Re: What Flows?

[mjakub@physics.otago.ac.nz (Marlyn Jakub)]

> Is there evidence combining these two ingredients causes confusion?
>
> -- 
> --James McLean
> jmclean@chem.ucsd.edu
> post doc
> UCSD

James, I don't have any evidence, but I do use a lecture-demo to minimize
first-year student's confusion over the thermal energy flow vs atomic
interaction concepts.
 
Materials: long table or multiple carts; about 60 wooden blocks; a bungy rope
Prodedure: line up 6 or 8 students back-to-front alongside the long
bench...each student has a pile of blocks which can be accessed by, say,
the left hand. The blocks will indicate the temperature of the particular
molecule involved in one-dimensional heat conduction.

With the other hand, each student holds onto the rope, which now can
transfer the energy from the hot boundary...to the row of molecules along
the rod. At the beginning, the first molecules begin to store energy and
having their temperature raised. Note that the conduction model equation
has been introduced so that each student knows that energy in requires a
temperature gradient, and energy out requires a temperature gradient in the
other direction. Each student thus needs to watch the temperatures of the
person in front and behind their temperature indication. 

By having the lecturer behave as the hot boundary, the up and down of the
rope passes the energy in the molecular kinetic energy fashion. Also the
lecturer can comment on the initial temperature patterns--pointing out the
thermal inertia period before the energy flow affects the last molecules at
the far end of the rod. 

Of course, the students eventually reach Steady State conditions of a
linear temperature profile where no further spatial variation of
temperature occurs.

Yes, this does take 5 to 10 minutes of lecture time, but I find it a useful
reference for further heat transfer topics, including the electrical
resistance model and conduction/convection effects at the boundary. 


Marlyn Jakub, mjakub@physics.otago.ac.nz