
#1
Aug3107, 07:18 AM

P: 45

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 insideresistance 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 3618=18 is the answer. WRONG 



#2
Aug3107, 07:59 AM

Mentor
P: 40,907

You cannot assume that the voltage across the lamp is 3Vnot 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.




#3
Aug3107, 08:13 AM

P: 45

Oh, I got it now, Thanks!



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