The half-life of strontium-90 is 28.1 years. calculate the percent of a strontium sample left after 100 years.
1. 76%
2. 63%
3. 0.34%
4. 8.5%
5. 82%

Explanation:

The half-life time of a radiactive isotope (radioisotope) is a constant value, meaning that the amount of the radioisotope that decays will be (1/2) raised to the number of half-lives passed.

Naming A₀ the initial amount to the radioisotope, you can build this table to find the amount left.

Number of half-lives           amount of radiosotope left

       0                                              A₀

       1                                               (1/2) × A₀

       2                                              (1/2)×(1/2)×A₀ = (1/2)² × A₀

       3                                              (1/2)³ ×A ₀

       4                                              (1/2)⁴ × A₀

       n                                              (1/2)ⁿ × A₀

Now calculate the number of half-lives the strontium-90 sample has passed after 100 years:

Hence, the amount of strontium-90 is:

In percent, that is:

Rounding to two significant figures, that is 8.5%.

Conclusion: The percent of strontium-90 left after 100 yeaers is 8.5% (choice number 4).

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current

lol idk man i guess u gotta ask bill nye the science guy