21. The speed v of the electron satisfies K
max
=
1
2
m
e
v
2
=
1
2
(m
e
c
2
)(v/c)
2
= E
photon
− Φ. Using Table 38-3,
we find
v = c
2(E
photon
− Φ)
m
e
c
2
= (2.998
× 10
8
m/s)
2(5.80 eV
− 4.50 eV)
511
× 10
3
eV
= 6.76
× 10
5
m/s .
21. The speed v of the electron satisfies K
max
=
1
2
m
e
v
2
=
1
2
(m
e
c
2
)(v/c)
2
= E
photon
− Φ. Using Table 38-3,
we find
v = c
2(E
photon
− Φ)
m
e
c
2
= (2.998
× 10
8
m/s)
2(5.80 eV
− 4.50 eV)
511
× 10
3
eV
= 6.76
× 10
5
m/s .