77.
(a) From P = V
2
/R we find V =
√
P R =
(10 W)(0.10 Ω) = 1.0 V.
(b) From i = V /R = (
E − V )/r we find
r = R
E − V
V
= (0.10 Ω)
1.5 V
− 1.0 V
1.0 V
= 0.050 Ω .