Thorium has atomic number = 90

 

Thus no. of protons in thorium nucleus = 90

 

so cahrge on thorium is = +90e

 

Where, e = positive electronic charge = 1.6 * 10^ (-19) Coulombs.

 

Charge on alpha particle = 2e (as it is helium nuclie)

 

So the Coulombic force on alpha particle and thorium nucleus is given by

 

F = (1/4₀) (90e) x (2e) / r²