physic 難題 15

Fot V1 delivers no current, the potential at the upper end of the 1 kilo-ohm resistor should be +4 v (assume the -ve terminals of the two batteries are at zero potential).

Hence, potential at the upper end of the 1 kilo-ohm resistor in series with V2 is also 4 v
Since potential at the lower end of that resistor = 9 v,
current delivered by battery V2 = (9-4)/1000 A = 0.005 A
Such current would flow to resistor RL
thus, RL = 4/0.005 ohms = 800 ohms