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Homework Help: How do I estimate hours of use and find power for appliances?

  1. Mar 24, 2014 #1
    1. The problem statement, all variables and given/known data

    Please look at the attachment, I'm not sure how to go about figuring out the estimated hours of use per week for the appliances, and the Power(wattage) as well. I figured to find out one you would need the other, they are given in the answers section but I need to know how to find out for myself. It should be really obvious but for some reason I'm just stumped on this.

    2. Relevant equations

    1kWh=1000J/s * 3600

    3. The attempt at a solution

    So the energy consumed per week for each appliance was easy to find out, by dividing the energy consumed per year with the number of weeks in a year.

    This gave, 5.08kWh/week for the TV, and 17.5kWh/week for the dryer
    Using these to multiply the given cost of electricity gave the Running cost per week, which was $0.30/week for the TV, and $1.05/week for the dryer.

    Now comes the part which asks for the estimated hours of use per week and Power, which I just couldn't seem to figure out where to start. Any guidance to redirect my train of thought could even be greatly helpful, or just straight up lay the math out for me would be even better.

    In the answers section, The "Estimate hours of use per week" was 45h/week for TV, and 5h/week for the Clothes dryer.
    The power for tv is 0.11kW or 111W , and for dryer is 3.5kW or 3500W
    I know the total energy spent per week divided by either gives the other, but how do i find out one without the other?


    Attached Files:

  2. jcsd
  3. Mar 24, 2014 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    You have kwhr/yr
    How many weeks are there in a year?
  4. Mar 24, 2014 #3
    52, which is what i used to find the energy consumption per week, but how do I apply that to find hours of use per week?
  5. Mar 24, 2014 #4
    You cannot find the actual power rating for the given data. The devices do not function continuously over the given period (1 year).
    You can just calculate some average power for the week or day. Is a misnomer to call these "power ratings" and to compare them is not too relevant. The TV is used for much longer than a dryer.

    But it seems that this is what they ask.
  6. Mar 24, 2014 #5
    So this means I cannot find the specific power rating of the device using the given data? If so how did the textbook arrive at the answers (attachment, answers.jpg). Also, does that mean I cannot actually find the amount of time in hours, that each device is used in a week, but rather the average amount of time its used on a weekly/daily basis? Sorry if I'm misunderstanding.

    In the part where I actually have to fill out a chart to submit for grading (attachment, fill.jpg) , it similarly asks for power, although it species "for one week" this time, and to estimate hours of use per week. Which is why I'm trying to figure out how they did that in original example that had the answers given. Thanks for taking your time out to help me.

    Attached Files:

  7. Mar 24, 2014 #6
    They estimate the hours per week. They are not calculated from the given data.
    It is your job to estimate (based on your experience) how many hours per week do you use a TV and a dryer. As an average, of course.
  8. Mar 24, 2014 #7
    Doh! That went right over my head. It never occurred to me I could just make that one up , or estimate it, so to speak.
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