Vab = Van - Vbn? Exploring the Basics

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In summary, Van is voltage at a referenced to n. Vbn is voltage at b referenced to n, and the potential difference between them is the voltage drop across phase a (Van) plus the voltage drop across phase b (Vbn).
  • #1
influx
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vbn.png


Why is Vab = Van -Vbn? I'm doing this after a while so maybe I'm getting something basic wrong but surely the potential difference between lines A and B is the voltage drop across phase a (Van) plus (rather than minus) the voltage drop across phase b (Vbn)?? So Vab = Van + Vbn?

Thanks
 
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  • #2
Notice orientations of '+' poles of source voltages Van and Vbn as you go through the loop a-n-b-a
 
  • #3
zoki85 said:
Notice orientations of '+' poles of source voltages Van and Vbn as you go through the loop a-n-b-a
I thought it had something to do with the signs but I've kind of forgotten. Could you elaborate please?

Cheers
 
  • #4
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?
 
  • #5
In a simple DC circuit, if one point is at +6 volts, and another point is at +6 volts, what is the potential difference between the two? Is it 12 volts, or is it zero?

If one point is at +6 volts and another is at -6 volts, what is the potential difference?

Do you add the readings, or do you subtract them?
 
  • #6
zoki85 said:
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?

Case I: Going from a to b is Van + Vbn ?
Case II: Going from a to b is Van - Vbn?
 
  • #7
NascentOxygen said:
In a simple DC circuit, if one point is at +6 volts, and another point is at +6 volts, what is the potential difference between the two? Is it 12 volts, or is it zero?

If one point is at +6 volts and another is at -6 volts, what is the potential difference?

Do you add the readings, or do you subtract them?
The first is 0V
The second is 12V?

How does that apply to this problem?
 
  • #8
influx said:
Case I: Going from a to b is Van + Vbn ?
Case II: Going from a to b is Van - Vbn?
:)
 
  • #9
zoki85 said:
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?
There is no difference. In either case the P.D. is given by the same equation.

Van is voltage at a referenced to n. Vbn is voltage at b referenced to n, that convention is clear.
So the P.D. is always Vab = Van - Vbn, and consistent with your output arrow direction.

Did you intend something different?
 
  • #10
Arrow denotes voltage Vab (missing in the drawings). Vbn changes polarity so the outputs Vab aren't same in both circuits
 
Last edited:

Related to Vab = Van - Vbn? Exploring the Basics

1. What is the meaning of "Vab = Van - Vbn"?

The equation "Vab = Van - Vbn" represents the potential difference between points A and B, which can be calculated by subtracting the potential at point B (Vbn) from the potential at point A (Van).

2. How is "Vab = Van - Vbn" related to electric circuits?

"Vab = Van - Vbn" is commonly used in electric circuit analysis to determine the voltage difference between two points in a circuit. It is also known as Kirchhoff's Voltage Law (KVL).

3. Can "Vab = Van - Vbn" be used for both DC and AC circuits?

Yes, "Vab = Van - Vbn" is applicable to both DC (direct current) and AC (alternating current) circuits as long as the potential at points A and B are measured at the same time.

4. What are the units of measurement for "Vab = Van - Vbn"?

The units for potential difference can vary depending on the context, but they are typically measured in volts (V) or millivolts (mV).

5. Are there any limitations to using "Vab = Van - Vbn"?

One limitation is that the equation assumes a linear relationship between voltage and current in a circuit. It may not accurately represent the behavior of non-linear circuits such as those with diodes or transistors.

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