Confusion with voltage polarities in circuit analysis.

[hassanjaved]

I am always confused with voltage polarities in circuit analysis.I can't really explain what I am confused about its just that i use intution rather than using a set rule to find voltage across some element bunch of elements.Most of the time the confusion is in applying KVL, and I am confused about terms +ve voltage drop,-ve voltage drop,terminal voltages,voltages across elements etc.
So,it would be of very great help if someone could explain what is voltage and how it is determined in circuit analysis,better still if you could provide a link that explains this in detail.
thanks a lot.

 

[UR_Correct]

It all depends on the direction of current. I always think of current going + to - (if you're assuming a voltage drop is positive [i.e., a source would give negative voltage since it is supplying the circuit when writing loop equations]). Suppose you have a branch going down with just a resistor in it (assuming part of a bigger circuit). If it asks you to find the voltage denoting a plus on top and negative on bottom, like down below
o
|
+
R
-
|
o

If you have current flowing down that branch, (+ to -) you will get a positive voltage. If your current is going up that branch, you will get a negative voltage, IN REFERENCE TO YOUR CONVENTIONS. You get a negative voltage value because your convention of current going plus to minus is the opposite polarity of the voltage on the resistor you're trying to get. It's all relative. You just have to make assumptions at the beginning of your problem and stick with them. Just denote at the end in a diagram which way you're assuming voltage polarities and current directions.

Does that help?

 

[sophiecentaur]

If you use the term 'potential' difference then the question is easily resolved. The difference between a + (or high)V and a - (or low)V will be Positive. ... and vice versa. This is just like the 'number line' which elementary Maths teachers use these days. Think of volts as up and down a hill, perhaps. I certainly try to draw circuits with the more positive potentials actually nearer the top of the paper and the lower or negative potentials near the bottom. This is not foolproof for 'seeing' the potential differences between points in the circuit but it is a good start.

 

[hassanjaved]

If you use the term 'potential' difference then the question is easily resolved. The difference between a + (or high)V and a - (or low)V will be Positive. ... and vice versa. This is just like the 'number line' which elementary Maths teachers use these days. Think of volts as up and down a hill, perhaps. I certainly try to draw circuits with the more positive potentials actually nearer the top of the paper and the lower or negative potentials near the bottom. This is not foolproof for 'seeing' the potential differences between points in the circuit but it is a good start.

Yes that's close to what I am confused about.Please explain a bit more the number line analogy and the term potential difference!