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Physics Electicty

  • Thread starter Hellohi
  • Start date
  • #1
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Homework Statement


A pgysics student wishes to determine the amount of electic enery consumed in one day at his school as a result of classsrom and hallway lighting. A quick survery revealed that there were approximately 200 40 W fluroescent lights opearting under a potentional difference of 240 V for 16 hours each day.

Hey guys, this isn't a hard question, I just dont know how to do it, could you please explain how?


Homework Equations


V=IR V= deltaEq/Q


The Attempt at a Solution



I dont know where to put the watts in my equations or what to do with it.
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
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Hello Hellohi! :smile:

(have a delta: ∆ wink:)

Homework Equations


V=IR V= deltaEq/Q
W = VI (= I2R) :wink:
 
  • #3
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Hey, I got my question right without the help but thank you very much for replying.( Doesnt know wether to drink or eat delta):D
 
  • #4
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Hey, I got my question right without the help but thank you very much for replying.( Doesnt know wether to drink or eat delta):D
I know u have solved the problm but just for the better understanding of everyone else who follows this link, I'd like to add..
W=VI is but only instantaneous power.
To find energy consumed u'd have to include the factor of time..
Evergy consumed =[tex]\Sigma[/tex] (power consumed per device*time in hours) in the units of watt-hour.

But electrically the question is incomplete as it does not tell the student if the system is DC or AC and expects the student to complete the assignment assuming the system to be DC. If the system is AC then it also require one additional factor called the Power factor or the relationship of lead/lag of current to voltage.
Also in the question, there is no actual need of the voltage information if the computation is to be done based on the wattage and time information, it is but a distraction to confuse the student into thinking it is relevant information.
The approximate Power consumed per day is;
= 200 x 40 x 16 watt-hour
where 200 is the number of lights
40 is the instantaneous wattage or power consumed by each of the 200 lights
16 is the number of hours the power was consumed
= ~128000 watt-hour or 128KW-hr where '~' stands for approximately as data states that the power consumed is approximately 40W. K stands for Kilo or multiplier of 1000

the average power consumed per hour is =128KW-hr/24 in unit of watt
= ~128000/24 watt
= ~5.333 KW
 
  • #5
tiny-tim
Science Advisor
Homework Helper
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( Doesnt know wether to drink or eat delta):D
You can get delta-wings :tongue2: …

barbecue them in sauce! :biggrin:
 
  • #6
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LOL, thanks for the laughs and help, and I should have been more clearer with my second reply. The thing is, the textbook expects me to use the formula P=VI but they dont say that in that part of the chapter, only after it. :rofl:
 
  • #7
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LOL, thanks for the laughs and help, a........textbook expects me to use the formula P=VI but they dont say that in that part of the chapter, only after it. :rofl:
Hi, but can you please share ur answer, if not the steps u used... Coz actually dont need to use the P=VI equation. I understand that the thread is over, but just wanted to know what the book expected you to do...
 
  • #8
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Hey man, soz for not replying earlier. The book actually expected me to use P=VI and no other formula. If I find any other way to solve this question ill let you know.

All the best wishes. :smile:
 
  • #9
42
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Just let me know if you want the answer, thanks.

All the best wishes.
 
  • #10
84
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yeah sure, let me know the answer as I have already posted my answer in the 4th post...
Just wanted to concur with what was required...
 
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