Date: Tue, 8 Jun 2010 18:59:49 -0700 (PDT)
From: "John M. Steele" <[email protected]>
Subject: [USMA:47581] Re: One unit only
To: "U.S. Metric Association" <[email protected]>
Cc: "U.S. Metric Association" <[email protected]>
Jim,
I agree with glassy-eyed and wobbly kneed, but this
is the ROOT of all energy vs power confusion.
Just ask, "So, after the grill has consumed 15000
BTU, it dies? That seems like a lot of money for a
grill with a one hour life."
I'm afraid it is up to the engineers to be
persistent PITAs on this matter. Proud to serve. :)
------------------------------------------------
From: James R. Frysinger <[email protected]>
To: [email protected]
Cc: U.S. Metric Association <[email protected]>
Sent: Tue, June 8, 2010 9:47:38 PM
Subject: Re: [USMA:47579] Re: One unit only
John, your last paragraph can be exemplified with
the ratings that one sees posted for gas grills.
They are usually rated as being, for example, "15
000 BTU". What is meant, though, is "15 000 Btu/h"
-- where I have fixed the error in the symbol and
have added the divisor. The former is an energy
value; the latter is a power value (the rate at
which chemical energy is converted to thermal
energy).
Caution: Experience has shown that if I try to
discuss this with the sales staff, they get
glassy-eyed and start to look wobbly in the knees.
Jim
John M. Steele wrote:
> Pat,
> I'm sorry but I must go back to your statement to
Stan, " It seems really odd to me that engineers,
who
> > probably know much better, are using a power
unit
> > when they are referring to energy."
> In the instance you cite, you are talking about
energy over a time period, and energy divided by
time is power. Annual energy usage has a dimension
of power, whether you use power units (watts) or
explicitly describe the energy and the time period.
> Stan is at least technically correct in using
watts. I have some misgivings about average power
vs peak power if the situation is not fully
explained.
> Power and energy have exactly the same
relationship between them as velocity and distance.
If either is described fully as a time function, I
can derive the other. Since I am retired, I drive
much less. Pardon the miles, but they are
unfortunately the units on my odometer. I am only
driving 4000 - 4500 miles per year. As there are
8760 hours in a common year, my average speed is
circa 0.5 MPH. That, of course is completely
useless as a description of my driving which is
normally at 25 - 75 MPH, plus many hours with the
ignition is off. My miles per annum is a speed
(just not terrible useful). 0.5 MPH or 4400
miles/annum encodes the same information.
> In the same sense 1600 PJ/annum and 50.7 GW
encode the same information. As I don't know how
evenly the 1600 PJ of coal is burnt over the year,
the utility of average power may be debatable but it
is technically correct. When energy usage over a
period is described, the period is so intimately
attached to the energy that it would be better to
drop both units than only one.
> I do understand that you meant petajoules per
annum, but I believe that omitting the per annum has
lead to some of the confusion that has existed here
in various notes about energy vs. power. It must be
completely explicit, or at least that is my view on
the matter.
>...
