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Re: [Phys-L] physics and potatoes

On 2015, Jan 14, , at 11:26, brian whatcott <betwys1@sbcglobal.net> wrote:

On 1/13/2015 11:51 AM, John Denker wrote:
On 01/13/2015 04:53 AM, Anthony Lapinski wrote:

Here's Hewitt's answer from the Instructor's Manual:

The main reason for serving potatoes wrapped in aluminum foil
is to increase the time that the potatoes remain hot after being
removed from the oven. Heat transfer by radiation is minimized
as radiation from the potatoes is

internally reflected,

So? I think the operative principle is the metal’s low emissivity.

The foil equilibrates w/ the potato v. quickly.

and heat
transfer by convection is minimized as circulating air cannot make
contact with the shielded potatoes.
In this example, as in so many others, Hewitt conducts a master
class in faulty reasoning. /clip/
It also fails the basic grade-school requirement to CHECK
THE WORK.
/snip/
Well, finally, that part of the answer contains a bit of
correct physics. It is still sloppy reasoning because it
fails to consider simpler and better ways of achieving the
same objective.
/snip/
Talking of grade-school - .a possible teaching moment?

If I inquire how thermal energy can be transferred between objects,
it was not so long ago that I would expect to hear about
Conduction, Convection and Radiation....
which led to puzzlement about the operating principle of the
Heat-Pipe

That’s advection.
In this case, the Elephant (or rather, the Hot Potato) in the Room
is Mass Transfer as a method more efficient than Conduction at times..
Brian Whatcott Altus
bc surprised no-one commented on the internally reflected.