Subscripts?
- Thread starter rattus
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LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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It seems to me that they're the same thing, the potential between points 1 and 2.rattus said:
I am used to using the second subscript as the reference point. V12 would be the voltage at point 1 with respect to point 2.
I'll bet different textbooks show it differently. Its possible (but I'm just guessing) that technical textbooks tend to use the opposite subscript, while engineering textbooks tend to use the notation above. (Similar to the way engineering textbooks are more likely to use conventional current flow, while technical books tend to use electron flow.)
Steve
I'll bet different textbooks show it differently. Its possible (but I'm just guessing) that technical textbooks tend to use the opposite subscript, while engineering textbooks tend to use the notation above. (Similar to the way engineering textbooks are more likely to use conventional current flow, while technical books tend to use electron flow.)
Steve
bphgravity
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I have no idea what you guys are talking about. Anyone care to give a basic explanation?
Basics:
Basics:
Bryan, measure the voltage on your car battery. It will read positive or negative depending on the placement of the red and black leads.
But if you define the battery voltage as Vbg (Vbattery re ground), Vbg will be positive, unless you have a positive ground, then it will be negative.
This is important when working with phase angles. Swapping leads on a phasemeter would introduce a 180 degree phase shift.
I agree with Steve, but I have seen it the other way too.
Basics:
bphgravity said:
Bryan, measure the voltage on your car battery. It will read positive or negative depending on the placement of the red and black leads.
But if you define the battery voltage as Vbg (Vbattery re ground), Vbg will be positive, unless you have a positive ground, then it will be negative.
This is important when working with phase angles. Swapping leads on a phasemeter would introduce a 180 degree phase shift.
I agree with Steve, but I have seen it the other way too.
winnie
Senior Member
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I'm certain that a couple of hours in a library would provide you a bunch of texts using either format 
The wikipedia reference is interesting, because if you look at different sections of wikipedia they you both forms
This wikipedia article uses r^hat_ab to mean the unit vector described by a-b/|a-b|, which happens to be the unit vector _from_ b to a. This would correspond to a voltage difference of a - b.
http://en.wikipedia.org/wiki/Law_of_universal_gravitation
This wikipedia article doesn't do a good job of describing the unit vector used in the equation, but the diagram clearly shows the vector R_12 as meaning the vector from 1 to 2, exactly the opposite of the above:
http://en.wikipedia.org/wiki/Coulomb's_law
I would suggest trying to find a reference such as a 'style manual' for whatever journal you are writing for, or if you are not writing for a journal, find one that is most commonly used in the field, and follow that style.
-Jon
The wikipedia reference is interesting, because if you look at different sections of wikipedia they you both forms This wikipedia article uses r^hat_ab to mean the unit vector described by a-b/|a-b|, which happens to be the unit vector _from_ b to a. This would correspond to a voltage difference of a - b.
http://en.wikipedia.org/wiki/Law_of_universal_gravitation
This wikipedia article doesn't do a good job of describing the unit vector used in the equation, but the diagram clearly shows the vector R_12 as meaning the vector from 1 to 2, exactly the opposite of the above:
http://en.wikipedia.org/wiki/Coulomb's_law
I would suggest trying to find a reference such as a 'style manual' for whatever journal you are writing for, or if you are not writing for a journal, find one that is most commonly used in the field, and follow that style.
-Jon
kingpb
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According to Elements of Power System Analysis, Fourth Edition, by William D Stevenson, Jr., the double subscript notation was devloped to replace the need for polrity marks for voltage, and direction arrows for current.
For current, the notation defines the direction of flow of current when the current is considered to be positive. For example, Iab would mean the current is flowing from point a to point b. Iab = -Iba.
For voltage, the double subscript denotes the nodes of the circuit between which voltage exists. As stated earlier, the convention is that the first subscript denotes the voltage of that node with respect to the node identified by the second subscript. Vab would be the voltage at node a with respect to voltage at node b, and that Vab is positive. Vab = Vba @ 180 deg = -Vab
Using Kirchhoff's voltage law, the order of the subscripts is the order of tracing a closed path around the circuit.
For current, the notation defines the direction of flow of current when the current is considered to be positive. For example, Iab would mean the current is flowing from point a to point b. Iab = -Iba.
For voltage, the double subscript denotes the nodes of the circuit between which voltage exists. As stated earlier, the convention is that the first subscript denotes the voltage of that node with respect to the node identified by the second subscript. Vab would be the voltage at node a with respect to voltage at node b, and that Vab is positive. Vab = Vba @ 180 deg = -Vab
Using Kirchhoff's voltage law, the order of the subscripts is the order of tracing a closed path around the circuit.
Thanks:
Thanks:
Thanks for the reference King. I am relieved to learn that I have been doing it right for all these years. I made a mistake once--thought I was wrong!
I would use the same notation when using Kirchoff's voltage law; that is, with phasors, I would start at the arrowhead and work toward the tail. Then the subscripts always have the same meaning.
Thanks:
Thanks for the reference King. I am relieved to learn that I have been doing it right for all these years. I made a mistake once--thought I was wrong!
I would use the same notation when using Kirchoff's voltage law; that is, with phasors, I would start at the arrowhead and work toward the tail. Then the subscripts always have the same meaning.
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Not at all authoritative. As a matter of maintaining purity of heart and cleanliness of body, I refuse to read anything on the wikipedia web site, and I decline to follow any links that lead to that site.Smart $ said:
Regards,
charlie ("purer and cleaner than thou") b.
- Location
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- Retired Electrical Engineer
I pulled my copy of Stevenson off the shelf, and confirmed Kingpb's statement. It's a bit disconcerting that we both have the Fourth Edition. Mine shows a copyright date of 1982. :shock:
kingpb
Senior Member
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charlie b said:
Mine has the same date, so either we're both getting old, or things havn't changed much. I prefer the latter.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
2, 3, 5, & 8?
kingpb
Senior Member
- Location
- SE USA as far as you can go
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- Engineer, Registered
I guess there are no more choices, Larry and Steve took them all...
Somebody needs to teach them how to share!
Somebody needs to teach them how to share!
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