# Electrical Circuit Problem

1. Aug 27, 2007

### bengaltiger14

1. The problem statement, all variables and given/known data
A real voltage source is of the Thevenin Type. The open circuit voltage of the source is 12 volts. If a 100 ohm load is added to the circuit, the voltage at the same terminals is 11.2 volts. What is the internal resistance of the source.

3. The attempt at a solution

I found the total current in the circuit to be 12/100 = 120mA

If there is .8 volts missing, the internal resistance should be: .8/120mA????

2. Aug 27, 2007

### Staff: Mentor

When they say "real voltage source", that means that you should not ignore the built-in output resistance of the source. So the total current will not be 12V/100 Ohms.... you need to include the Rout in that equation as well....

3. Aug 27, 2007

### bengaltiger14

I need to find the internal resistance before I can include it in the equation right? After doing .8/120mA=6.66 Ohms. The new total current could be 12V/(100 + 6.66)

4. Aug 27, 2007

### Staff: Mentor

I'm not sure if that's right, but try it this way -- use the voltage divider equation, since you know the voltage source is 12V, and the divided down voltage across Rout and 100 Ohms is 11.2V.....