How old is a radioactive specimen if the half-life is 5,000 years and only 12.5% of the original material is remaining? 20,000 years 15,000 years 10,000 years 5,000 years

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.

you get 2.3=1.21*\frac{a}2 or 4.6=1.21a or a=3.802 (b)

explanation: