# Kirchoff's Problem.

1. Feb 16, 2013

### Pork5

Hi, I'm new here. I've seen that other people have gotten homework help before. For this problem, I have to fine the current, in amperes, across the R, 80, 20, and 70 resistor. The R resistor is 236.6 ohms. I'm not sure where to start because of the three batteries. Here is a picture of my problem
http://i.imgur.com/JvKWegd.gif

2. Feb 16, 2013

### SammyS

Staff Emeritus

Hello Pork5. Welcome to PF !

Do you know Kichhoff's Rules?

It can also be solved using superposition, if that's allowed for you.

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3. Feb 16, 2013

### Pork5

I know that the sum of the currents entering is the same as leaving. Also that the sum of potential differences is 0. I'm just not sure where to start with three batteries, plus one that is oriented different than the other two. Some of the videos that I saw on this topic had only 1 battery.

4. Feb 17, 2013

### CWatters

Basically with problems like this you have a number of unknowns (currents and voltages). You need to mark these on the diagram then use KVL and KCL to write enough simultaneous equations to solve for the unknowns.

At the outset you might not know which direction current will flow in any particular branch of the circuit nor which end of a resistor is +ve but that doesn't matter. Just as long as you are consistent when writing the equations.

If at the end you discover one of the calculated currents or voltages is negative that just means your initial arbitrary choice for the direction was wrong. There is no need to change anything.