I'm confused about the terminals in the picture above simply because I've seen the exact same thing mean 2 different things in schematics, namely:
(1) it is an open circuit with voltage Vi(t)
(2) The terminals are connected to some other circuit and thus have a voltage/current source, and the circuit is closed.
This further confuses me, because it would be so much clearer to just use a voltage source symbol for case (2), instead of it looking like an open circuit.
So I guess my question is, how do I know what the schematic wants this symbol to represent? An open circuit, or a voltage source that is closed so that current is coming out of it?
Why don't we just use a voltage source instead? Is it supposed to be implied that the schematic symbol means case (2) (because otherwise vi(t)=0)