I’ve been reading about HL, but I still don’t get why would you use it. So, if half life of coffee is 5h, how that info is relevant when we know that full life is 10 (roughly)? On top of that, how do you get the half-life of a material other than waiting to be completely ‘dead’ and say, ok full life is X, the half life is X/2.
Also, let’s take uranium which in Earth’s crust has a half-life of almost 4.5 billion years.. how did we get this number?
Thank you!
In: Chemistry
If half life is 5h then full life is not 10h. It is always true that half is gone by the half life, no matter how much you start with. Say you drink 100mg of caffeine, about a cup. After 5h you would have 50mg left in your body. But this would be equivalent to drinking half a cup. So after another 5h, 10h after drinking the coffee, you would have 25mg of caffeine left in your body. After 15h you would have 12.5mg and so on. Mathematically the caffeine never have a full life as you theoretically never get to zero. It only have a half life.
You do not have to wait a full half life in order to measure the half life. You can measure a shorter time and then expect that the rate remains the same. For example if you drink 100mg of caffeine, and then we wait an hour and measure that you have 87mg of caffein left in your body. We see that 13% of your caffeine is metabolised every hour so that means that by hour 2 you would have 75mg of caffeine, by hour 3 you would have 65mg. Notice how the amount of caffeine metabolised every hour goes down, because 13% of 100mg is 13mg while 13% of 75mg is only 10mg. By hour 4 you would have 57mg and finally by hour 5 you would have 50mg, half of what you started with.
We can do the same with Uranium. Measure the exact ratio of Uranium isotopes in some ore and then leave it for a year and measure it again. You would notice a tiny decrease in the amount of radioactive Uranium isotopes. You can then expect this to continue for 4.5 billion years and then you have your half life.
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