# Homework Help: Equivalent Voltage for Thevenin Equivalent Circuit

1. Oct 12, 2012

### Kurushimi

1. The problem statement, all variables and given/known data
This isn't for a class I'm actually taking. Just self study.

I'm reading a book on electronics. When I got to the part about Thevenin's theorem I got a little confused. Specifically, I'm confused about the equivalent voltage for the Thevenin equivalent circuit. It said the equivalent voltage would be the open-circuit voltage, which the book defines as "potential at the output terminals when the output current is zero".

The reason this confuses me is that when I think of the equation V = IR, when you have a 0 current you should have 0 voltage. The book showed an example for calculating the equivalent voltage but they didn't explain how they arrived at their equation, so I figure it's something basic that I'm missing.

3. The attempt at a solution
I went to wikipedia's page on Thevenin's theorem used the concept of a voltage divider to show an example of how to calculate the equivalent voltage. Doing this also yields the equation my book got for its example of calculating equivalent voltage. However, my understanding of a voltage divider relies on the existence of a current through the circuit. Even the proof given the wikipedia page linked uses the assumption of current. So I don't see how it makes sense to apply this concept when calculating equivalent voltage with the assumption that there is no current. How does this make sense?

2. Oct 12, 2012

### Staff: Mentor

Electric potential can exist without current flowing. Take for example an isolated 10V battery (no connections). There's still a potential difference of 10V between its terminals even with a load connected. This will be true even if the battery is not ideal and has an internal resistance (no current means no drop across the internal resistance).

The idea with the Thevenin voltage for a given source and network is that you first remove any load and consider the potential that the source and network "presents" to the point where a load was (or will be) connected.