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Determining value of resistor

  • #1

Homework Statement



A flashlight bulb rated at 1.4W and 4.0V is operated by a 9.0V battery. To light the bulb at its rated voltage and power, a resistor is connected in series as shown in the figure. ( i dont know how to paste the picture but its only a resistor and then light bulb in series with a 9v battery attached)

What value should the resistor have?
Express your answer using two significant figures.





Homework Equations


P=IV
V=IR


The Attempt at a Solution



I know that when resistors are in series the current is equal throughout.. so i used P=IV to solve for current, then pluggled that into V=IR using to find R but that is obviously wrong i am going crazy!
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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Welcome to PF!

Hi alwaysdazed! Welcome to PF! :smile:
A flashlight bulb rated at 1.4W and 4.0V is operated by a 9.0V battery. To light the bulb at its rated voltage and power, a resistor is connected in series as shown in the figure. ( i dont know how to paste the picture but its only a resistor and then light bulb in series with a 9v battery attached)

What value should the resistor have?
Hint: the flashlight will only take 4.0V …

so you need to use up 5.0V on the reisistor.

use Kirchhoff's rules for potential differences. :wink:
 

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