1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

1.5V/3W lightbulbs with a 9V battery. Will they burn out?

  1. Feb 25, 2014 #1
    1. The problem statement, all variables and given/known data

    Imagine that you have a box of 9V batteries and a box of flashlight bulbs labeled 1.5V/3W for operational voltage/power. What is the ohmic resistance of the bulbs? Can you turn on the 1.5V bulbs without burning them out by using the available 9V batteries?

    2. Relevant equations
    P=ΔW/t=ΔVI=ΔV2/R=I2R
    I = P/V
    R = V/I

    3. The attempt at a solution
    I in the bulbs will be 3W/1.5V which is 2A
    R in the bulbs will be 1.5V/2A which is .75ohms

    If you attach one of the bulbs to a 9V battery, the voltage drop will be = to IR which is (2A)(.75ohms) which is 1.5V. So using a 9V battery the bulb will only get 1.5V which will not cause the bulb to burn out. Is this correct?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 25, 2014 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    What they really mean is: It's a dark and lonely night, and all you have is a 9v battery and a box of 1.5v bulbs. Are you going to be stuck in the dark all night, or can you create some steady light? If so, how can you do it?

    Warning: if you cause any light bulb to experience (across its terminals) a voltage that exceeds its rated voltage, the bulb will burn out in too short a time to be useful here.
     
  4. Feb 25, 2014 #3
    I think I see. You would need to connect 6 lightbulbs to each 9V battery. This is because each lightbulb is rated for 1.5V. In order to create a voltage drop of 1.5V across each bulb and have the total voltage drop sum to zero, you would need 9/1.5 bulbs, which is 6 bulbs. Hopefully this is correct.
     
  5. Feb 25, 2014 #4

    NascentOxygen

    User Avatar

    Staff: Mentor

    Yes, I guess that's what was intended by this question. I'd call that a total voltage drop of 9v.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: 1.5V/3W lightbulbs with a 9V battery. Will they burn out?
Loading...