# Energy Questions, Really Easy. I Need Help!

## Homework Statement

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?

## The Attempt at a Solution

1. attempt failed.

Last edited:

Hootenanny
Staff Emeritus
Gold Member
Welcome to PF bruinboy28,
1. 100watt x 0.30 x 0.30
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?

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

Hootenanny
Staff Emeritus
Gold Member
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
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.

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.

Hootenanny
Staff Emeritus
Gold Member
do you have any hints/ suggestions? not trying to lazy out of this. I wouldve posted my attempt if I knew where to start.
How much energy does a 100W bulb expend in one month?

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:
Hootenanny
Staff Emeritus
Gold Member
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

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.
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.

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.

Redbelly98
Staff Emeritus
Homework Helper
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

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)?

> 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.

Redbelly98
Staff Emeritus
Homework Helper
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.

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

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:
Redbelly98
Staff Emeritus
Homework Helper
ok so

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

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.

1000 J/s

259,200,000=259200 Joules
Whoah! Huh?

if i needed to convert the seconds into joules...

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)?

^ 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:
Redbelly98
Staff Emeritus
Homework Helper
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 "?"

259,200,000 x (.30) =77,760,000

.30 = 77,660,000 / 259,200,000

Redbelly98
Staff Emeritus
Homework Helper
Hint:

Efficiency = (Energy output) / (Total energy used)

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:

Redbelly98
Staff Emeritus
Homework Helper
Had to log off the computer after my last post, but I'm back now for a little while.

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%
That's right. And we know that:
Efficiency = 30%
(useful energy or work output) = 259.2 MJoules

That leaves just one unknown thing in the equation: (total energy input)
You can solve for that unknown, once you have plugged in the 30% and 259.2 MJ in the right places.

Efficiency= (259.2 MJoules)(.30) / 259.2 MegaJoules , right? what am I doing wrong?

The problems with that statement are:

"Efficiency" should be replaced with its known, given value of 0.30 or 30%
The "0.30" on the right hand side does not belong there, because useful energy
is 259.2 MJ, not "(259.2 MJ)(.30)"
The final 259.2 MJ, written after the "/", does not belong there. "Total energy input" belongs here, we do not know it's value yet, it is what we are trying to find.

I mean, the total energy used is the energy that the 100w bulb is running for a month,which is 259.2 MegaJoules
Yes, true.
See my first block of comments above for how to go about solving this.

doh! thank you redbelly! you have been a really great help. I need to understand what they are asking for sometimes. if that's what they are looking for then, we are done!