1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Headlight Power Question

  1. Aug 24, 2013 #1
    1. The problem statement, all variables and given/known data
    The headlights on a car operate typically at 60 W and the parking lights typically at 5 W. Assuming that there are two main headlights and four parking lights, what length of time will it take to discharge a 60 Ah battery, if the lights are left on?

    2. Relevant equations
    I = Q/t

    V = W/Q

    P = W/t

    P = I2*R

    P = V2/R

    P = IV

    3. The attempt at a solution
    I have gotten no where with this question. :frown:

    All that I could do was find that the total power consumed by the lights:

    PT = (2*10) + (4*5) = 140 W

    , and the charge of the battery:

    1 Ah = 3600 C
    ∴ 60 Ah = 216,000 C

    , and that's it.

    I have tried simultaneous equations by substitution and elimination but that doesn't work because all of the equations are derived from the base equations I = Q/t, V = W/Q and P = W/t, so I end up getting P = P all the time.

    Hint's anybody?

    Thank you.
  2. jcsd
  3. Aug 24, 2013 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Not sure, but maybe you are suppose to assume that the voltage of the battery is the typical voltage for a car battery.
  4. Aug 24, 2013 #3


    User Avatar
    Homework Helper

    You need the voltage of the car battery to find the current. Look at Automotive battery - Wikipedia, the free encyclopedia, or just see the battery in your car...


  5. Aug 25, 2013 #4
    Hmm... It seems like it... Based on this link, which is the worked solution to the question.

    Thank you TSny and ehild.

    So much for "complex, challenging and novel"... :rolleyes:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted