OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Radioactivity: (h) radioactive dating, e.g. carbon-dating.
Radioactive dating uses the known decay of a radionuclide to estimate the age of a material.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 11 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Radioactive dating uses the known decay of a radionuclide to estimate the age of a material. In carbon dating, living things exchange carbon with the atmosphere, so the proportion of carbon-14 remains roughly constant while they are alive. When they die, exchange stops and the carbon-14 decays with a half-life of about 5730 years. Measuring the remaining carbon-14 activity or ratio allows the age to be estimated using N = N₀e^(−λt) or the half-life relationship. The method is limited to materials that were once living and to ages up to roughly 50 000 years, because beyond that too little carbon-14 remains to measure reliably.
What earns the marks
- Living organisms exchange carbon with their surroundings, maintaining a roughly constant carbon-14 proportion until they die.
- After death, carbon-14 decays with a known half-life of about 5730 years, so the remaining proportion indicates the time elapsed.
- The age can be estimated using N = N₀e^(−λt) or by counting how many half-lives have passed.
- Carbon dating is limited to once-living material and to ages up to roughly 50 000 years because the remaining activity becomes too small to measure accurately.
- Other radionuclides with longer half-lives are used to date rocks and minerals over much longer timescales.
Common mistakes and how to put them right
Thinking carbon-14 is stable in dead material.
Put it right: It continues to decay, which is exactly why the method works.
Assuming carbon dating works for any material.
Put it right: It applies only to things that were once living and exchanged carbon.
Ignoring the upper age limit.
Put it right: After many half-lives the remaining carbon-14 is too small to measure reliably.
Confusing half-life with the age of the sample.
Put it right: The age is found from how much has decayed, not from the half-life alone.
Exam tips
- State the assumption that the initial carbon-14 proportion is known and constant in living things.
- Show the calculation clearly, using either the decay equation or the number of half-lives.
- Mention the limitation on age range when discussing reliability.
Try the lesson
The start of the Radioactivity: (h) radioactive dating, e.g. carbon-dating. lesson, made from checked teaching notes for this specification point.
Listen
Radioactive dating uses the known decay of a radionuclide to estimate the age of a material. In carbon dating, living things exchange carbon with the atmosphere, so the proportion of carbon-14 remains roughly constant while they are alive. When they die, exchange stops and the carbon-14 decays with a half-life of about 5730 years. Measuring the remaining carbon-14 activity or ratio allows the age to be estimated using N = N₀e^(−λt) or the half-life relationship. The method is limited to materials that were once living and to ages up to roughly 50 000 years, because beyond that too little carbon-14 remains to measure reliably. A strong answer earns marks for these points. Living organisms exchange carbon with their surroundings, maintaining a roughly constant carbon-14 proportion until they die. After death, carbon-14 decays with a known half-life of about 5730 years, so the remaining proportion indicates the time elapsed. The age can be estimated using N = N₀e^(−λt) or by counting how many half-lives have passed. Carbon dating is limited to once-living material and to ages up to roughly 50 000 years because the remaining activity becomes too small to measure accurately. Exam tip: State the assumption that the initial carbon-14 proportion is known and constant in living things.
Key words
Say what each word means before you turn the card.
Radioactive dating
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Carbon-14 and half-life
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Exponential decay in context
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Limitations of dating methods
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See every definition
- Radioactive dating
- A method that uses the known decay of a radionuclide to estimate the age of a material.
- Carbon-14 and half-life
- While alive, organisms keep a roughly constant carbon-14 proportion; after death it decays with a half-life of about 5730 years.
- Exponential decay in context
- The age can be estimated using N = N₀e^(−λt) or by counting how many half-lives have passed.
- Limitations of dating methods
- Carbon dating applies only to once-living material and to ages up to roughly 50 000 years, because the remaining activity becomes too small to measure accurately.
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