Re: [Phys-l] cooling hints

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

It would be interesting to see if they gave the same response for the
following problem:
1.  You want to accelerate a frictionless cart to velocity V, so you do an
experiment.  A. You give it one big push for 10s, or B you give it 10
shorter pushes.  Which one uses more energy or C. do they both use the same
amount of energy.

The paradigm might be a race paradigm.  Let us say you are racing for 1/4
mile.  Ignore air resistance.  You do one of the 2 following.  A.
Accelerate to cruising speed and stay there.  B.  You accelerate to cruising
speed then take your foot off the accelerator 1/3 of the way down the track,
so you slow down for a while and then accelerate back to a high speed.  If
it takes the same time in both cases which one requires more energy?
Everyone knows the answer to this, so they may be transferring this type of
situation as an analogy.  

But if instead of equal time, you require that you just go back up to the
same cruising speed, they will probably get it wrong and pick B, while C is
the correct answer.  If you don't ignore air resistance the B takes less
energy.

John M. Clement
Houston, TX

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu 
> [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf 
> Of James Mackey
> Sent: Thursday, August 25, 2011 9:44 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] cooling hints
>
> The student argument was usually this: Since the unit had to 
> run at full
> power longer when the system was totally off than when the 
> level was simply
> reduced (which is true) it must use more total energy over 
> the total cycle.
> This was is opposition to the fact that the power-time curves 
> showed less
> total energy used.  However this was only one of numerous spurious
> viewpoints about energy production and consumption that my students
> expressed during the class.  It was an interesting experience 
> to teach this
> general education class from a physicist's perspective.
> James Mackey
>
> On Wed, Aug 24, 2011 at 11:23 PM, John Clement 
> <clement@hal-pc.org> wrote:
>
> > The student misconception probably comes from some concept 
> they alread have
> > and are misapplying.  It might be instructive to try to 
> figure out what
> > that
> > concept might be.  At this moment I can not think of one.
> >
> > John M. Clement
> > Bellaire, TX
> >
> > > Years ago, during 70s energy crisis (la plus sa change, la
> > > meme la chose!) I
> > > did a simulation in lab with a variac and a homemade AC 
> box that was
> > > connected to a simple immersion heater placed in a an
> > > insulated beaker and
> > > an uninsulated beaker.  Part of the lab was to compare energy
> > > consumption
> > > between complete AC turn off and reducing the set point
> > > temperature.  A
> > > complete cycle was run to simulate a constant AC setting, a
> > > temp reduction
> > > setting and a complete AC off by starting all runs at a fixed
> > > temperature
> > > and running through the 3 cycles for fixed times and then
> > > returning to the
> > > initial temperature.  Invariably, minimum energy 
> consumption for the
> > > complete cycle was lowest with the  complete turn off cycle.
> > > Probably not
> > > terribly applicable to today's units, but students found it very
> > > surprising.  Most students believed you used more energy to
> > > "catch up" if
> > > you turned off the heater.
> > > James Mackey
> > >
> > > On Wed, Aug 24, 2011 at 3:31 PM, John Clement
> > > <clement@hal-pc.org> wrote:
> > >
> > > > I know someone who was working for the Army and in the
> > > winter they used to
> > > > open the windows to combat the overheating of their work
> > > space.  But during
> > > > one year when there was a jump in energy prices the Army
> > > sealed the windows
> > > > shut to prevent this.  Why they didn't just turn down 
> the heat is an
> > > > unsolved mystery.  So the employees found they could 
> have comfort by
> > > > turning
> > > > on the AC.
> > > >
> > > > John M. Clement
> > > > Houston, TX
> > > >
> > > > > Ah! Bitten by the "Law" of Unintended Consequences.
> > > >
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