Half-life: carbon-14 and the age of old wood
1 min 46 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Radiometric dating and half-life, with the past questions on it.
What Dr. Mars says (the words of the video)
How does carbon-14 tell the age of old wood?
Wood holds carbon-14, a radioactive kind of carbon.
After the tree dies, no new carbon-14 comes in.
Radioactive atoms decay into another element.
Carbon-14 decays into nitrogen-14.
Half-life: the time for half of the radioactive atoms to decay.
For carbon-14, it is five thousand seven hundred thirty years.
Watch: atoms decay one at a time, at random.
Dr. Mars moves the nitrogen-14 aside and writes the count.
Decayed atoms are not lost: they changed into nitrogen-14.
After one half-life, thirty-two are left: one half.
In the next half-life, half of the rest decay.
After two half-lives, sixteen are left: one quarter.
One more half-life: half of the sixteen decay.
The fraction left tells the age.
One eighth left: three half-lives, or seventeen thousand one hundred ninety years.
Check yourself! Wood has one quarter of its carbon-14 left. How old is it: A, B, or C?
Think: how many half-lives make one quarter?
Answer: B! Two half-lives: eleven thousand four hundred sixty years.
Dr. Mars moved the decayed half aside twice.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.