Given :
The ball of a ballpoint pen is 0.5 mm in diameter and has an ASTM grain size of 12.
To Find :
How many grains are there in the ball?
Solution :
Volume of ball of the ballpoint is :
[tex]V = \dfrac{4 \pi r^3}{3}\\\\V = \dfrac{4\times 3.14 \times 0.5^3}{3}\ mm^3\\\\V = 0.523\ mm^3[/tex]
Now, grain size of 12 has about 520000 grains/mm³.
Therefore, number of grains are :
[tex]n = 520000\times 0.523\ grains\\\\n = 271960\ grains[/tex]
