1. The problem statement, all variables and given/known data A circuit: Battery (1.5v) -- Battery (1.5v) -- Lamp(18 Ohm) and back to the first battery. The circuit is producing 0.25 Watt to the lamp. what is the inside-resistance of the batteries? 2. Relevant equations P=VI IR=V 3. The attempt at a solution 1.5+1.5=3Volt P=0.25Watt 0.25=3*I I=1/12 (1/12)*R=3 R=36 we already have 18 Ohms from the lamp so the remining 36-18=18 is the answer. WRONG
You cannot assume that the voltage across the lamp is 3V--not when the batteries have internal resistance. Instead, find an expression for power consumption in the lamp in terms of I and R, and solve for the current.