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

[Richard Tarara <rtarara@saintmarys.edu>]

Really, you need to be careful in every instance to be very clear.  An 
example that can have some consequences would be talking about a natural 
gas fired electrical power plant.  The rated electrical power output of 
the plant might be 1000 MW (and of course a 1000 MW of electrical power 
is seldom actually delivered to the end-users).  Suppose an 
environmentally minded lay-person is building a house in this area and 
the builder asks whether electrical heat or gas heat is desired.  The 
person might think that it doesn't really matter since his electrical 
power is being produced by natural gas (one could even assume 
--wrongly--that electrical energy and natural gas are going for the same 
price per therm produced in the house).  We all know that the thermal 
conversion of the natural gas is running at 2400-3000 MW in order to 
produce the 1000 MW electrical output and therefore an environmentally 
concerned person should really opt for the gas fired furnace!  Similarly 
thinking about a plug-in electrical car being powered by a coal-fired 
plant might not be as friendly as one might hope and might not be much 
better than driving a high-mileage gasoline car.

In other words, talking about Power here always involves energy, but how 
detailed we want to be with the energy conversions involved in a given 
situation will dictate how detailed we should be with our power 
descriptions.

rwt

On 12/1/2014 12:56 PM, John Denker wrote:
> Here is some evidence that people define power one way and
> use it another.
>
>    Note that I use wikipedia as a reflection of how the
>    general population thinks, not as a source of objective
>    facts about the subject matter.  I have been told by
>    wikipedia officials that this is how they /want/ it to be.
>
> Let's look at the wikipedia article on power
>    http://en.wikipedia.org/wiki/Power_%28physics%29
>
>   a) In the first sentence, it explicitly defines power as
>    the rate of doing work (where work is F•dx).
>
>   b) In the second sentence, it asserts that this is equivalent
>    to energy consumed per unit time.  This is wildly inconsistent
>    with the previous definition, as you can see by considering
>    something like a refrigerator, where the energy dissipated
>    is not anywhere close to the F•dx work done.

--
Richard Tarara
Professor Emeritus
Saint Mary's College

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