAS occured but rather how much decay is going on when it is being measured. There is no such device as a "Decay-o-meter" and his use of the term "radioactive decay" is in error. He has improperly phrased the answer and no reference to "past-tense" is infered Who's right in this matter? Please help.
    Last edited: Jan 30, 2007
  2. jcsd
  3. Jan 30, 2007 #2
    Well, as things get older, the number of radioisotopes decrease. And fewer radioisotopes means fewer radioactive decays. Assuming that I'm understanding this definition of radioactive decay correctly, it looks to me like the answer is #4.

    Then again, #2 would also be a good choice, assuming that the phrase "exact ages" allows for the fact that all scientific data have an associated error.
  4. Jan 30, 2007 #3
    I am hung up with the first part of the sentence which states it is being measured. Therefore, wouldn't the measured rate of decay be less the older the rock gets?
  5. Jan 30, 2007 #4
    What is the proper use and meaning of "Radioactive decay"? How much decay HAS occured? or how much decay IS occuring?
  6. Jan 30, 2007 #5
    The teacher is right, and in the context of the problem #4 is the best answer. I would call your concept "the rate of radioactive decay".
  7. Jan 30, 2007 #6
    Y'know, you could have just got your English teacher to explain this one.
  8. Jan 30, 2007 #7


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    The teacher is WRONG. And that's only because the wording of choices 3 and 4 is ambiguous, and from a physics point of view, gibberish.

    The teacher is applying the meaning that (for #4) the greater the age the more the object has decayed. You have interpreted it to say (in the case of choice #3) that the smaller the age, the greater is the decay rate. Of these two interpretations, I'd pick yours any day over the teacher's (which would be the better interpretation if this were a layman conversation, not a science question). Think about what the teacher's interpretation means and you'll see that were the question intended that way, choice #4 would be trivially true, and the question would not, in any way, test your knowledge of science. On the other hand, your interpretation requires the student to understand that the rate obeys first order kinetics, or that it is porportional to the amount of undecayed stuff left. That's a question that tests understanding of a scientific concept.

    Besides, any measurement of radioactivity only measures the decay rate, from which amounts that have decayed are calculated. This alone is sufficient to show (reading carefully, the first part of the choices 3,4) that the OP is correct and the teacher wrong.
    Last edited: Jan 30, 2007
  9. Jan 31, 2007 #8

    Andrew Mason

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    I would have to disagree.

    As I understand radiometric dating, the measurement of age of a rock is not determined by its present radioactivity but by the relative concentrations of different nuclei. A rock with a high concentration of U will be more radioactive than a rock with a low concentration of U but the first may be much older than the latter. This relative proportion of nuclei is a measure of how much decay has occurred since the rock crystals were formed.

    In Uranium-Lead dating, for example, Zircon crystals incorporate U when formed but not lead. So the Pb present in the Zircon is from the decay of U to Pb. By measuring that proportion of U to Pb (not the absolute amount of U or Pb or its present radioactivity) we can determine how long it has been since the Zircon crystals were formed.

  10. Jan 31, 2007 #9
    I agree with Gokul - The 1st reading of the question I picked out 4, then reread 3 to see why it might be right.

    Of course I score very low in aural attention
    e.g. "Your driving a bus at an average of 30mph, at the 1st stop 3 people get on 2 people get off, at the 2nd stop 4 people get on 1 person gets off, What is the name of the driver?"
    Always gets me...

    I would say that whilst 3 maybe correct the wording of the question allows for ambiguity so unless it is testing your skills at interpretation and grammer it is a poor question.

    Award yourself an honoury A+ and develop a deep rooted social problem based around this injustice. I did, never did me any harm.
  11. Jan 31, 2007 #10
    The question is ambiguous. All those answers can be correct based on perfectly reasonable and correct interpretations of the term 'radioactive decay'. The term has been improperly used in this question - radioactive decay is a process, you don't have an 'amount' of it, that doesn't make any sense.
  12. Jan 31, 2007 #11


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    I tend to agree with Andrew on this one, although I also agree with Gokul that the least one can say, is that this question is badly and ambiguously phrased.

    Indeed, what matters is the amount of decay that occurred from an initial amount of "unstable" material. So the necessary quantity to be measured is "what fraction has decayed" independently of how much there is overall (and hence what is the actual decay rate, which is only dependent on the total amount of parent material present, and is not a function of how old the stuff is for equal amounts of final parent product). Usually, the "current decay rate" is almost unmeasurable for very long-lived isotopes, and it is a chemical analysis which indicates how much decay product has formed. So the more decay product has formed, AS COMPARED TO AMOUNT OF PARENT NUCLEUS that is somehow estimated (that's the difficult part!), the older the rock.
  13. Jan 31, 2007 #12


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    I'm going to say that I don't think that answers 3 and 4 are poorly phrased at all. Gokul, you threw the word "rate" in there, but that word isn't there in either usage and the context doesn't imply it should be (more on that below). The words "degree of radioactive decay" is another way of saying "the more the object has decayed". Key word: degree. A degree is not a rate, it is a discrete point on a scale. http://dictionary.reference.com/browse/degree
    Now s/he left the word "degree" off the second time s/he used it, but since it is still in the same sentence, it is reasonable to expect people to consider the usage to be the same.

