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Energy Questions, Really Easy. I Need Help!

  1. May 18, 2008 #1
    1. The problem statement, all variables and given/known data

    1. Suppose you left a 100 watt light bulb on for one month. If the electricity generation and transmission is 30% effective how much chemical energy in joules was wasted at the power plant for this oversight ?

    2. If electricty is generated at 10,000 volts and is to be transmitted at 130,000 volts what should be the turns ratio of the transformer?


    2. Relevant equations



    3. The attempt at a solution

    1. attempt failed.
     
    Last edited: May 18, 2008
  2. jcsd
  3. May 18, 2008 #2

    Hootenanny

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    Welcome to PF bruinboy28,
    I'm afraid that this doesn't constitute an attempt. You must have some idea of how to approach this problem, what are your thoughts/initial attempts?
     
  4. May 18, 2008 #3
    haha I shouldn't even have even put that. I've been reading the chapter in regards of figuring this problem, but still have no idea....that just an example that i THOUGHT I could have followed.\


    nm...yea, i shouldn't even have put that. thats a bad example to follow
     
  5. May 18, 2008 #4

    Hootenanny

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    I still can't believe that you have no idea how to solve either question. You say the you have read the relevant chapters and examples, you must have some idea of how to start.
     
  6. May 18, 2008 #5
    Hootennany, sadly... I still don't. I wish I can blame the book, but that would a be false accusation. I looked over the lecture notes and the book and I can't seem to put it all together.

    do you have any hints/ suggestions? not trying to lazy out of this. I wouldve posted my attempt if I knew where to start.
     
  7. May 18, 2008 #6

    Hootenanny

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    How much energy does a 100W bulb expend in one month?
     
  8. May 18, 2008 #7
    correct me if I am wrong

    E=Power(delta)time

    so...assuming a month is 30 days 30(24)=720
    so Energy=100watt(720)= 720,000 = 720 kWH

    is this right Hootenanny? Thanks for the help btw, I am not good with physics. Thus I am trying to take an intro to get a better grasp at it.
     
    Last edited: May 18, 2008
  9. May 18, 2008 #8

    Hootenanny

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    You're on the right lines. However, one Watt is one Joule per second, therefore you need to convert 30 days into seconds, not hours.
     
  10. May 18, 2008 #9
    i see. But is it safe to say that a month consist of 30 days? I mean, to get more technical, a month could consist of 29 or 31 days? so how do we conclude how many days a month is consist of?

    so my conversion is this:

    30 day x 24hr/1 day x 60 min/1 hr x 60 seconds/ 1 min= 2,592,000 seconds = joules.
     
  11. May 18, 2008 #10

    Redbelly98

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    30 days per month is fine.

    Okay, you've shown that a month (of 30 days) is 2,592,000 seconds. That's good.

    The "= joules" you wrote does not make sense, just tacked on like that. Use the equation you had earlier,

    E=Power(delta)time
     
  12. May 18, 2008 #11
    kk. So

    Do i keep it in joules to proceed to this problem? Since it ask for joules, I should covert the seconds into joules.



    when it says the electricity generation and transimission is 30% how much chemical in joules was wasted at the power plant for this oversight?, how do i compute these into the problem

    Do i multiply the Energy by (.30)?
     
  13. May 18, 2008 #12
    > Do i keep it in joules to proceed to this problem? Since it ask for joules, I should covert the seconds into joules.

    Keep what in joules?

    You haven't actually calculated E yet. You calculated the number of seconds in one 30 day period, but didn't plug that back into your equation.
     
  14. May 18, 2008 #13

    Redbelly98

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    Let's get the energy used by the light bulb correct first before proceeding.

    You have the bulb power in joules/sec. You have the time interval in seconds. What do you get for energy,

    Energy = Power x time

    After you calculate what this energy is, then we'll worry about where the 0.30 comes in.
     
  15. May 18, 2008 #14
    ok so

    Energy= 100watts (2,592,000 seconds)=259,200,00
    am I doing it, right? :C eee..

    1000 J/s

    259,200,000=259200 Joules
    if i needed to convert the seconds into joules...

    Energy= 100watts x 259200 = 25,920,000


    -------------------------------------------------------------BETTER ANSWER--------------------------

    let me try to make it so that I understand it

    Energy= Power x Time
    E= (100 J/s)(2,592,000 s)= 259200000 J
     
    Last edited: May 18, 2008
  16. May 18, 2008 #15

    Redbelly98

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    If you meant 259,200,000 after that "=" sign, and if you tack on the correct units of Joules ... then yes, you did it right, you have correctly calculated the energy used by a 100W light bulb in 1 month.

    What is this about?

    Whoah! Huh?


    No, you can't convert between seconds and Joules. You can convert seconds only into other time units, eg. hours, days, milliseconds, etc. Just like you can convert meters into other length units, eg. cm or inches.

    But you can't convert a time unit (like seconds) into units of energy (like Joules). Seconds can only be converted into other time units. Only other energy units can be converted into Joules.

    Hope that makes sense.

    Okay, moving on ...

    Now that we know the light bulb uses 259.2 Megajoules in one month, the next question is:

    At 30% efficiency, how many Joules are need to produce an output of 259.2 MJ (and run that light bulb for a month)?
     
  17. May 18, 2008 #16
    ^ I am confused. So let me get something sorted out

    The energy of a 100 watt on for a month produces an energy of 259.2 megajoules.
    but since its' only at 30% efficiency.

    then... (energy) x (efficiency) = x = 77,760,000
    So.... 259.2 J - x= ?

    am i making sense at all??
     
    Last edited: May 18, 2008
  18. May 18, 2008 #17

    Redbelly98

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    We are getting there.

    That 30% efficiency means that 0.30 is the ratio of two different energies. One of those energies is the energy used by the light bulb in one month.

    So,

    0.30 = ? / ?

    what goes in place of each "?"
     
  19. May 18, 2008 #18
    259,200,000 x (.30) =77,760,000

    .30 = 77,660,000 / 259,200,000
     
  20. May 18, 2008 #19

    Redbelly98

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    Hint:

    Efficiency = (Energy output) / (Total energy used)
     
  21. May 18, 2008 #20
    that's weird, when i punched in the numbers i got it the first time.

    I can see the equation in the book that you used as a hint for me

    Efficiency= (useful energy or work output/ total energy input ) x 100%

    Efficiency= (259.2 MJoules)(.30) / 259.2 MegaJoules , right? what am I doing wrong? I mean, the total energy used is the energy that the 100w bulb is running for a month,which is 259.2 MegaJoules

    sorry for being such an idiot......:|
     
    Last edited: May 18, 2008
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