# Homework Help: How to find voltage in an open circuit

1. Mar 14, 2013

### tooperoo

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

Hi guys, attached is a photo of the circuit. Apologies for the bad drawing, but paint is hard :P

What I am wondering is, how do you find the voltage of the open circuit with circuits like these? I have used Thevenin and Norton equivalent circuits before, but the circuits didn't look like this. Any guidance is appreciated! Thank you

2. Relevant equations

3. The attempt at a solution

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2. Mar 14, 2013

### phinds

With an open circuit, how much current is flowing through R2 and R3 ? What is the voltage across them?

3. Mar 14, 2013

### tooperoo

There isn't any at all, right?

What i usually do for the Voc is combine the resistors with a voltage divider and then multiply it by the voltage source. But u thought you need at least two resistors to do that approach.

4. Mar 15, 2013

### CWatters

You don't have a voltage divider in the circuit when the output is unconnected.

Look at it this way... If there is no current flowing through R2 and R3 the voltage drop across R2 and across R3 is zero. You can remove and replace R2 and R3 with wire.

5. Mar 15, 2013

### tooperoo

So say if the voltage source was pumping out 5v, the Voc would be 5v?

6. Mar 15, 2013

### phinds

Are you asking or telling?

7. Mar 15, 2013

### tooperoo

Hahaha definitely asking. I'm nearly positive it would be 5v but I want to check if I'm correct.

8. Mar 17, 2013

### Staff: Mentor

That's the open-circuit voltage. Now what is the short-circuit current?