# Homework Help: Internal Resistance

1. Sep 17, 2008

### nslinker

How can I go about calculating internal resistance? I have variables I, R, and V but no idea on how to find the 'internal' resistance. Is this some sort of special case?

2. Sep 17, 2008

### edziura

Your question needs to be a bit more specific.

Internal resistance is a concept used to account for the fact that the terminal voltage of a a real battery (as opposed to an ideal voltage source, whose voltage is constant, regardless of the current drawn from the source) drops as the current drawn from the battery increases. A simple electrical model of a battery is an ideal voltage source in series with a resistor representing the internal reistance. As current increases,, more voltage is dropped across the internal R and the battery terminal voltage decreases.

3. Sep 17, 2008

### nslinker

Well, the question says "A 20 V battery delivers 107 mA of current when connected to a 74 ohm resistor. Determine the internal resistance of the battery."

4. Sep 18, 2008

### Defennder

Just draw a circuit consisting of the 74 ohm resistor, the unknown resistance r in series and the 20V batteries. Then solve for r given the info in the question.

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