# Kirchoff's law?

1. Apr 22, 2012

### gmx

kirchoff's law??

Hi

Sometimes in kirchoff's law questions it gives the circuit without showing the direction of the current so how do I determine the direction of the current (not the direction that I choose to solve the question)?

I have exam tomorrow

2. Apr 22, 2012

### Staff: Mentor

Re: kirchoff's law??

If you don't know the direction of the current in a certain section of the circuit, simply make a guess, and set up your equations accordingly. If you guessed the wrong direction, the current in that section will come out negative, but the magnitude will still be OK, so remove the minus sign and flip the current-direction arrow around on your diagram.

3. Apr 22, 2012

### gmx

Re: kirchoff's law??

thank you very much
I have another question if you don't mind
how do I know from where does I1 start and end? also for I2 and I3

4. Apr 22, 2012

### Staff: Mentor

Re: kirchoff's law??

Generally, the different currents start and end at junction points. You should always start out by assuming different currents between different junction points, or along different paths between the same two junction points.

5. Apr 22, 2012

### DrummingAtom

Re: kirchoff's law??

My circuits professor always assumes that all current is going away from each node; so the sum of all the currents at a node equal 0. I personally haven't adopted this method because I find it confusing but and professor and some of my friends live by it. Here's a link that explains this method. The good thing about is you don't have to worry about signs until the end.

http://www.mathonweb.com/help/backgd5.htm

6. Apr 22, 2012