Question

A rule of thumb used to determine the internal resistance of a source is that it is the open-circuit voltage divided by the short-circuit current. Is this correct? Why or why not? 

Answer 1

Answer

Thumb rule describes that the source’s internal resistance is the voltage of the source at open circuit condition divided by current flows through internal resistance at short circuit condition.

At open circuit condition the voltage of battery source is equal to emf of the source. The battery source consists of internal resistance The formula to find current flows through the internal resistance at short circuit condition using ohm’s law is I=εr .

From the current expression, the expression to find source’s internal resistance is

r=εI

• ε is the voltage of the source at open circuit condition,
• I is the current flows through internal resistance at short circuit condition.