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3 Electrictity Problems

  1. Mar 18, 2008 #1
    1. Compute the cost per hour at 8 cents per kw-hr of electrically heating a room if it requires 1 kg per hour of anthracite coal having a heat of combustion of 8000 kcal/kg.

    2. The lights on a car are left on. They dissipate 95 watts. How long will it take for the fully charged 12-volt battery to run down if the battery is rated a 150 ampere-hours (A-hr)?

    3. What is the cost of electrically heating 50 liters of water(50kg) from 40 degrees C to 100 degrees C at 8 cents per kw-hr?

    3. For the first 2 I honestly don't know how to start or what to do. For the 3rd one, I multiplied 50*4.18*60 to get the joules. Than I multiplied that by the 8 cents. The answer I got was 1 dollar. I'm not sure if this is right, the problem does not give the time it takes, so I don't know how the 8 cents per kw-hr would work.
  2. jcsd
  3. Mar 18, 2008 #2
    1. you know how much anthracite it takes per hour to heat the room. You know how much a kg of anthracite produces if you burn it. you can compute how much energy you need in an hour. you have to convert that to Kilowatthours. Mmake sure you get the units right.

    1 kcal is the energy to heat 1 kg of water 1 degree celsius (or kelvin) and is equal to 4184 Joules.
    1 kilowatr-hr is the energy that a powersource of 1 kilowatt produces in an hour.
    it is equal to 3600000 Joules

    2. compute how many amperes the lights use. In an hour they will use that amount of amperehours

    3. 1 dollar is wrong.
    - if you use kilograms of water you will get an answer in kilojoules. You must then determine how much Kilowatthours that energy is, and multiply that by the price of a kilowatthour.
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