A battery-battery-resistor circuit

[zimo]

Homework Statement

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?

Homework Equations

P=VI
IR=V

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

 

[Doc Al]

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.

 

[zimo]

Oh, I got it now, Thanks!