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Physics 12
Lab Module #11
Half-life
Module 11
4/21/2015
Name: ________________________
Group Member(s): _______________
Introduction
Some substances contain radioactive elements and they have a property called half-life.
Half-life is the time it takes for half of atoms in the element to decay or change into another
substance. The atoms do not decay in any set order. Some radioactive elements have a half-life of
5000 years. This means that after 5000 years, only, half the remaining atoms will decay.
Geologists use -half-life in radioactive dating of some rocks. They compare the amount of
the radioactive element in a rock to the amount that has changed into another substance. By
knowing the half-life of the element, the approximate age of the rock can be determined. For
example, uranium is radioactive. Through a series of steps it breaks down to lead at a known rate.
Its half-life is used to calculate the age of the rock. The half-life of uranium is the amount of time
it takes half of any amount of uranium to break down to lead .
By measuring the ratios between uranium and lead, we can estimate the ages of rocks that
are millions of years old. The measurements are not perfect, but that have provided a time scale
that is more accurate than any previous one. Similar calculations have been made using radioactive
potassium and rubidium.
Carbon-14 is radioactive carbon formed from nitrogen in the atmosphere. It has been used
to date plant and animal remains. All living things use carbon-14 in the life processes. After death,
however, the carbon-14 gradually reverts to nitrogen and disappears. The rate is its half-life which
is approximately 5770 years. In another 5770 years, half of the remaining carbon-14 will disappear
and so on.
Organic matter younger than 1000 years has lost too little radioactive carbon for the
difference to be measured. Organic matter between 1000 to 50 000 years can be dated by the
amount of carbon-14 it contains. This information is extremely useful to geologists,
anthropologists, and archeologists.
Materials
- Simulats (simulated atoms)
- Shaker
Procedure
1. Assemble the shaker that you have been provided.
2. Place the entire collection of simulats into the shaker. Shake the shaker covering the top with
one hand and then dump the simulats onto a table or desk.
3. All of the simulats that land with their white side up should be moved to one side, and black
side up to another.
4. Count the number of white simulats and record into Table 1.
5. Return the black simulats to the shaker and the process repeated, recording data into Table 1
6. This should be continued until all simulats have decayed (landed white side up!)
7. Return simulats to their bag and flatten the shaker. Return to your teacher.
Physics 12
Lab Module #11
4/21/2015
Table 1
# of Tosses
No. of simulats that
have decayed (White)
1
2
3
4
5
6
7
1. Plot the data Number of Tosses Vs. White Simulats.
2. Take the square below and assume it represents a mass of carbon-14. Divide the square to represent the amount of carbon-14 left after 5700 years (Use 5700 years as the approximate halflife of carbon-14). Continue to divide the square to show the amount of carbon 14 left after
11400, 17100 22800, and 28500 years
3. What fraction of carbon-14 still remains in charcoal burned in a primitive man’s campfire
approximately 28,000 years ago? You may check your answer with the divided square.
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Physics 12
Lab Module #11
4/21/2015
4. What fraction of carbon 14 still remains in an animal frozen in a glacier 18,000 years ago. Use
your graph (back page) to assist you
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5. Estimate the age of pollen found in peat swamps left by a flakier from your graph. Assume that
only 1/8 of the original carbon-14 remains in the pollen. (Check your answer with the square)
____________________________________________________________________________
Conclusions
6. How many simulats changed or decayed by the end of the experiment? ________________
7. (a) Compared to the original number of simulats you started with, approximately how many
were left after each shake?
___________________________________________
(b) What does this indicate about the rate at which half-life occurs?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
8. What are some inaccuracies of this experiment in demonstrating half-life?
____________________________________________________________________________
____________________________________________________________________________
9. Will all carbon 14 in nature eventually disappear? _________ Explain your answer.
____________________________________________________________________________
____________________________________________________________________________
Physics 12
Lab Module #11
4/21/2015
250
Decayed Simulats
200
150
100
50
0
0
1
2
3
4
5
6
7
8
9
10
Number of Tosses
Plot information from Table 1 in this above graph.
50000
45000
40000
Years sinceDeath
35000
30000
25000
20000
15000
10000
5000
0
Fraction
To help you with problems, and to complete this assignment, plot the fraction of C-14 remaining
versus the time that has passed. The source data for this comes from question 2