    In fact, since an understanding of how radioactive dating works precludes the use of a rate in either case, the word "degree" isn't even required the first time. The words "radioactive decay" can mean either a rate or a total, depending on the context - and in this context, it is always a total.

    The only quibble I have with the question is that I don't see what is wrong with answers 1&2 - both look correct to me. But if I can only pick one, it is clearly answer #4 based on how specific it is.
    Last edited: Jan 31, 2007
  14. Jan 31, 2007 #13


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    Okay, admittedly, I'm completely wrong on what is the best way to interpret "degree of decay". But, in that case, the question is silly and translates to: 'Given that something is decaying with time, does it take more time to have decayed more?'

    Russ, I threw the word "rate" into the interpretation to help the question qualify as science.

    And then there's this:

    1. http://www.google.com/search?hl=en&q=phys+rev+decay+radioactive&btnG=Search
    Nearly 400,000 hits on Google for search terms: phys+rev+decay+radioactive

    2. http://www.google.com/search?hl=en&q=phys+rev+"degree+of+decay"+radioactive&btnG=Search
    5 hits on Google for search terms: phys+rev+"degree of decay"+radioactive

    Dictionary definitions are virtually useless in real science. All that matters is how the scientific community defines a term (not how Merriam-Webster does).
    Last edited: Jan 31, 2007
  15. Jan 31, 2007 #14


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    Well, it is high school... :tongue:
    Yeah, I know - absent other contextual clues, there really are two different ways to interpret the words "radioactive decay".
    Yes, a geologist likely would have worded the question differently.
  16. Jan 31, 2007 #15
    I see the ambiguity, although I think it has been established that #4 is the correct answer. I found this link in the earth sciences forum, it basically explains the methods of radiometric dating to a layman.

  17. Jan 31, 2007 #16
    I see your point, and I confess that the question was poorly worded. As others have suggested, this seems to be a problem for English majors and grammar nazis rather than physicists.

    Let me just say this: it is true that older objects will tend to be less radioactive. But ultimately, the age is determined by the ratio of C-14 to other isotopes of carbon. I haven't taken English since high school (yes, I even got out of Freshman comp in college), so I won't say anything beyond that for fear of confusing you.
  18. Jan 31, 2007 #17

    Andrew Mason

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    Carbon 14 dating is only used to date living tissue and it does not work for geological time. It works because we know what the relative proportion of Carbon 14 to carbon 12 is when the organism dies (thereafter no new C14 is taken up) and we know what the C14 half life is so we can determine how long it has been since death occurred.

    For rocks, it is not so easy. And it is not necessarily true, but it is generally true, that radioactivity decreases with age. An exception would be if there is natural fission - which has occurred at least once in earth's history.

  19. Jan 31, 2007 #18
    I disagree with your teacher also.

    It's pretty clear that you understand the idea of radioisotope dating and that is what should count. Unfortunately your teacher chooses to use multiple choice questions rather than essay. This is a poor choice. The ability to explain concepts should be the goal of a science course and that is what you should be asked to do. Writing a number of sound-alike answers to try to trick you is a poor educational device.
  20. Feb 1, 2007 #19


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    I agree with you in principle, but from the teacher's PoV, multiple choice questions have two big advantages:
    1) it grades quickly and easily (right/wrong)
    2) there shouldn't be any discussion about poorly worded half maybe correct essays.

    Now, there's nothing worse than a confusingly worded multiple choice question, as in this case.

    From the student PoV, it is of course a bad and horrible technique, but it is not the student who decides :biggrin:
  21. Feb 1, 2007 #20
    I think answer #4 is most correct

    #4. 'It uses a technique in which the degree of radioactive decay is measured, the older the rock the more radioactive decay.'

    To me when he says the 'degree of radioactive decay', he means the total aggregate decay that has happened in the rock. Older rocks on average must have had more decays than newer rocks, relatively of course. As someone pointed out earlier, its still a very real possibility that the newer rock has undergone more decays than the older rock, since the newer rock could have started out with higher concentrations of decaying nuclei than the concentration of decaying nuclei that the older rock did. But, to me the question sort of implies an 'on average' or 'relative to' hidden clause in it. So this is why I think #4 is the best answer to a poorly worded question.

    The real question is why this teacher phrases questions like he's writing an english exam; they are so open to interpretation and filled with airy fluff that they are sure to confuse his students. It seems inappropriate to have a pretty rigid scientific discipline such as chemistry, treated with a not so cut-n-dry question.

    my two cents
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