# Use Joules to find Energy

1. Feb 27, 2006

### edal01

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.

Last edited: Feb 27, 2006
2. Feb 27, 2006

### Pengwuino

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?

3. Feb 27, 2006

### edal01

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.

4. Feb 27, 2006

### chroot

Staff Emeritus
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

5. Feb 27, 2006

### Pengwuino

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

6. Feb 27, 2006

### edal01

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.

7. Feb 27, 2006

### Pengwuino

If those calculations are correct, I suspect they are using the nutritional concept of "calorie" which is actually kilocalorie which is 4186J per Calorie.

8. Feb 27, 2006

### Tide

Pen,

I think that's 4.187 to 4 significant digits (from 4.1868...) - not trying to be picky! :)

9. Feb 27, 2006

### Pengwuino

Tell my textbook that! :P