Understanding Kirchoff's Laws: Troubleshooting Common Application Problems

[junior_J]

I understand Kirchoffs law ( its statement and what it means) but when it comes to application of the laws I am lost ...

Heres what i gathered from some books :

1. I have to choose a direction . Any current through a resistor that opposes this direction is -IR , the ones that follow this direction are +IR
?

2. When I follow the direction I've chosen emf is positive if i find that the potential is decreasing (conventional current) and negative if i find that the potential is increasing ...

Am i right with these ??... I am solving some problems that i just can't get right ... please help !

 

[OlderDan]

I understand Kirchoffs law ( its statement and what it means) but when it comes to application of the laws I am lost ...

Heres what i gathered from some books :

1. I have to choose a direction . Any current through a resistor that opposes this direction is -IR , the ones that follow this direction are +IR
?

2. When I follow the direction I've chosen emf is positive if i find that the potential is decreasing (conventional current) and negative if i find that the potential is increasing ...

Am i right with these ??... I am solving some problems that i just can't get right ... please help !

It's not sounding right to me. It sounds backwards, but if you do it consistently backwards, it would work if you interpreted the signs correctly. Consider the simple case of a battery connected to a resistor. You would probably pick the direction to go around the loop as going through the battery from - to + (increasing potential) and you would naturally assume the current (conventional current or positive current) flows through the resistor in the direction you are going. In that case, if E is the emf of the battery (E is positive) you would have E - iR = 0. The battery increases the potential, and the resistor decreases the potential. If you go around the loop the opposite way, you would say the battery reduces the potential and the resistor increases the potential and write -E + iR = 0. Either way leads you to the same equation.

If you assumed the current was in the other direction, you would have E + iR = 0 and the solution for i would be negative, indicating that in fact the current was opposite what you had assumed.

 

[QuantumCrash]

3 things to remeber when doing Kirchoff's problems:
1. direction of current
2. direction of loop
3. change in energy.
Basically the 3rd one is the most important. If you can get that, then you've half-mastered it. Just imagine the change in energy levels as you travel along the wire. Increase means adding, decrease means subtracting: +ve and -ve.

 

[junior_J]

It's not sounding right to me. It sounds backwards, but if you do it consistently backwards, it would work if you interpreted the signs correctly. Consider the simple case of a battery connected to a resistor. You would probably pick the direction to go around the loop as going through the battery from - to + (increasing potential) and you would naturally assume the current (conventional current or positive current) flows through the resistor in the direction you are going. In that case, if E is the emf of the battery (E is positive) you would have E - iR = 0. The battery increases the potential, and the resistor decreases the potential. If you go around the loop the opposite way, you would say the battery reduces the potential and the resistor increases the potential and write -E + iR = 0. Either way leads you to the same equation.

If you assumed the current was in the other direction, you would have E + iR = 0 and the solution for i would be negative, indicating that in fact the current was opposite what you had assumed.

So in essence I've to choose a direction using which ill traverse the loop and in doing so if the potential is decreasing the emf is positve(again conventional current) and viceversa ... and ill be using that very same direction and checking to see if the voltage drop in a resistor is positive if the current follows in the direction and negative if it oppses the direction I've chosen .

QuantumCrash mentioned increase means adding decrease means subtracting ... is it the same thing that I am trying to say ?

 

[junior_J]

part of the problem is that I don't understand enough to ask these questions !

PS How do other members link pics to Circuit diagrams and Free Body Diagrams ... are there any software for that which i could use ?

 

[OlderDan]

part of the problem is that I don't understand enough to ask these questions !

PS How do other members link pics to Circuit diagrams and Free Body Diagrams ... are there any software for that which i could use ?

Some people use free image hosting sites and post links. Others include attachments in their post. Attachments require time for approval before they become visible. In advanced mode you will see the place for adding attachments.

So in essence I've to choose a direction using which ill traverse the loop and in doing so if the potential is decreasing the emf is positve(again conventional current) and viceversa ... and ill be using that very same direction and checking to see if the voltage drop in a resistor is positive if the current follows in the direction and negative if it oppses the direction I've chosen .

QuantumCrash mentioned increase means adding decrease means subtracting ... is it the same thing that I am trying to say ?

I still think you have this backwards from the way everyone else does it, and that is why you are having difficulty. When you go around the loop in whichever direction you have chosen, a voltage increase is positive and a voltage decrease is negative. Voltage always increases when you go through an ideal battery from - to + Voltage always decreases when you go through a resistor in the direction of the conventional current.

Here is someone else's problem you could look at as a good example.

https://www.physicsforums.com/showthread.php?t=138751

You could continue the discussion over there. Why don't you use that diagram and try to write the correct equations using Kichoff's laws. We can continue the discussion over there where the diagram already exists.

 

[junior_J]

im there .