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==Key Stage 4 Foundation==
===Meaning===
[[File:HalfLifeGraph1.png|right|300px|thumb|A [[graph]] showing the [[fraction]] of [[Unstable Isotope|unstable isotope]] left compared to the '''half life'''.]]
A '''half life''' is the time it takes for half of the [[Unstable Isotope|unstable isotopes]] in a [[radioactive]] sample to [[Radioactive Decay|decay]].
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the [[Count Rate|count rate]] has halved.'''
<math>200 \div ÷ 2 = 100</math>
<math>100 \div ÷ 2 = 50</math>
<math>50 \div ÷ 2 = 25</math>
The [[Count Rate|count rate]] has halved 3 times.
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>224 \div ÷ 2 = 112</math>
<math>112 \div ÷ 2 = 56</math>
<math>56 \div ÷ 2 = 28</math>
<math>56 \div ÷ 2 = 28</math>
The number of [[Unstable Isotope|unstable]] [[atom]]s has halved 4 times.
| style="height:20px; width:300px; text-align:center;" |'''2. Divide the time taken by the number of times the [[Count Rate|count rate]] has halved.'''
<math>90 \div ÷ 3 = 30</math>
'''Half Life''' = 30 days.
| style="height:20px; width:300px; text-align:center;" |'''2. Divide the time taken by the number of times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>18 \div ÷ 4 = 4.5</math>
'''Half Life''' = 4.5 [[minute]]s
===Finding Half Life and Count Rate from a Table===
{| classborder="wikitable1" style="border-collapse:collapse"
| style="height:20px; width:100px; text-align:center;" |Time /s
| style="height:20px; width:50px; text-align:center;" |0
:1000/2 = 500Bq
:The [[isotope]] took 20 seconds [[second]]s to half in [[Count Rate|count rate]]::'''Half life''' = 20 [[secondssecond]]s.
2. Predict the [[Count Rate|count rate]] at 90 seconds[[second]]s.:Since the '''half life''' is 20 seconds [[second]]s the [[Count Rate|count rate]] 20 seconds [[second]]s earlier can be halved. :At 70 seconds [[second]]s the [[Count Rate|count rate]] was 88Bq. Therefore at 90 seconds [[second]]s the [[Count Rate|count rate]] would be:
:88/2 = 44Bq
3. Predict what the [[Count Rate|count rate]] was 10 seconds [[second]]s before the [[sample]] was first tested.:Since the '''half life''' is 20 seconds [[second]]s the [[Count Rate|count rate]] 20 seconds [[second]]s later can be doubled. :At 10 seconds [[second]]s the [[Count Rate|count rate]] was 707Bq. Therefore at -10 seconds [[second]]s the [[Count Rate|count rate]] would have been:
:707x2= 1414Bq
{| classborder="wikitable1" style="border-collapse:collapse"
| style="height:20px; width:100px; text-align:center;" |Time /min
| style="height:20px; width:50px; text-align:center;" |0
==Key Stage 4 Higher==
===Meaning===
[[File:HalfLifeGraph1.png|right|300px|thumb|A [[graph]] showing the [[fraction]] of [[Unstable Isotope|unstable isotope]] left compared to the '''half life'''.]]
A '''half life''' is the time it takes for half of the [[Unstable Isotope|unstable isotopes]] in a [[radioactive]] sample to [[Radioactive Decay|decay]].
<math>R = \frac{R_I}{2^n}</math>
Where:
<math>R</math> = New [[Count Rate]] or number of [[atom]]s of an [[Unstable Isotope|unstable isotope]] left.
| style="height:20px; width:300px; text-align:center;" |'''1. Calculate the number of half lives which have occurred.'''
<math>17.5 \div ÷ 2.5 = 7</math>
n = 7 '''half lives'''
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>25 \times 60 \div ÷ 300 = 5</math>
n = 5 '''half lives'''
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the [[Count Rate|count rate]] has halved.'''
<math>200 \div ÷ 2 = 100</math>
<math>100 \div ÷ 2 = 50</math>
<math>50 \div ÷ 2 = 25</math>
The [[Count Rate|count rate]] has halved 3 times.
