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Potential energy to watts

  1. Mar 30, 2016 #1
    • OP warned about not using the template
    Could anyone help me with this sum - To generate electricity, 180000 kg water is dropped from a height of 50 m (let g = 10 m/s^2). If half the gravitational potential energy is converted to electric energy, how many 100 W bulbs can be lit ?

    Formula - Ep = mgh

    I calculated the potential energy to be 90000000 J. Half of it is electric energy = 45000000 J. I am at a loss from here.
     
    Last edited: Mar 30, 2016
  2. jcsd
  3. Mar 30, 2016 #2

    DrClaude

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    The units don't work. Energy should be in joules. You will also need a time so you can convert the power consumption of the light bulbs into a total energy consumption.
     
  4. Mar 30, 2016 #3
    So, is the question wrong ?
     
  5. Mar 30, 2016 #4
    why not use the conversion table of watt and joule? one joule per second is equal to one watt!
    when water is coming down -it should have some rate of flow - so you should not write Js only -whether s is for time or its joules?
     
  6. Mar 30, 2016 #5

    DrClaude

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    Not necessarily. It is not very well formulated, but there are different ways of coming up with a reasonable answer.
     
  7. Mar 30, 2016 #6
    s is for seconds
     
  8. Mar 30, 2016 #7
    Lets say 180000 kg of water fall per hour. Now, can we find an answer ?
     
  9. Mar 30, 2016 #8
    does it mean joule in a second then it is J s^-1 or J/s -it means the rate of flow is there -then you can convert directly to watt and calcilate number of light bulbs.
     
  10. Mar 30, 2016 #9

    DrClaude

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    Yes. You first need to figure out how much gravitational potential energy there is in that mass of water.
     
  11. Mar 30, 2016 #10
    Using the mgh formula, I multiplied 180000 kg, 50 m and 10 m/s^2. It gave 90000000 kgm^2/s^2. I made a mistake, kgm^2/s^2 equals just Joules and there is no seconds.
     
  12. Mar 30, 2016 #11

    DrClaude

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    Correct. Now you need to convert it to power, using the assumption you have made about the rate of flow.
     
  13. Mar 30, 2016 #12
    Ok, so 90000000 J is converted to 45000000 J of electric energy. As 1 watt = 1 joule per second, 45000000 J/ 3600 s gives 12500 W. So, is the answer 125 bulbs (12500 W/ 100 W) ? If it is, Thanks a lot.
     
  14. Mar 30, 2016 #13

    DrClaude

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    That sounds fine.
     
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