What Resistor Value Is Needed to Power a 1.4W, 4.0V Bulb with a 9.0V Battery?

[alwaysdazed]

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 don't 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!

 

[tiny-tim]

Welcome to PF!

Hi alwaysdazed! Welcome to PF!

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 don't 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.