How Do You Solve Electric Circuit Problems with Multiple Voltage Sources?

[alexmath]

Homework Statement

http://iceimg.com/i/2a/b2/df5d993eb7.png

Homework Equations

Sorry for the big picture...

The Attempt at a Solution

Every time i see more then 1 generator i get in trouble... i don't understand here when to add and when to substruct voltages. Is there any difference between Uab and Uba...

For the first one i get R=29 which is wrong.
-15V-I(2+R)+6=18

 

[gneill]

Every time i see more then 1 generator i get in trouble... i don't understand here when to add and when to substruct voltages. Is there any difference between Uab and Uba...

Yes, Uab = -Uba. Look at it as a KVL "walk" from one node to another. In one case you're "walking downhill", dropping overall in potential from +18V to 0V, and in the other case "walking uphill" from 0V to +18V.

For the first one i get R=29 which is wrong.
-15V-I(2+R)+6=18

I think you'll find that the above resolves to R = -29 Ohms, which is clearly not a physically correct value for resistance (at least not at this course level ).

Are you "walking uphill" or "walking downhill" overall in this problem?

 

[alexmath]

So, the first one should be:
18v-15v+6v=I(2+R)

second one:
If i go from + to - will be a negative sign, but the current is going to the left, and I'm going to the right... so it will be 18V+30 or 18V-30V

signes are so confusing to me :(

 

[gneill]

So, the first one should be:
18v-15v+6v=I(2+R)

Yes, that looks fine.

second one:
If i go from + to - will be a negative sign, but the current is going to the left, and I'm going to the right... so it will be 18V+30 or 18V-30V

signes are so confusing to me :(

If you pencil in the signs of the voltage drops across the resistors according to the current direction first, it should make it easier to assemble your terms when you do your "KVL walk" along the path. The signs of the drops will respect the current's effect according to Ohm's law.