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Homework Help: Recharging a battery when Lightbulb and TV are in parallel

  1. Jul 3, 2016 #1
    1. The problem statement, all variables and given/known data
    In a household, the wife reads a book that uses a light bulb that has a resistance of 100Ω and is connected to a 100 V line while the husband watches a TV that draws 100 W of power. They do these actives for 200 mins. Power comes from a wind generator that charges batteries. The wind supplies 10 amperes at a voltage of 100 V.

    How long must the wind blow to recharge the battery.
    2. Relevant equations
    P= V2/r

    3. The attempt at a solution
    Modeled the circuit to have the light bulb and tv in parrallel. Using the power equation, the tv also has 100Ω of resistance which doesn't make to much sense to me.

    Power is J/s so we know that 200 min is 12000s which tell us 1.2 MJ is required to charge the battery.
  2. jcsd
  3. Jul 3, 2016 #2


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    Gold Member

    Think about W consumed and W generated, that may make it clearer for you. You don't need a circuit diagram to solve this. You are missing an equation for wind power - what is it? That is key to seeing the solution in the simplest way. Write three power equations -

    TV Power =
    Light Power =
    Wind Power =

    That should help.

    I see two potential answers - one (trivial) answer assuming the wind is blowing while the appliances are drawing power, and a different answer assuming the wind only starts blowing after the appliances have been turned off.
  4. Jul 3, 2016 #3
    Thank you for the response.

    The missing equation is P=IV which helps us calculate the Wind Power

    I've calculated the following:
    TV Power = 100W
    Light Power = 100W
    Wind Power = 1000W

    Since a watt is a J/s then the TV and Light require 2.4x106 J
    Divide power required by wind power and we get 2400 s which is 40 minutes? Does that sound correct?
  5. Jul 3, 2016 #4


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    Science Advisor
    Homework Helper
    Gold Member

    40 mins is correct.

    You can also get to same answer using
    Energy out = Energy In
    200W * 200mins = 1000W * 40mins

    Ok so the Watt.min isn't an SI unit but that doesn't matter.
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