Rather simple diode problem

[Learnphysics]

Homework Statement

[PLAIN]http://img27.imageshack.us/img27/4475/q12r.png [Broken]

Homework Equations

KCL/KVL

Mesh current, node voltage

ohm's law

The Attempt at a Solution

for b)
I understand how diodes work, and i'v deduced that diode 2 is definitely not conducting as the voltage on it's anode is 0, and the voltage on it's cathode MUST be higher than 0.

To find weather diode 1 is off or not, i need to figure out the voltage at the node where the 12v and 16v lines meet. I tried using KVL, however the path to the 12 volt source has no resistance, and therefore results in me dividing by 0.

KCL can't be used as there's no useful closed loops in this.

Could anyone give me an idea on how to determine the voltage at that node?

I have similar problems with part a), I need to find the voltage of the node on the left, but one of it's arms have no resistance so i can't use node voltage or KCL. there is really not loops to use KVL.

 

[Tweedle_Dee]

I would begin by assuming that D1 is conducting. If D1 is conducting then you immediately know the voltage across R2 since the voltage across an ideal diode is constant. From there you can calculate the current in each node. If D1 is not conducting, then the resulting currents and voltage drops will not obey KCL/KVL and you'll know something is wrong.