Re: Joules and MegaWatts

bryan borich [email removed] writes:
> Does anyone know the conversion of Joules to Megawatts?  (formula wise)
> And if so what is it <grin>.

Bryan, a gentle reminder. Joules and Megawatts are different dimensions,
so you are comparing apples and oranges. A Joule is a measure of Energy,
whereas a Watt is a measure of Power. Power is defined as the rate of
Energy use.

For an example, an automobile's gasoline tank holds gasoline, which has
an intrinsic fixed value of pontential energy per unit volume. I don't
know the real numbers, but assume that 1 kg of gasoline when combusted
with oxygen, will yeild 1MJ of energy. Now, this energy can be expended
at minute rates or huge rates, but the same quantity of potential energy
is released.

A Watt is defined as the use of 1 Joule per second. So, 1 kg of gasoline
fully combusted (1MJ) over 20 seconds would yeild an energy rate (power
dissipation) of 1MJ/20s = 50kJ/s = 50kW. If instead you squandered the
1kg of gasoline in .2 seconds, you would yeild a power of 1MJ/.2s =
5MJ/s = 5MW.

Pop quiz:
	1) does an automobile travelling up a hill at 20m/s have greater
	   ENERGY that one with the same amount of gas traveling down a
	   hill at the same altitude at 60m/s?
	2) does an automobile travelling up a hill at 20m/s generating
	   greater POWER that one with the same amount of gas traveling
	   down a hill at the same altitude at 60m/s?

Answers:
	1) no, both vehicles have the same amount of gas in the tank, so
	   chemical energy is the same; they have the same gravitational
	   potential energy (same altitude) and kinetic energy (same
	   speed).
	2) yes, the vehicle travelling up the hill is working harder.
	   It's using gasoline at a greater rate to increase it's
	   gravitational potential energy (altitude) while maintaining a
	   constant kinetic energy (speed).

You can also roughly think of energy as the capacity to perform work,
and power as the rate that work is getting done.

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