# V =0, I=0 , OC or SC?

1. Oct 27, 2012

### swayam007

When Voltage across two points is 0 & current flowing through that connection is 0. Is it a short circuit or open circuit?

2. Oct 27, 2012

### Studiot

Short circuit means there is zero impedance (resistance) between the two points.
This may lead to zero voltage and no current or a lot of current.

Open circuit means that there is infinite impedance between the two points.
Current flow is always zero but the voltage is indeterminate and may be any value including zero

The condition v=0, I=0 can be either so it is impossible to determine which without knowledge of the connecting impedance.

3. Oct 29, 2012

### mistermotown

'When Voltage across two points is 0 & current flowing through that connection is 0'
That means I have an amp meter, and a volt meter. I measure the voltage between two points, and the voltage is zero. I then take my amp meter and measure the current that flows through both those nodes, and that current is zero.

That means your circuit is off buddy, you could just as well have a wire lying on the table, not connected to anything and measure that. You could measure V=0 and I=0 off a glass cup.
:P

If you are talking about circuit theory:
then V=0 is a short circuit AND there can be current flowing through it
I=0 is an open circuit AND there can be a voltage across the two non-connected wires.
But those ONLY count when your circuit is ON in some way.

What are you trying to do? Are you trying to calculate resistance of a circuit, or using super position?

4. Oct 29, 2012

### sophiecentaur

If R=V/I and both V and I are zero then you get
R = 0/0
which is indeterminate. You need to specify either V or I.

5. Oct 29, 2012

### Ratch

swayam007,

A short or open is determined by physical contact, not by what voltage is across, current through, or resistance between the contacts. So if the contacts physically touch each other, then you have a short circuit across the contacts.

Ratch

6. Oct 30, 2012

### sophiecentaur

True but, if all you know is that measurements gave you V=0 and I=0, you cannot determine anything about the resistance - infinite / zero / 347Ω.

7. Oct 30, 2012

### Ratch

sophiecentaur,

If you are able to insert an ammeter between the contacts, then you should be able to determine whether they are touching or not, and thereby know whether they are open or closed. The quality of the contacts, such as whether they are dirty or corroded does not mean they are open or shorted. Only a physical inspection can determine that for certain. The resistance of the contacts are a a good indication, but not a certainity of their status.

Ratch

8. Oct 30, 2012

### sophiecentaur

According to the title of the thread, your Ammeter reads zero and your voltmeter reads zero. If the only info you have is those readings, then you can tell nothing. You are going for a very practical way of finding out the condition of one particular pair of contacts. Fair enough but unless you have some way of finding current and volts, you can't be sure.

9. Oct 30, 2012

### Ratch

sophiecentaur

Well, assuming a perfect ammeter and voltmeter, the ammeter is going to short out the switch so the voltmeter will always read zero whether the switch is open or closed. So, knowing the current and voltage is no sure way to determine the switch status. For me, physical inspection is the definitive determination.

Ratch

10. Oct 30, 2012

### sophiecentaur

But who, in their right mind, would ever connect an Ammeter in PARALLEL???

11. Oct 30, 2012

### Ratch

sophiecentaur,

Last edited: Oct 30, 2012
12. Oct 30, 2012

### sophiecentaur

13. Oct 30, 2012

### Ratch

sophiecentaur,

Nope, I make no assumptions about whether the switch is connected to a circuit or stands alone. All I am saying is that you cannot definitively determine whether the switch is open or closed by just examining the voltage across it or the current through it. A physical examination of the contacts is required to definitively determine whether the switch is open or closed.

Ratch

14. Oct 31, 2012

### swayam007

actually It is about a Galvanometer in a balanced wheatstone bridge . Current passing through it & Voltage across it is zero. Is it open circuit or short circuit?

15. Oct 31, 2012

### sophiecentaur

So all those 'black box' exercises we did at School and all the measurements done, daily, on components are a waste of time? We have to see what it actually looks like before we can tell the impedance of a circuit element.

Interestingly, it turns out that the scenario was, as I suspected, the situation in a bridge circuit.

16. Oct 31, 2012

### Studiot

It is neither open circuit nor short circuit.

I have already outlined the conditions for both and your circuit meets neither.

Note the conditions do not depend on voltage or current.

17. Oct 31, 2012

### Ratch

sophiecentaur,

The exercises were theoretical problems. The measurements were determining highly probable conditions. The observation of the contacts are the definitive status.

You can determine the impedance from the theoretical position of the switch, or what you think it is.

Ratch

18. Oct 31, 2012

### sophiecentaur

I'm not sure of your point here. There are many examples of systems into which we cannot look. Opening them up could even destroy them or alter their state. A philosophy of measurement surely can't be based on what we infer from the outward appearance of a component.
The point was made in a very early post that 0/0 is indeterminate and that's really an end to it. You need to have at least a finite value for one or the other in order to infer anything about the resistance of a component. 'Visual' observation is just not reliable.

