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Electrical Energy and Power

  1. Mar 7, 2007 #1
    1. The problem statement, all variables and given/known data
    The cost of electricity varies widely throughout the United States; $0.120/kWh is a typical value. At this unit price, calculate the cost of (a) leaving a 40 W porch light on for 2 weeks while you are on vacation, (b) making a piece of dark toast in 3 min with a 970 W toaster, and (c) drying a load of clothes in 40 min in a 5200 W dryer.

    3. The attempt at a solution
    I have my attempt but I doubt I doing this right because it doesn't make sense at all. Can you please tell me where I'm messing up? Thanks in adv.! :smile:

    [tex]a) 40W*\frac{1kW}{1000W} = 0.04 kW[/tex]
    [tex] 2 weeks * \frac{7days}{1week}*\frac{24 hours}{1 day}=336h[/tex]
    [tex] \frac{0.120}{kWh}*0.04kW*336h= 1.6128[/tex]

    Is my method for (a) right? I got $1.6128 and it seems too little.. =[
  2. jcsd
  3. Mar 7, 2007 #2
    14 days times 40 w; thats 24*14 *.04=13.4 KW-hrs times 12 cents=what you say. Post the rest of the results,I'd be curious.
  4. Mar 7, 2007 #3
    I'm not a maths expert, but you're logic seems to make sense, even though the answer is quite small. I worked it out in a similar way to you:

    energy transfered or total units (Kw/h) = power (kw) x time (h).


    total cost = number of units x cost per unit.

    Perhaps, 40W is not a lot of power? The average hairdryer is 1.5kw
  5. Mar 7, 2007 #4


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    Your answer is correct to part a.

    Part of the reason we consume such an enormous amount of resources in the US is precisely because the price of those resources is so low compared to other places in the world. 40W is currently an almost insignificant amount of power in terms of its cost, but this might not always be so.

    - Warren
  6. Mar 7, 2007 #5
    Thanks for all the replies. =]
    As to denverdoc, here's what I got for part b) and c):

    [tex]b) \frac{0.120}{kWh}*0.970kW*0.05h= 0.00582[/tex]
    b) $0.00582 o_0 is this correct??

    [tex]c) \frac{0.120}{kWh}*5.2kW*0.667h= 0.416208[/tex]

    b) $ 0.416208
  7. Mar 7, 2007 #6
    Thanks. Theres something to be said for air drying ones clothes the old fashioned way.

    My only issue I see with the answers are the number of significant figures; rremember that the answer should have no more sig figs than the number with the least:


    So answer b should be .01, etc
    Last edited: Mar 7, 2007
  8. Mar 7, 2007 #7
    No problem. =] Thanks for reminding me of the sig figs :smile:
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