# Is it reasonable to convert energy to power over time?

I have recently been talking with another person on instructables about converting 33 megajoules into a standard comparable electric unit, watts.  Now the formula for joules just so happens to be watts times seconds, but he still thinks this is wrong.  He says I can't just assume time.  Now I can assume any amount of power for any amount of time I wan't to as long as it adds up to 33,000,000 joules in the end right.  So if I wan't to consume one watt every second, then I have to set the amount of time as 33,000,000 seconds or roughly 9,166.7 hours.  This all seems fine to me right, because one watt for 33,000,000 seconds is equivalent to 33 megajoules.

Now I originally didn't understand all of this so I just fumbled around and ended up converting to hp and got roughly 12, which is very close for being a rounded off number.  It comes out over one hour to equal just over 32 megajoules.

I can't seem to understand what the problem is, the DEFINITION OF ENERGY/WORK IS POWER *(OR OVER THE PERIOD OF) TIME
Is there really such flawed logic here that "joules could be converted to cubic meters?"

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lemonie5 years ago

Watts and Joules are defined, if those names are used there is no ambiguity.
Watts and Joules are different terms though, there is a conversion if you choose a measure of time, the assumption (if any) relates to something you were discussing that you haven't mentioned to us yet.

Agree upon what your terms mean and better define better what your actual argument is (between yourselves)

L
jj.inc (author)  lemonie5 years ago
I am sorry you didn't understand, it seems that you manage to find a way to not understand any questions I ask. Basically someone said the biggest railgun was 33 MJ which could power a city, I found a conversion thing and found that as equivalentish to 12 hp over a time of one hour. I said, that couldn't power my house for a day. Someone new came on and said you can't convert that, they are two different measures, one energy and one power. He continued his argument and ended with this statement. "with your logic you could convert joules to cubic meters" which I didn't really need to know. I wanted to know if my argument was a reasonable one, or if I was wrong, and why.
In the end I think I was correct, and I did convert joules to cubic meters of gasoline. It wasn't a real conversion though, I just found the energy in a liter of gas.
kelseymh5 years ago
You can't compare power directly to work. But if you already know the amount of time (duration) that the power is going to be delivered, then yes, you certainly can compute the total energy delivered during that time, which is what you computed above.

One watt delivered for two minutes is just about the same as a hundred watts delivered in one second. However, you aren't going to be able to use that "one watt" to heat something up as effectively as the "hundred watts." The longer time means that your target can dissipate the heat, conduct it away, etc.
lemonie5 years ago

For Joules to cubic metres, you could do "cubic metres of-", convert to mass and use E=MC2.
That's my best answer for that one.
(Or entropy of a 1m3 vacuum?)

L
steveastrouk5 years ago
Power is a rate of doing work. If you have a certain amount of energy stored, and you release it all very quickly, a higher power is dissipated than when you do it slowly.

You are quite right, if you do work at the rate of 1 J/sec, and you store the energy released for 33,000,000 seconds, you have 33 MJ.

Where is this "assumption" of time ?
5 years ago
+1

It really helps to equate the units in their true form of 1 joule/second = 1 watt = blablabla Then when you convert the units all make sense.

http://en.wikipedia.org/wiki/Watt

Formulas expressed there. An amount of energy is not the same as the amount of energy expended over a given amount of time. one is a total, the other is a flowrate.
5 years ago
quote from same wiki article:

Confusion of watts, watt-hours, and watts per hour

The terms power and energy are frequently confused.[citation needed] Power is the rate at which energy is generated or consumed.

For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt-hours (W•h), 0.1 kilowatt-hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours. A power station would be rated in multiples of watts, but its annual energy sales would be in multiples of watt-hours. A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 MJ.

Terms such as watts per hour are often misused.[16] Watts per hour properly refers to the change of power per hour. Watts per hour (W/h) might be useful to characterize the ramp-up behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations.

Major energy production or consumption is often expressed as terawatt-hours for a given period that is often a calendar year or financial year. One terawatt-hour is equal to a sustained power of approximately 114 megawatts for a period of one year.

The watt second is a unit of energy, equal to the joule. One kilowatt-hour is 3,600,000 watt-seconds. The watt-second is used, for example, to rate the energy storage of flash lamps used in photography.