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Re: [Phys-l] [PTSOS] Re: entropy help, please


People!


On 2011, Dec 03, , at 09:14, jonathanhanna wrote:

Hi Valerie,

I hear you! Entropy is a tough topic and I've struggled in teaching it (and understanding it!) as well.

I agree!


I just finished my lessons on entropy in Physics and AP Physics and here's some resources I've used:

1. Heat flows from hot to cold.

This is a reification (similar to phlogiston that is generally agreed by members of the PHYS-L (list,) as a no, no. (proper as a verb; the more correct form is a certain type or form of energy.) Note: J.D., AFAIK, doesn't use that word, except to advise not using it in isolation. For example: he uses heat capacity, and specific heat many times.

16.1 Definitions


The term “heat” is a confusing chimera. It is partly energy, partly entropy, partly temperature, and partly who-knows-what. It shows up in a variety of idiomatic expressions, as discussed in section 16.2.

The word “heat” has at least five sensible and widely-used but mutually-inconsistent technical meanings (not to mention innumerable nontechnical and metaphorical meanings). It is not worth arguing about the relative merits of these meanings, except to say that each has some merit. I observe that a typical thoughtful expert will use each of these meanings, depending on context. It would be nice to have a single, universally-accepted meaning, but I doubt that will happen anytime soon.

You may be wondering how it is possible that a concept as important as “heat” could lack a clear, unique meaning? Well, the answer is that “heat” just isn’t that important. When we want to quantify things, it is better to forget about “heat” and just quantify energy and entropy, which are unambiguous and unproblematic.

Heat [Ch. 16 of Modern Thermodynamics]

I recommend careful reading of his thermo. exposition:


Modern Thermodynamics

http://www.av8n.com/physics/thermo/

Here's a great physlet I've used for this: http://jersey.uoregon.edu/vlab/Thermodynamics/therm1a.html
It doesn't say anything about entropy, but it shows the random motion of molecules and how they reach thermal equilibrium. It can be a lesson on the "disorder" part of entropy. I've also used an activity with flipping pennies to explain entropy and relating it to disorder, but the students find it very confusing and don't make the connection well. Here's the activity (maybe you'll have better luck): http://facultyweb.berry.edu/ttimberlake/entropy/

2. The Carnot Engine and Q/T
The Mechanical Universe has some decent videos explaining heat engines and how they relate to entropy: http://www.learner.org/resources/series42.html
Look at video #47 on Entropy. It has some good visuals explaining how in a Carnot Engine entropy is conserved and in a real engine entropy increases. The film uses the relationship Q/T to explain why. In a Carnot Engine QH/TH = QC/TC (where H represents the hot reservoir and C the cold reservoir). However, since real engines can never be as efficient as a Carnot Engine, less heat (QH) goes into work and QC is larger as a result. Since QC is greater in a real engine, QC/TC is greater than QH/TH and overall entropy increases. That's my simplistic explanation, as I still do not feel 100% comfortable with entropy! The video also talks about entropy in melting ice and how it relates to disorder. In addition, the Khan Academy has a video that has helped me understand entropy a little bit better: http://www.youtube.com/watch?v=xJf6pHqLzs0

Speaking of the Carnot Cycle, here's a couple of great sites which shows a simulation of the Carnot Cycle, but unfortunately do not talk about entropy:
http://galileo.phys.virginia.edu/classes/109N/more_stuff/flashlets/carnot.htm
http://science.sbcc.edu/physics/flash/heatengines/Carnot%20cycle.html

Hope that helps!

-- Jonathan

--- In PTSOS@yahoogroups.com, Valerie Risk <valrisk@...> wrote:

Hi,

I'm teaching entropy in AP Physics on Monday and finding it challenging to explain, bot mathematically and conceptually.

How do you make a connection between statements like "Heat will not flow spontaneously from a cold object to a hot object" or "The total entropy of any system increases", and the equation S = deltaQ/T? What does this equation say, anyways?

Do you use the Carnot cycle to talk about entropy? What do you do to convey the concept to your students?

Any help would be much appreciated.

Thanks,

Valerie



I pray J.D.'s "Modern Thermodynamics" will be helpful.

bc
I I jj__._,_.___

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