IB thermodynamics: FAQ


Send me questions, and I'll answer them here. I've put them in rough order of how often I am asked.

Q1. What sort of question can we expect on the exam?

Q2. How can we decide what quanities we can take for granted to estimate values? I do not remember any sets of numbers, such as the energy required by a person in a day, ionisation energies, energy in food, etc. I look them up in books, or on the packets, etc.

Q3. This estimation is fun and all very well. But will we get steamrollered on the exam by regular questions?

Q4. What is with the sign you wear?

Q5. But by wearing the sign, aren't you making a political statement in lecture?

Q6. Is Equilibrium Thermodynamics (also by Adkins) a suitable replacement for Introduction to Thermal Physics?

Q7. Have you ever seen a jam-filled torus donut, and if so where? I want one.

Q8. This estimation is fun and all very well. But don't we need to know exact calculations? I thought physics was an exact science.

Q9. I have heard that thermodynamics is the most fundamental of all the physical theories that describe the world: the second law is times arrow and all that jazz. Will you convey this fundamentalism of the ideas to us in the course?

Q10. Would anything actually have to be learnt to do estimations, or is it just ways of thinking?

Q11. If we will not do much formal material, what is the purpose in attending the lectures?

Q12. Secondly, why is your course called Classical Thermodynamics? It seems you are teaching a course on "thinking in physics" with a slight thermodynamic twist.

Q13. The purpose of "equation pushing" (as you put it) in lectures is so that you can be fed the information, and in your own time make of it what you understand it to be. What is your opinion on this idea?


Q1. What sort of question can we expect on the exam?

The thermodynamics part of the physics paper will have 3 quickie questions (like section A), 2 longer questions (a la section C on last year's exam) and 1 essay/notes question. I forget, or rather have no clue, how much choice you have among all the questions. Probably the examiners are debating that issue now, since the whole IB course is new this year.

The section A questions will be like the shorter examples we do in class, e.g. estimate the heat of vaporization of water by any reasonable method. The longer questions will be like the longer examples from class, perhaps explaining a bit of everyday physics, perhaps even using an equation or two, maybe even an integral (once entropy appears, which will be soon, integrals are hard to avoid). As for the notes/essay one, I don't know. But you'll have practice with all the types in the problem sheets and I will submit questions for a mock exam.

Q2. How can we decide what quanities we can take for granted to estimate values? I do not remember any sets of numbers, such as the energy required by a person in a day, ionisation energies, energy in food, etc. I look them up in books, or on the packets, etc.

Good question. The fewer quantities you look up the more you will learn, but don't torture yourself. If you do look up a value, think how you could have figured it out.

For example, an ionization energy may be a horrible value in Joules. Convert it to eV and you'll find that it's maybe 4-10 eV. How could you know that? Visible light photons are about 2 eV and they don't ionize most substances, whereas UV light does. So ionization energies are somewhat bigger than 2 eV, and 4-10 eV is plausible.

If you analyze each quantity after you look it up, you'll eventually learn them, and remembering them will not be an issue.

Q3. This estimation is fun and all very well. But will we get steamrollered on the exam by regular questions?

No. I propose the exam questions. The examiners have final say (`final responsibility for the exam rests with the examiners' is the official phrasing), but I will propose questions like what we do in class and on the problem sheets.

Q4. What is with the sign you wear?

It says "Oiligarchy" in big red letters, derived from oligarchy or `rule by the few'. I added the "i" to say that the Iraq war shows we (the Americans directly and us indirectly) are ruled by a few oil men. Since physics enables America and Britain to invade and rob Iraq, as a physics lecturer and an American and British citizen I feel a triple obligation to protest.

Q5. But by wearing the sign, aren't you making a political statement in lecture?

Yes, and feel free to agree or disagree with it. Political statements are hard to avoid. Many people wear clothes with a Nike logo, thereby supporting a company that subcontracts to factories paying children sub-poverty wages. (If I wore a T-shirt with a large "Oiligarchy" logo, many would think it is more acceptable than wearing a sign, because T-shirts are a more normal form of dissent. But I don't want to paint a sign on all my shirts!) Or look at the convention that lecturers dress nicely. Many, including me sometimes, wear a jacket and tie. As a class-based dress style, it too makes a political statement.

