How to Transform a Circuit to One Resistor?

[builder_user]

All I am saying is, if the terminals at U2 are open (not connected to a source), then there is no loop and no current flows (through terminals U2).

When i=0?

 

[gneill]

When i=0?

No current flows through an open circuit. If the terminals at U are open, then current i = 0.

 

[builder_user]

No current flows through an open circuit. If the terminals at U are open, then current i = 0.

How show on the scheme that terminals are opened?

 

[gneill]

How show on the scheme that terminals are opened?

By having them not connected to any component. In the attached figure, the terminals at U1 are not open. The terminals at U2 are open.