Physics
The green ray
Weightnessless in the elevator
The periodic table
Physics of the spaghetti
Albert Einstein
First stage to the invisibility

If one calls N0 a number initial of radioactive atoms of sample and c the half-life of atom considered (i.e. the time with the end of which half of the atoms are disintegrated), then there are at any moment t :
First relation
There is also the law :
Second relation
These two relations make it possible to determine the age of object whose one can measure the relationship between two elements (of which one is obtained by disintegration of the other), with the proviso of knowing the initial ratio.
For example, one knows the initial ratio carbon 12/carbone 14 bus this one is constant since the night of times. By measuring it ratio in a rock, one can evaluate its age. One can also use the couple Rubidium 87/Strontium 87, of the couples containing sodium... Moreover, the half-life are of duration extremely different from one core to another (certain species last a few seconds, others like uranium several million years). One can thus choose an isotope adapted to the size-order of the age which one wants to calculate to obtain the most precise possible results: for example, the mummies are often dated with carbon-14 because this one has a half-life of the order of the century.

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