Question

When a 20.0-ohm resistor is connected across the terminals of a 12.0-V battery, the voltage across the terminals of the battery falls by 0.300 V. What is the internal resistance of this battery?

Answer 1

think of the battery as a resistor and a power source

Answer 2

First off, draw out the circuit and label everything to be something like this:

So I've designated
R=20.0 ohm
E=12.0 volts
r=internal resistance
and the voltage drop was .3 volts.