2. Our calculation is similar to that shown in Sample Problem 43-1. We set K = 5.30 MeV = U =
(1/4πε
0
)(q
α
q
Cu
/r
min
) and solve for the closest separation, r
min
:
r
min
=
q
α
q
Cu
4πε
0
K
=
kq
α
q
Cu
4πε
0
K
=
(2e)(29)(1.60
× 10
−19
C)(8.99
× 10
9
V
·m/C)
5.30
× 10
6
eV
=
1.58
× 10
−14
m = 15.8 fm .
We note that the factor of e in q
α
= 2e was not set equal to 1.60
× 10
−19
C, but was instead allowed to
cancel the “e” in the non-SI energy unit, electronvolt.