Re: [Phys-L] Conservation of Energy vs Constant Energy

["John Clement" <clement@hal-pc.org>]

Isn't this how they were taught at one time?  Conservation was probably
defined as saying energy is constant.  There is the problem that energy is
often taught as something that is transformed from one type to another,
which sets up the idea that it is then "something" different.  Modeling
takes the approach that energy is just transferred from one place to
another.

Conservation can be thought of as energy being constant in a closed system,
but in an open system it is defined differently as the energy in + energy
initial= energy out + energy final.  In both cases you are thinking abount
conservation as the law that energy does is neither created nor destroyed,
merely transferred.

So it may be that they are just using a restricted definition of energy
conservation.  Or perhaps they are conflating  the given system with a more
general closed system.  It may be double think.  This happens when you ask
students to predict which direction a coat on top of a car goes when the car
starts moving to the right and accelerates foreward.  You tell them to view
the event from the point of view of someone standing on the sidewalk.  The
will practically always say the coat falls to the left.  They think of what
they would see if they are in the car and not what they would see from the
sidewalk.  You show them the video and have someone put their finger by the
coat at the beginning.  Many of them still insist that the coat went to the
left even though it is finally on the right side of the finger marker.  So
they may be thinking of two systems at once.  So these are two suggestions
as to why there is a problem.

I would suggest having them do bar charts for two systems.  One is a closed
system, and the other is a subset of the closed system which is actually
open.  Then in between bar charts they have to put an arrow indicating
whether energy goes in or out, and also a bar showing how much energy.  The
bar would be negative for energy out.  There are 2 ways to do bar charts,
either as absolute energy or as energy changes.  MOP does energy changes,
but with students below the formal operational level I think that energy
changes poses difficulties, so I usually use bars that indicate the amount
of energy rather than the change in energy.  They should do this for a
variety of systems.  For example bottles of oxygen & propane heating
something, and then just the something heated.  The reason for both bottles
is that the energy is not in the propane, but in the propane, oxygen system.
Or alternately you can use propane/air system.  Somewhere along the line you
also need to do phase changes with bars for internal potential energy and
internal kinetic energy (heat as the street name).

John M. Clement
Houston, TX


> Hello.
> I am in an awkward spot.  I am trying to convince a group of 
> hs physics teachers that energy is always conserved. 
> Regardless of open or closed systems. 
> E(total initial) + W + Q = E (total final) They accept this 
> as the first law of thermo without question. I point out that 
> the gas (or the piston depending on your view) is the system 
> and other items either do W or Q to the system.
>
> They do not seem to accept this as a statement of 
> conservation of energy where W & Q are ways for energy to 
> transfer into or out of an open system.
> They insist energy is constant (which they equate to 
> conserved) for closed systems.  
>
> Good lord.
> How do I convince them that energy is conserved regardless of 
> an open or closed system?
> Do folks have authoritative resources I can point to?
>
> Thanks for your help.
>
> Paul.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@www.phys-l.org
> http://www.phys-l.org/mailman/listinfo/phys-l