Re: [Phys-L] physics and potatoes

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

Hmmm.  Is convection really minimized, as the air circulates around the
potato with or without foil?  I would say the retention of moisture is a big
factor because evaporation can cool things rapidly.  This may be the largest
effect here.  But since some cooks claim it cooks faster in foil there may
be other answers.  As to Hewitt's answer, it depends on what the cook wants,
and is not just a matter of physics.

I still say it is a discussion question which may not have a unique answer.
As such it can be a very good question.  It is an example of an open ended
question.  Only having students ponder questions which have one unique
approved answer kills a lot of thinking skills.  If I used this question, I
would probably not tell them the "approved" answer, but I would try to get
them to use good physics ideas rather than everyday reasoning, even if no
definite answer emerges.

John M. Clement
Houston, TX 

> 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, and heat transfer by convection is minimized as 
> circulating air cannot make contact with the shielded 
> potatoes. The foil also serves to retain moisture.
>
> (Conceptual Physics, 10th ed, Ch 16, Ex 8)
>
>
> Phys-L@Phys-L.org writes:
> > So has anyone done the experiment????
> > What did Hewitt say the answer was? 
> >
> > 1. Trapping the steam is a reasonable answer compared to a 
> lot of the 
> > others.  Evaporation will certainly cool the potato, but 
> some moisture 
> > will still escape through the folds in the foil.
> > 5,6 are also possibilities.
> >
> > I suspect this is not designed to be a question with a 
> definite answer, 
> > but rather one to promote discussion.
> > Foil will also reflect radiation, while the darker potato skin will 
> > absorb it better, but also radiate it better.
> > The type of oven may also be important, electric vs gas vs 
> convection.
> > Baking may be assuming it is not a grill.
> >
> > It has certainly been successful on this list, but much of the 
> > discussion has not been about resolving which answers make 
> more sense.
> > The factor of mushy vs crisp skin has certainly gone around, but the 
> > time to cook difference has not been well established.
> >
> > John M. Clement
> > Houston, TX
> >
> > > I saw a question in Hewitt about why potatoes are wrapped 
> in foil for 
> > > baking. I searched online and found some interesting/conflicting 
> > > answers:
> > >
> > > 1. The foil will trap the steam inside, making the potato 
> cook faster.
> > > 2. The foil is a good conductor, making the potato cook faster.
> > >
> > > 3. The foil should be wrapped tightly around the potato to 
> minimize 
> > > air inside, which is a poor conductor.
> > >
> > > 4. Put the shinier side of the foil toward the potato. This will 
> > > reflect more heat back to the potato, making it cook faster.
> > >
> > > 5. The foil does nothing during baking, only to create less mess.
> > >
> > > 6. The foil does nothing during baking. But after baking, 
> wrapping a 
> > > hot potato in foil will keep it warmer because the foil traps the 
> > > heat (less energy lost via radiation).
> > >
> > > etc...
> > >
> > > I'm a bit puzzled with all of this. Are there multiple ideas about 
> > > heat transfer happening here? Has anyone actually tried 
> this cooking 
> > > method, and is there a significant difference?
> > >
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