19. Oct 31, 2012

### Ratch

sophiecentaur,

Just what I said before. You cannot be sure of the status of a switch just by knowing the current existing through it or the voltage across it. I hope the last sentence was clear and concise.

True that is, but lack of accessibility does not change the uncertainty.

The philosophy would have to include a caveat stating that the status of the switch cannot be determined with complete confidence from the current/voltage measurement previously mentioned.

And my point was that any value of current/voltage other than 0/0 still has a degree of uncertainty.

An inference is not the same as knowing for sure.

Visual inspection of the contacts is 100% reliable. It is not always available or practical.

Ratch

20. Oct 31, 2012

### sophiecentaur

Surely it is obvious that you need to know both current and PD in order to measure the resistance. Resistance is actually defined in terms of the two variables taken together.
You can know nothing, "for sure". One can only infer, from appropriate measurements.
Your visual inspection issue is a total red herring. It is not part of the Set of 'electrical measurements'. Visual inspection of contacts is not sufficient at all. They could be made of a shiny insulator, for all you could know. (And, in any case, the OP did not introduce the state of a switch into the argument). You would need extra chemical analysis, at least. On the other hand, an electrical measurement of V and I would tell you how your test subject will behave 'electrically', which is what we want to know (because we are seeking to find the resistance).
You are welcome to have the final word on this as I'm not sure whether you are 1. Disagreeing with any or all of the above or 2. Agreeing ditto.

21. Oct 31, 2012

### Ratch

sophiecentaur,

For resistance yes, but a switch that is supposed to be closed can be defective and have a high resistance. Similarly, an open switch can be shorted. So resistance by itself is not a positive determining factor as to the status of the state of the switch.

Correct, but electrical measurements are not the defining factor in determining the state of the switch.

Physical positioning of the contacts is the absolute determining factor of the switch state. A switch is not manufactured with insulators for contacts.

Actually he did. In post #1, he asks about whether there is a conduction path or not between two points.

Why?

That would involve isolating the switch from the circuit, applying its rated current and knowing its specified voltage drop. That is what the factory quality control does, not me. You can measure the resistance with an ordinary ohmmeter and determine the state of the switch with a high degree of confidence. But to truly know, you have to look at the contacts.

Ratch

22. Nov 1, 2012

### sophiecentaur

@ratch
Does that (original) post in any way imply a 'mechanical switch'? Why are you obsessed with your special case of a metallic switch with 'visible' contacts? What has that got to do with the OP?

When will you ever find even the mechanical switch in your argument where there is a pure short circuit? Resistance may be relevant for any switch you care to choose so what has the position of a lever got to do with the question.
We have agreed that you need to know V and I to be certain of the electrical characteristic so I still don't know what extra point you are trying to make that has any bearing on the OP.
We can either discuss the Maths of 0/0 or discuss the business of electrical measurement without any power supply but they both end up with the same conclusion. That conclusion is not allowed to contain the concept of visual inspection because that is just one special case.

23. Nov 1, 2012

### Ratch

sophiecentaur,

Yes, it does. When he talks about open or short across a connection, that implies a switch.

Because that is the way physical switchs are made.

Defective mechanical switches sometime short out.

Certainly, but it is not necessarily definitive as to whether a defective switch is open or closed.

I never agreed that the electrical characteristics determine definitively whether the switch is open or closed. The OP asked about whether the electrical measurements could be used to determine whether a two contacts were open or shorted. That question ties the OP to the statement I make.

Without visual inspection of the contacts, it cannot be determined with certainty what the status of the state of the switch is.

Ratch

24. Nov 1, 2012

### sophiecentaur

So a piece of wire is a switch? (That's a short circuit)
The OP is about two points in a circuit. You are merely implying that he means a switch (and, if you read later, he is definitely not talking about a switch.)
Switches don't always do what you expect from their appearance but the will always have some value of resistance. The proof of the pudding is in the eating. If I find that a sealed switch happens to have a low resistance then for my purposes, the state of the switch is closed and the circuit that it is put into is likely to agree with me.
Why are you so much in love with switches? They are only extremes of a variable resistance. You can't determine anything about either without applying some I or some V.
It helps to stick to the point when possible rather than to chase wild geese.

25. Nov 1, 2012

### Ratch

sophiecentaur,

No, how did you deduce that from what I said?

The OP has more or less described a switch.

Likely yes, but not for certainty.

Besides being an important electrical component, it is what we are talking about here. You can determine their state by visual observation of the contacts.

Yes, that is what I am doing. If it is not interesting to you, that does not mean if is off point.

Ratch