# B Conversion between Watt and Joule

1. Sep 18, 2018

### jamiebean

example)
E = 2734.2 joules per gram
P = 2.73 x 109 watt/gram

q: How can this conversion work?????

E = 21000 joules per gram
P = ?

2. Sep 18, 2018

### lomidrevo

what do you want to convert? Joule is unit of energy, Watt is unit of Power. Those are different physical quantities.

3. Sep 18, 2018

### darth boozer

1 watt = 1 joule per second

4. Sep 20, 2018

### jamiebean

E=energy
P=power

s the conversion above possible??

5. Sep 20, 2018

### lomidrevo

As I said, those are different quantities, so I miss any meaningful reason for such conversion. Maybe, what you want to rather know, is the relationship between them. At the most basic level, we can define Power as the rate of doing work (using energy), or amount of work done per time interval and the relationship between the units is $W = \frac{J}{s}$

6. Sep 20, 2018

### A.T.

If you know the time.

7. Sep 20, 2018

### lomidrevo

I don't think that conversion is the correct word here. We could also ask (as analogy to Energy and Power): What is the conversion between Distance and Speed? It doesn't have any physical meaning. But we can say there is close relationship between them, when describing motion of an object.

8. Sep 20, 2018

### CWatters

If the two systems are similar I would try using ratios (scaling)

P2/E2 = P1/E1
So..
P2 = (P1/E1) * E1

P2 = (2.73 * 10^9 / 2734.2) * 21000

Without more information it's not possible to say if this is the correct approach. Not everything scales linearly like this.

9. Sep 20, 2018

### Mister T

$\frac{2734.2 \ \mathrm{J/g}}{2.73 \times 10^9 \ \mathrm{W/g}}\approx 1.00\times10^{-6} \ \mathrm{s}$

This calculation tells us that if you delivered energy at a rate of 2.73 x 109 J/s for 1.0 x 10-6 seconds you'd deliver a total of about 2734.2 J of energy. There are no conversions involved here.

It's like saying your position changes by 60 miles every hour when you move at a speed of 60 mi/h. If you do this for, say 2 hours, you'll travel a distance of 120 miles. Note that I'm not converting 60 mi/h to 120 mi. Again, there are no conversions here.

Conversions are present when two quantities are measuring the same thing but in different units. When you measure a speed of 60 mi/h you're measuring a speed, not a distance. And when you measure a distance of 120 mi you're measuring a distance, not a speed. Speed is the rate at which distance changes.

The watt is a unit of power, not energy. The joule is a unit of energy, not power. Power is the rate at which energy is transferred.

10. Sep 20, 2018

### OmCheeto

Can't see that anyone has done it so far, but I'd like to point out that the equations are incorrect.
Joules/gram is "specific energy", not just energy.
Likewise, watts/gram is "specific power". Which happens to be such an obscure thing, that wiki doesn't even have an entry for it.
They do list two examples though, in their "power density" page:
Stars/hydrogen: 1.84 watts/gram
Plutonium: 1940 watts/gram​

This might seem trivial, and it appears that everyone has simply worked around it, as "grams" simply cancels out, but it confused me at first.

In my confusion, I decided to find out the specific energy of something, and randomly picked apples.
Serendipitously, they have a surprisingly similar specific energy to your number: 2200 joules/gram
From that, I determined that I'd have to eat 6,700 apples per second, at the given "specific power".
Which is when I think I went back to your original post, as that seemed like a lot of apples to consume, per second.

11. Sep 20, 2018

### Staff: Mentor

Yes, but that's just the chemical energy released in digesting apples. You'll need a lot less if you fuse them.

12. Sep 20, 2018

### Mister T

All you'd need is a tiny piece of one of those apples, provided you had a comparable amount of anti-apple.

13. Sep 21, 2018

### CWatters

Specific power isnt totaly obscure, it's used for engines or batteries where weight matters, such as aircraft.

14. Sep 21, 2018

### CWatters

I think we need the OP to provide more context to his question.

15. Sep 21, 2018

### OmCheeto

Yes. I remember the "power to weight" ratios listed for cars, back when I was a youngster.
Interesting how "specific power" is mostly only a concern of rocket scientists, 16 year olds, and electric car manufacturers.
hmmmm........
Yes.
Where is that OP?

16. Sep 24, 2018

### zanick

a watt-second is a joule.

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