Half-Life (OCR GCSE Physics A (Gateway)): Revision Note

Half-Life

• It is impossible to know when a particular unstable nucleus will decay

• But the rate at which the activity of a sample decreases can be known

  • This is known as the half-life

• Half-life is defined as:

  The time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half

• In other words, the time it takes for the activity of a sample to fall to half its original level

• Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length

Using Half-life

• Scientists can measure the half-lives of different isotopes accurately:

  • This means it would take 704 million years for the activity of a uranium-235 sample to decrease to half its original amount

  • Uranium-235 has a half-life of 704 million years

Uranium atoms decay whilst the number of lead atoms increases

• Carbon-14 has a half-life of 5700 years

  • So after 5700 years, there would be 50% of the original amount of carbon-14 remaining

  • After two half-lives, or 11 400 years, there would be just 25% of the carbon-14 remaining

• With each half-life, the amount remaining decreases by half