in the attachment there are four circuit diagram (A,B,C,D)
questions:
1). What are the condition for the diode to conduct in circuit A,circuit B,circuit C , circuit D?
example : in A the VRA must be bigger than VA for the diode to conduct.




please help me , i know that diode only allows current to flow in one direction.. but the problem is there are resistor and another voltage source connected here ... please help me..

 

The "example" statement makes no sense. I sure hope that wasn't a given part of the problem.

You need to show your efforts at solving YOUR homework. We can then use that as a starting point for discussion.