Calculate Internal Resistance: I, R, V & More

AI Thread Summary
To calculate internal resistance, one must recognize that it accounts for the voltage drop in real batteries under load. The internal resistance can be determined by analyzing a circuit with the known variables: a 20 V battery, a 74-ohm resistor, and a current of 107 mA. By drawing the circuit and applying Ohm's law, the internal resistance can be calculated as the difference between the ideal voltage and the voltage drop across the external resistor. This approach illustrates how internal resistance affects terminal voltage as current increases. Understanding this concept is crucial for accurate battery performance analysis.
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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?
 
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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.
 
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."
 
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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