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Ellie
12/8/2025
Physics
Half-life
340
•
Dec 8, 2025
•
Ellie
@ellie_yb
Ever wondered why radioactive materials become less dangerous over time?... Show more
Think of radioactive decay like popcorn popping - you can't predict exactly when each kernel will pop, but you can estimate how long it takes for half of them to pop. Nuclear decay works the same way as a completely random process.
Half-life is the time it takes for half the nuclei in a radioactive sample to disintegrate, or for the activity to drop to half its original value. This measurement is incredibly useful because whilst we can't predict when individual atoms will decay, we can accurately predict the behaviour of large groups of atoms.
Different isotopes have wildly different half-lives. Carbon-14 (used in dating ancient objects) has a half-life of 5,715 years, whilst Francium-223 decays so quickly it only lasts 20 minutes. Understanding these timescales helps scientists choose the right isotopes for different applications.
Remember: Count rate isn't the same as activity - your detector only measures radiation coming towards it, not the radiation shooting off in all directions!
Setting up a half-life experiment is straightforward but requires careful measurement. You'll use a detector and counter to measure how many radioactive particles hit your equipment per minute (the count rate).
The first crucial step is measuring background radiation - the natural radioactivity that's always present around us. You subtract this from all your readings to get the corrected count rate, which shows only the radiation from your sample.
When you plot your results on a graph, you'll see a beautiful curved pattern. The activity consistently takes the same amount of time to halve - whether it's dropping from 1000 to 500 counts per minute, or from 500 to 250. This predictable pattern makes half-life calculations reliable and useful.
Top Tip: Always remember to subtract background radiation from your measurements, or your calculations will be completely wrong!
Half-life problems become easy once you master the pattern. In the first example, activity drops from 64kBq to 2kBq in 60 minutes - that's 5 half-lives (64→32→16→8→4→2). So each half-life equals 60÷5 = 12 minutes.
Working forwards in time is just as straightforward. If 8MBq becomes 0.5MBq after 4 half-lives, and each half-life is 12 hours, then 48 hours (2 days) have passed. The key is recognising that every half-life cuts the activity in half.
Fraction calculations follow a clear pattern too. After 8 half-lives, only 1/256 of the original activity remains. Each half-life creates a fraction: 1/2, then 1/4, then 1/8, and so on. You can calculate this as (1/2)^n, where n is the number of half-lives.
Pro Tip: Draw a simple table showing activity versus number of half-lives - it makes even complex problems much clearer!
Sometimes you need to work backwards to find what the count rate was in the past. The rock example shows this perfectly - if it's 25 c/m now and was much more active 150 years ago, you can calculate backwards.
Working backwards means doubling the activity for each half-life you go back in time. From 2020 back to 1870 is 150 years, which equals 5 half-lives (150÷30). Going backwards: 25→50→100→200→400→800 c/m.
This reverse calculation technique is incredibly useful in carbon dating and forensic science. Scientists can determine how old objects are by measuring their current radioactivity and calculating backwards to estimate when they were alive or created.
The beauty of half-life is its consistency - whether you're predicting future decay or reconstructing the past, the mathematical relationship stays exactly the same.
Real-World Application: This backwards calculation method is how archaeologists date ancient artifacts and determine the age of fossils!
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
App Store
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
Ellie
@ellie_yb
Ever wondered why radioactive materials become less dangerous over time? It's all about half-life - a crucial concept that helps scientists predict how quickly radioactive substances decay and lose their activity.
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Think of radioactive decay like popcorn popping - you can't predict exactly when each kernel will pop, but you can estimate how long it takes for half of them to pop. Nuclear decay works the same way as a completely random process.
Half-life is the time it takes for half the nuclei in a radioactive sample to disintegrate, or for the activity to drop to half its original value. This measurement is incredibly useful because whilst we can't predict when individual atoms will decay, we can accurately predict the behaviour of large groups of atoms.
Different isotopes have wildly different half-lives. Carbon-14 (used in dating ancient objects) has a half-life of 5,715 years, whilst Francium-223 decays so quickly it only lasts 20 minutes. Understanding these timescales helps scientists choose the right isotopes for different applications.
Remember: Count rate isn't the same as activity - your detector only measures radiation coming towards it, not the radiation shooting off in all directions!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Setting up a half-life experiment is straightforward but requires careful measurement. You'll use a detector and counter to measure how many radioactive particles hit your equipment per minute (the count rate).
The first crucial step is measuring background radiation - the natural radioactivity that's always present around us. You subtract this from all your readings to get the corrected count rate, which shows only the radiation from your sample.
When you plot your results on a graph, you'll see a beautiful curved pattern. The activity consistently takes the same amount of time to halve - whether it's dropping from 1000 to 500 counts per minute, or from 500 to 250. This predictable pattern makes half-life calculations reliable and useful.
Top Tip: Always remember to subtract background radiation from your measurements, or your calculations will be completely wrong!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Half-life problems become easy once you master the pattern. In the first example, activity drops from 64kBq to 2kBq in 60 minutes - that's 5 half-lives (64→32→16→8→4→2). So each half-life equals 60÷5 = 12 minutes.
Working forwards in time is just as straightforward. If 8MBq becomes 0.5MBq after 4 half-lives, and each half-life is 12 hours, then 48 hours (2 days) have passed. The key is recognising that every half-life cuts the activity in half.
Fraction calculations follow a clear pattern too. After 8 half-lives, only 1/256 of the original activity remains. Each half-life creates a fraction: 1/2, then 1/4, then 1/8, and so on. You can calculate this as (1/2)^n, where n is the number of half-lives.
Pro Tip: Draw a simple table showing activity versus number of half-lives - it makes even complex problems much clearer!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Sometimes you need to work backwards to find what the count rate was in the past. The rock example shows this perfectly - if it's 25 c/m now and was much more active 150 years ago, you can calculate backwards.
Working backwards means doubling the activity for each half-life you go back in time. From 2020 back to 1870 is 150 years, which equals 5 half-lives (150÷30). Going backwards: 25→50→100→200→400→800 c/m.
This reverse calculation technique is incredibly useful in carbon dating and forensic science. Scientists can determine how old objects are by measuring their current radioactivity and calculating backwards to estimate when they were alive or created.
The beauty of half-life is its consistency - whether you're predicting future decay or reconstructing the past, the mathematical relationship stays exactly the same.
Real-World Application: This backwards calculation method is how archaeologists date ancient artifacts and determine the age of fossils!
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
