Applying KVL: Understanding the Process and Confusions

[sciman]

hello

what is the process to apply KVL exactly?

we know nothing in a loop, apart from that it consists of 2 batteries with known polarity and 2 resistors.

what do we do?

we assign arbitrarily the currents to each of the branches that compose the loop? and then we assign polarities to the resistors?

or we assign arbitrarily polarities to the resistors in the loop and apply kvl based on that and if we find negative values, we just invert the polarity?

I am a bit confused

To give you an example:

1) according to the first method (we take resistor polarities based on the arbitrarily taken currents)
in left loop, we have V1-Vr3-Vr1=0
in right loop, we have V2-Vr2+Vr3=0
2) according to the second method (we assign polarities of resistors arbitrarily)
in left loop, we have V1-Vr3-Vr1=0
in right loop, we have V2-Vr2+Vr3=0

which are the same, but the website of this example gives:

and

WHICH IS DIFFERENT!thanks

 

[CWatters]

but the website of this example gives:

Which website?

 

[sciman]

Which website?

http://en.wikiversity.org/wiki/Electric_Circuit_Analysis/Kirchhoff's_Voltage_Law

look in Part 4 please

 

[SammyS]

hello

what is the process to apply KVL exactly?

we know nothing in a loop, apart from that it consists of 2 batteries with known polarity and 2 resistors.

what do we do?

we assign arbitrarily the currents to each of the branches that compose the loop? and then we assign polarities to the resistors?

or we assign arbitrarily polarities to the resistors in the loop and apply kvl based on that and if we find negative values, we just invert the polarity?

I am a bit confused

To give you an example:
View attachment 82039

1) according to the first method (we take resistor polarities based on the arbitrarily taken currents)
in left loop, we have V1-Vr3-Vr1=0
in right loop, we have V2-Vr2+Vr3=0
2) according to the second method (we assign polarities of resistors arbitrarily)
in left loop, we have V1-Vr3-Vr1=0
in right loop, we have V2-Vr2+Vr3=0

which are the same, but the website of this example gives:

and

WHICH IS DIFFERENT!thanks

I think it's just a typo.

He later writes:

and

which implies using opposite signs for V_R3 .