From the daily exposure to radioisotopes of organisms, scientists discovered to carbon date fossils. In carbon dating, the age of organic matter can be determined from the relative proportions of C-12 and C-14 i.e. isotopes of Carbon (carbon 12 and carbon 14). This ratio changes over the years because carbon-14 decays and is not replenished as there is no exchange with the atmosphere.
12 V is the f.e.m. of the battery. The potential difference that is applied to the motor is actually the fem minus the voltage drop on the internal resistance r: this is equal to the voltage drop on the resistance of the motor R: so we can write: and using and we can find the current I:
of the battery. The potential difference that is applied to the motor is actually the fem minus the voltage drop on the internal resistance r:
and 
we can find the current I: