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Homework Help: Physical Science Question on Electrical Power and Energy

  1. May 15, 2012 #1
    1. An electric space heater draws 15.0 A from a 120 V source. It is operated, on average, for 5.0h (hours) each day. a) How much power does the heater use and b) At $.10 per kWh, what does it cost to operate the heater for 30 days?
    2. Suppose each person in the United States uses 12,000 kWh of electricity a year and the power plants using coal to generate electricity are 40% efficient. If 1.0 kg of burned coal releases 33.0 mJ a) how much coal will be burned during a year for a family of four b) how large a pile of coal would that be (using 2620 kg/ m^3) c) how much heat is given off to the environment

    power= current x voltage, energy= power x time

    I can find a) at 1800 w (watts), but I have no idea where to start on part b.
    For 2, I'm unsure of where to star. I know the family of four would use 48,000 kWh a year, but after that, I'm lost
  2. jcsd
  3. May 15, 2012 #2


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    Staff: Mentor

    A kWh is the energy equivalent to one kilowatt for one hour duration. As you may recall, a Watt is equivalent to a Joule per second (J/s). So a kWh is 1000 W x 1hr = 3.6 x 106 J.

    For the first question you should be able to work out how many hours the heater runs in the 30 day period, and then find the kWh, etc.
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