How to define resistor polarities and when to use standard labeling in KCL and KVL

[thuc]

Hi guys,
I'm having some troubles while doing KCL and KVL exercies. I have two question:
1. Do I have to define the polarities of resistors and how to do that ?
2. When do i need to use the STANDARD LABELING ?
For example in this exercie

 

[axmls]

The way I do it is, when writing, for instance, a loop equation, moving from - to + results in a positive term, and moving from + to - results in a negative term. Pick your currents at the beginning of the problem and stick with them. The voltage drop in a resistor due to a current always goes from + to -. It just takes some practice to get used to it.

 

[thuc]

oh thanks, but I have another question: when i have 2 power sources which power source will generate the current ?
for example in this exercie

 

[berkeman]

oh thanks, but I have another question: when i have 2 power sources which power source will generate the current ?
for example in this exercie View attachment 96046

They both can contribute. Can you show us the KCL equation that you get by writing that the sum of all currents leaving the center node is equal to zero?

 

[sophiecentaur]

Pick your currents at the beginning of the problem and stick with them.

Yes. Annotate the diagram with I₁, I₂,I₃ etc (with arrows) and give all the nodes numbers. It's essential to be totally consistent throughout. The 'polarity' can be any way you want because the answer will yield the correct sign, whichever.

 

[CWatters]

There is a similarity with problems involving motion. The sign of the answer only makes sense when taken together with the definitions you make at the outset (eg "I define UP as positive"). If you don't make those definitions before writing KCL/KVL equations you will come unstuck.