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>224 \div ÷ 2 = 112</math>
<math>112 \div ÷ 2 = 56</math>
<math>56 \div ÷ 2 = 28</math>
<math>56 \div ÷ 2 = 28</math>
The number of [[Unstable Isotope|unstable]] [[atom]]s has halved 4 times.
| style="height:20px; width:300px; text-align:center;" |'''2. Divide the time taken by the number of times the [[Count Rate|count rate]] has halved.'''
<math>90 \div ÷ 3 = 30</math>
'''Half Life''' = 30 days.
| style="height:20px; width:300px; text-align:center;" |'''2. Divide the time taken by the number of times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>18 \div ÷ 4 = 4.5</math>
'''Half Life''' = 4.5 [[minute]]s
===Finding Half Life and Count Rate from a Table===
{| classborder="wikitable1" style="border-collapse:collapse"
| style="height:20px; width:100px; text-align:center;" |Time /s
| style="height:20px; width:50px; text-align:center;" |0
:1000/2 = 500Bq
:The [[isotope]] took 20 seconds [[second]]s to half in [[Count Rate|count rate]]::'''Half life''' = 20 [[secondssecond]]s.
2. Predict the [[Count Rate|count rate]] at 90 seconds[[second]]s.:Since the '''half life''' is 20 seconds [[second]]s the [[Count Rate|count rate]] 20 seconds [[second]]s earlier can be halved. :At 70 seconds [[second]]s the [[Count Rate|count rate]] was 88Bq. Therefore at 90 seconds [[second]]s the [[Count Rate|count rate]] would be:
:88/2 = 44Bq
3. Predict what the [[Count Rate|count rate]] was 10 seconds [[second]]s before the [[sample]] was first tested.:Since the '''half life''' is 20 seconds [[second]]s the [[Count Rate|count rate]] 20 seconds [[second]]s later can be doubled. :At 10 seconds [[second]]s the [[Count Rate|count rate]] was 707Bq. Therefore at -10 seconds [[second]]s the [[Count Rate|count rate]] would have been:
:707x2= 1414Bq
{| classborder="wikitable1" style="border-collapse:collapse"
| style="height:20px; width:100px; text-align:center;" |Time /min
| style="height:20px; width:50px; text-align:center;" |0
:3968x2= 7936Bq
===References===
====AQA====
:[https://www.amazon.co.uk/gp/product/1782945598/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945598&linkCode=as2&tag=nrjc-21&linkId=ad276ad49df77ab4b40ab4fd0fe09977 ''Half-life, page 200, GCSE Combined Science; The Revision Guide, CGP, AQA '']
:[https://www.amazon.co.uk/gp/product/178294558X/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=178294558X&linkCode=as2&tag=nrjc-21&linkId=f0dfb66dafcb0c6e9449e7b1a4ae1ac236 ''Half-life, page 46, GCSE Physics; The Revision Guide, CGP, AQA '']
:[https://www.amazon.co.uk/gp/product/019835939X/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=019835939X&linkCode=as2&tag=nrjc-21&linkId=57e96876985fc39b1a3d8a3e3dc238b6 ''Half-life, pages 100-101, 109, GCSE Physics; Third Edition, Oxford University Press, AQA '']
:[https://www.amazon.co.uk/gp/product/1782946403/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782946403&linkCode=as2&tag=nrjc-21&linkId=32a0abb60dff015b15b50e9b1d7b4644 ''Half-life, pages 118-120, GCSE Combined Science Trilogy; Physics, CGP, AQA '']
:[https://www.amazon.co.uk/gp/product/0008158770/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=0008158770&linkCode=as2&tag=nrjc-21&linkId=ec31595e720e1529e49876c3866fff6e ''Half-life, pages 118-19, GCSE Physics; Student Book, Collins, AQA '']
:[https://www.amazon.co.uk/gp/product/1782945970/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945970&linkCode=as2&tag=nrjc-21&linkId=a120d24dcc7cc7a58192069a3aafc1d2 ''Half-life, pages 130-132, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA '']
