Consider a battery that has Vt 140 V when it is not connect

Consider a battery that has V_t = 14.0 V when it is not connected to a circuit when a 10.0 Ohm resistor is connected with the battery, the potential difference across the battery\'s terminals drops to 12.9 V. What is the internal resistance of the battery?

Solution

current=14/(10+r)

Potential difference across battery=14-ir=12.9

1.1=14r/(10+r)

11=12.9r

r=0.8527 ohm