How Do You Calculate the Internal Resistance of a Thevenin Voltage Source?

[bengaltiger14]

Homework Statement

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.

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?

 

[berkeman]

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...

 

[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)

 

[berkeman]

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...