:[https://www.amazon.co.uk/gp/product/0008158770/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=0008158770&linkCode=as2&tag=nrjc-21&linkId=ec31595e720e1529e49876c3866fff6e ''Half-life; definition, page 118, GCSE Physics; Student Book, Collins, AQA '']
:[https://www.amazon.co.uk/gp/product/1471851370/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1471851370&linkCode=as2&tag=nrjc-21&linkId=01c69b0ae058f809cf636033e6ba793e ''Half-lives, page 99, GCSE Physics, Hodder, AQA '']
:[https://www.amazon.co.uk/gp/product/1471851354/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1471851354&linkCode=as2&tag=nrjc-21&linkId=9012a0d354024419214fb3ad5ac44ba0 ''Half-lives, pages 248-9, GCSE Combined Science Trilogy 1, Hodder, AQA '']
:[https://www.amazon.co.uk/gp/product/1471851370/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1471851370&linkCode=as2&tag=nrjc-21&linkId=01c69b0ae058f809cf636033e6ba793e ''Half-lives; and stability, page 100, GCSE Physics, Hodder, AQA '']
:[https://www.amazon.co.uk/gp/product/1471851370/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1471851370&linkCode=as2&tag=nrjc-21&linkId=01c69b0ae058f809cf636033e6ba793e ''Half-lives; measurement of, pages 99-100, GCSE Physics, Hodder, AQA '']
====Edexcel====
:[https://www.amazon.co.uk/gp/product/1782945741/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945741&linkCode=as2&tag=nrjc-21&linkId=30da4f2178da182547b62a7329d13b57 ''Half-life, page 176, GCSE Combined Science; The Revision Guide, CGP, Edexcel '']
:[https://www.amazon.co.uk/gp/product/1782945733/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945733&linkCode=as2&tag=nrjc-21&linkId=2a2dbec9db6bf5766c0458d908fa0a52 ''Half-life, page 53, GCSE Physics; The Revision Guide, CGP, Edexcel '']
:[https://www.amazon.co.uk/gp/product/1782948163/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782948163&linkCode=as2&tag=nrjc-21&linkId=0fdbfd5dd397d6e24a9dfb250f08587f ''Half-life, pages 161-163, GCSE Physics, CGP, Edexcel '']
:[https://www.amazon.co.uk/gp/product/1782948163/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782948163&linkCode=as2&tag=nrjc-21&linkId=0fdbfd5dd397d6e24a9dfb250f08587f ''Half-life; effect on safety, page 167, GCSE Physics, CGP, Edexcel '']
:[https://www.amazon.co.uk/gp/product/1292120223/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1292120223&linkCode=as2&tag=nrjc-21&linkId=068ecf40278c32406a7f1c6e66751417 ''Half-lives, pages 102-103, GCSE Physics, Pearson Edexcel '']
:[https://www.amazon.co.uk/gp/product/1292120193/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1292120193&linkCode=as2&tag=nrjc-21&linkId=572df39392fb4200db8391d98ae6314e ''Half-lives, pages 366-367, GCSE Combined Science, Pearson Edexcel '']
====OCR====
:[https://www.amazon.co.uk/gp/product/0198359837/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=0198359837&linkCode=as2&tag=nrjc-21&linkId=3c4229e8b023b2b60768e7ea2307cc6f ''Half-life, pages 176-177, 183, Gateway GCSE Physics, Oxford, OCR '']
:[https://www.amazon.co.uk/gp/product/1782945695/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945695&linkCode=as2&tag=nrjc-21&linkId=ceafcc80bcad6b6754ee97a0c7ceea53 ''Half-lives, page 199, Gateway GCSE Combined Science; The Revision Guide, CGP, OCR '']
:[https://www.amazon.co.uk/gp/product/1782945687/ref=as_li_tl?ie=UTF8&camp=1634&creative=6738&creativeASIN=1782945687&linkCode=as2&tag=nrjc-21&linkId=9a598e52189317a20311d7a632747bc9 ''Half-lives, pages 77, 79, Gateway GCSE Physics; The Revision Guide, CGP, OCR '']