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Re: [Phys-L] definition of "power"

I define power as the rate at which energy is transferred or transformed. I tell students about the use of the word "dissipated" but other than to warn them that they will encounter it I try to avoid it as it suggests that energy disappears. I don't recall having thought of the advection question in terms of power. Hot gas flowing into a volume of space that I have defined as a system certainly represents energy flow into the system and I would indeed call the rate at which the energy flows into the system that way, power. The question of gasoline flow is a tough one. I don't think the location of the energy can be nailed down--it is somewhere between the gasoline and the oxygen with which it might react. We tend to assign the energy to the gasoline but suppose we had a methane atmosphere and oxygen was something we found underground; I'm guessing we would assign the energy to the oxygen.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of John
Denker
Sent: Tuesday, December 02, 2014 2:50 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] definition of "power"

On 12/01/2014 01:23 PM, Richard Tarara wrote:

Really, you need to be careful in every instance to be very clear.

1) That's excellent advice.

Different people define power differently. This horse escaped from the
barn a long time ago. For the foreseeable future, the R.T. approach seems
like the only game in town.

I've been known to use long-winded expressions such as "F dot v
mechanical power" and "I dot V electrical power".

On 12/02/2014 10:59 AM, Carl Mungan wrote:
How about defining power as rate of converting energy from one form to
another? This could be an internal conversion (burn chemical energy of
gasoline to get kinetic energy of motion of a car) or an external
transfer (in the form of work, heat, and/or mass transfer from one
object or system to another). I'm somewhat undecided about whether a
transfer of energy without a change in form should count (eg. the cue
ball hits the 8 ball elastically) but I suppose it could so one
doesn't have to worry too much about what one means by the vague term
"forms." Just consider any measurable change in a a system to involve
energy and hence power.

2) YMMV, but if you want to know my personal preferences, the case that
has me conflicted is advection. For example, suppose there is a small leak in
the gas tank of a car. Overnight a small amount of fuel leaked out. Is that
"power"?

To say the same thing again, consider the equation:
dE = F dX + T dS + advection

If you leave out the advection term, I am perfectly happy to let the rest of
dE/dt be power, i.e. mechanical power plus thermal power. Advected
power seems weird to me.
I could probably be persuaded either way.

3) I'm not sure I care. See item (1). I've pretty much written off this word as
one of those supposedly "technical" terms without any clear meaning. It's
nowhere near being the worst offender in this category.
https://www.av8n.com/physics/weird-terminology.htm
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