A battery-battery-resistor circuit

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The discussion revolves around calculating the internal resistance of batteries in a circuit with two 1.5V batteries connected to an 18 Ohm lamp, producing 0.25 Watts. The initial approach incorrectly assumed the voltage across the lamp was 3V, leading to an erroneous calculation of internal resistance. The correct method requires expressing power consumption in terms of current and resistance, taking into account the internal resistance of the batteries. The realization that the voltage across the lamp is affected by the batteries' internal resistance is crucial for accurate calculations. Ultimately, understanding the relationship between power, current, and resistance is key to solving the problem correctly.
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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:confused:
 
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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.
 
Last edited:
Oh, I got it now, Thanks!
 

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