# Thevenin Theorem Question

1. Jan 15, 2009

### thelovemonkey

1. The problem statement, all variables and given/known data

hey guys. im new to the board. ive seen this problem on a website of which i'll list below. im not sure how came up with that formula for e.

http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/thevenin.html -> is the link to the page.

i understand the rest of the problem, thaz teh only thing thaz giving me a hard time. thanks in advance for ur helps. cheers.

2. Jan 16, 2009

### tiny-tim

Welcome to PF!

Hi thelovemonkey! Welcome to PF!

Do you mean e = V1R3/(R1 + R3) ?

Since no current is flowing through R2, the voltage across AB is the same as the voltage across R3, which you can calculate from Kirchhoff's rules

3. Jan 16, 2009

### thelovemonkey

oh okay okay i udnerstand that. but i dont quite understand the actual equation e = V1R3/(R1 + R3) as in how it was derived.

4. Jan 16, 2009

### ravioli

When nodes a and b are open, no current will flow through R2. Since there is no current through R2, there will be no voltage drop across R2. So therefore, the open circuit voltage is simply the voltage across R3. Which if you do KVL, or just a simple voltage divider equation, you'll get to the equation that's in question.

What does your Kirchoff equation look like?

Last edited: Jan 16, 2009
5. Jan 16, 2009

### thelovemonkey

aw man. i think thats my problem right there. i dont know about voltage divider equatiosn and stuff.

6. Jan 16, 2009

### ravioli

Ok, try to sum up the current at the point between R1 and R3. We know from the link that tiny-kim gave us, that this summation will equal zero. Even more helpful is the formula V=I*R. , so we'll get something like ( V_oc / R3 ) as the current through the resistor R3. V_oc means Voltage of open circuit. Now what will the current be for R1? What is the voltage difference between the source voltage and V_oc? Then from there, what's the current knowing the resistance is R1? Is this current the same current as that flowing through R3? Why? Once you get an equation in terms of voltages and resistances, then it becomes some algebraic maneuvering to achieve the desired formula.

There's a a few details in dealing with whether or not the current's sign should be positive or negative, which can be addressed using the passive sign convention.

http://en.wikiversity.org/wiki/Passive_sign_convention There's a good link for you to look at and digest.

7. Jan 18, 2009

### thelovemonkey

hurray for b00bies i found it. this link helped sort the voltage divider thing for me