# Confusion with short circuits, and when you can ignore a loop

For the image above, when t<0, howcome it's safe to ignore the loop with the 50V source, and the 60 and 200 ohm resistors?

How do you know when you can ignore a loop, or when to ignore a part of a circuit due to short/open circuit?

• At t<0 according to your circuit, that "loop" is disconnected from the rest of the circuit. With the switch up there. So you can "ignore" it when analyzing that rest of the circuit. Jul 23, 2018 at 21:26
• But current can still go there though, and it can be recycled back to the circuit. Jul 23, 2018 at 21:33
• Do you know KVL? Anything that could possibly "go" there, will get out of there as well. To clarify things you can even "collapse" the bottom "wire" of that loop into a single point. Jul 23, 2018 at 21:34
• Yeah lol. But isn't that true for that small loop? Jul 23, 2018 at 21:35

You can ignore the loop because it's only connected to the rest of the circuit at a single point or electrical node. For anything to be considered "part" of a circuit it must connect two DIFFERENT nodes.

• Is there any similar rule for nodes? Jul 24, 2018 at 20:16
• A node is a common point for two or more components.
– Norm
Jul 25, 2018 at 13:52

This applies if there is no inductive high voltage contact arc across the contacts as it moves from one contact to the next. So on old distributor caps they were very far apart to each plug wire. Otherwise assume the “pole” opens, Briefly between “throws”. As in a SPDT switch shown here.

• Please elaborate on what you mean by "high voltage contact arc", and "old distributor". Jul 23, 2018 at 21:43
• When current is opened instantly on inductors the stored energy creates an arc. But not in your circuit. Cars 30 yrs. ago had a mechanical HV switch rotating called a Distributor. So prevent arcs between contacts they were far apart. Jul 23, 2018 at 22:03