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A battery-battery-resistor circuit

by zimo
Tags: circuit
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zimo
#1
Aug31-07, 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 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
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Doc Al
#2
Aug31-07, 07:59 AM
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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
#3
Aug31-07, 08:13 AM
P: 45
Oh, I got it now, Thanks!


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