Use Joules to find Energy


by edal01
Tags: energy, joules
edal01
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#1
Feb27-06, 10:10 PM
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Please Help...

Given: A person walks about 3.6 calories/minute.
How much energy is in a 40 Watt light bulb, that is lit for 5.6 hours?

Answer: I think it is 144000 J/hr. I am not sure what to do with the 5.6
hours. I think it should be used to caluculate the total J which
would be 806400.

Question: I need to find the amount of time it takes to walk that equals the energy of the light bulb (in hours) using the calorie amount given.

I have taken the total J (806400) and divided by the calorie/hr (I converted the 3.6 calorie/min to calorie/hour). Then I tried to solve for the total number of hours. I keep getting the wrong answer.
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Pengwuino
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#2
Feb27-06, 10:28 PM
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Well, "how much energy is in a 40 watt lightbulb" doesn't make sense. It's correct to say "How much energy is used by a 40 watt lightbulb".

What answer are you getting and did you convert from calories to joules?
edal01
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#3
Feb27-06, 10:38 PM
P: 3
Thanks for the reply...

When converting calories to J: 3.6 calories =0.251 J/s

0.251 J/s = 903.6 J/hour

Since I already have the total Joules: 806400 J for 5.6 hours, I used it by dividing the 806400 by 903.6. I am getting 892.43 hours.

chroot
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Feb27-06, 10:48 PM
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Use Joules to find Energy


First, notice the units. 1 watt is defined as one joule per second. A 40 watt bulb consumes 40 joules per second. 5.6 hours is 20,160 seconds. 40 joules per second * 20,160 seconds = 806,400 joules. (Not joules/hr[/b].)

- Warren
Pengwuino
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Feb27-06, 10:49 PM
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Uhm, 1 calorie = 4.186J. Calorie is not a rate.

You need to determine how many Joules of energy per second are being used if theres 3.6 calories/minute being used. I think they maybe using the biological use of "calorie" which is usually Kilocalorie but still denoted as calorie. If this is the case, 1 Calorie = 4,186J
edal01
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#6
Feb27-06, 11:30 PM
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I converted 3.6 calories/minute to Joules of energy per second. I got 0.251 J/s. Then I divided the total energy (806400) by 0.251. This gave the number of seconds which is 3,212,749. I coverted this to hours and got 892.43 hours. It is wrong. I am not sure what I am doing wrong.
Pengwuino
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Feb27-06, 11:40 PM
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If those calculations are correct, I suspect they are using the nutritional concept of "calorie" which is actually kilocalorie which is 4186J per Calorie.
Tide
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Feb27-06, 11:44 PM
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Pen,

I think that's 4.187 to 4 significant digits (from 4.1868...) - not trying to be picky! :)
Pengwuino
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Feb27-06, 11:46 PM
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Tell my textbook that! :P


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