Hi!

I don't know how to attack this one. Find I and V. The teacher said the diode on top is on and the two other are off, but didn't explain why. How do I know which one is on and off?

-Thanks

 

You know because diodes have a fixed voltage drop in the forward biased condition.

 

Try on and off for all the diodes and then analyse the circuit.
For eg. Assuming the lowest diode (across which 1V is applied) is on and other two are off.
So, V=1 volt(assuming Ideal diode).
But if V=1V then other diodes must also be on.
So this assumption is wrong.
In the same way you can try out other combinations.

 

First you assume that you have a ideal diode with Vd = 0V. Or the real one with Vd = 0.7V. Next for example, you can assume that the top diode is ON. And use a circuit theory to check if your assumption is correct ( find V voltage and check if middle and lower diode can be ON).