On watts and joules
In the latest Iron Man movie, when said man gets his perma-electo-magnet mabob he launches into a hooey based conversation with his surgeon regarding the power source glowing in his chest. I don’t remember the lines exactly, but it went something like this:
Almost Iron Man: It produces 50 GJ/s
Cave Surgeon in Dapper Suit: That would power your heart-o-mabob for a hundred years!
Except for the fact that there’s not a single mention of capacity. Silly Cave Surgeon, Almost Iron Man expressed a rate, not a quanitity. However, shortly after this scene a lot of shit blew up, including the surgeon, so we can forgive this slight. I think AIM might have said 50 GW/s, which is a rate of a rate, and that makes even less sense, but then he started to fly, so again, it doesn’t really matter.
However, our fine friends at the MIT Tech Review posted an article proclaiming the benefits of formic acid fuel cells, in which they state
Beller points out that using formic acid to make hydrogen also has drawbacks. Compared with methane and methanol, formic acid has much less hydrogen. If you use all the hydrogen in a kilogram of methanol, you get 4.19 kilowatts of power, while the hydrogen in formic acid gives 1.45 kilowatts.
I’m guessing the good Dr. Beller stated kilowatt-hours.
To review:
- A joule is the basic unit of energy.
- The rate at which energy is transferred is a watt.
- The former tells us how much we have, the latter tells us how fast we can use it.
- Think gas tank vs. engine.
Engineers generally use watt-hours as a term of capacity, as it saves us from having to divide by 3600 when we have to figure how long we can run our batteries for. Scientists, who are increasingly interested in the minute, tend to use joules.