A battery-battery-resistor circuit

  • Thread starter zimo
  • Start date
  • #1
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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:
 

Answers and Replies

  • #2
Doc Al
Mentor
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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:
  • #3
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Oh, I got it now, Thanks!
 

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