Q6. Is Equilibrium Thermodynamics (also by Adkins) a suitable replacement for Introduction to Thermal Physics?

No. It is long, abstract, and abstruse. I don't know 90 per cent of it myself and I am a happy guy. But here is an alternative to Introduction to Thermal Physics.

Q7. Have you ever seen a jam-filled torus donut, and if so where? I want one.

In my naive youth (i.e. until Jan 2004), I thought that all jelly donuts were filled with jam. Now I know better. In American jelly donuts, the jelly is a torus inside the dough torus. In America, you can also get cream-filled donuts. It is the land of the free, at least if you are hungry for topologically correct donuts. Yet another reason to go on the MIT exchange!

Q8. This estimation is fun and all very well. But don't we need to know exact calculations? I thought physics was an exact science.

Exact calculations are dull. Sometimes you have to do them: for example, a conservation law accurate to a factor of 2 is not useful. I'll do a few exact, legitimate calculations to please the physics gods. However, most of the Cambridge physics course emphasizes derivations and exactness. This lecture course is an exception, as is my Part II course and Malcolm Longair's Part II course, and is a chance to apply physics to the world around you. The world is messy so approximation is necessary. And it's fun.

Q9. I have heard that thermodynamics is the most fundamental of all the physical theories that describe the world: the second law is times arrow and all that jazz. Will you convey this fundamentalism of the ideas to us in the course?

I will try. We will soon discuss the second law. No matter how much one discusses it, it is mysterious. Just like the double-slit experiment in quantum mechanics. From strictly classical thermodynamics, the second law is empirical. You've never seen it violated (for example, teacup shards do not unshatter themselves and jump onto the table as a teacup), so you assume it is a law. To understand it more deeply, you need statistical mechanics, the subject of the following course taught by Bill Allison. Meanwhile see the beautiful book, The Second Law by P.W. Atkins.

Q10. Would anything actually have to be learnt to do estimations, or is it just ways of thinking?

Both! I am trying to show that good ways of thinking allow you to do most of thermodynamics, so you have little formal material to learn beyond good ways of thinking. The formal bits include, for example, the thermal flux equation (with thermal conductivity), entropy and adiabatic versus isothermal expansions, but I am more interested in the ways of thinking than the formal material. Plus much of the formal material is derivable from ways of thinking, as we are doing with the thermal flux equation.

Q11. If we will not do much formal material, what is the purpose in attending the lectures?

To practice using and arguing about physics ('speaking physics') -- like the way you become fluent in a language. For example today, to find many different ways of estimating the heat of vaporization of water, each including a different piece of physics. Almost no textbook teaches the art of approximation, so for now it's learnt in lectures by participating in it and also seeing what I do. Another purpose of lecture is to practice explaining and defending your ideas about physics. That's how physicists spend their days and make progress. Lecture isn't the only place to get that -- you can argue with other physicists in your college, but lecture is also one place to meet physicists outside your college.

Q12. Secondly, why is your course called Classical Thermodynamics? It seems you are teaching a course on "thinking in physics" with a slight thermodynamic twist.

That is mostly right. I think most important to learn are the confidence and methods to fight your way out of the jungle. I happen to be teaching Classical Thermodynamics, so I'll use examples related to energy, power, flux, and a bit of entropy, in order to teach those habits of mind.

Q13. The purpose of "equation pushing" (as you put it) in lectures is so that you can be fed the information, and in your own time make of it what you understand it to be. What is your opinion on this idea?

I think it's a waste of taxpayer (e.g. your parents') money to pay me to feed you equations when you can get them in a book; and a waste of your time when you can read the book on your own. Most people learn from examples; then once the pattern is understood, by formalizing it in an equation. If the equation comes first, it too easily turns into symbol pushing because the knowledge is not backed by examples. If you understand the examples and why they produce the equation, then you can always rederive the equation. That's why I emphasize the habits of thinking useful in any problem. You still have to spend a lot of time on your own struggling with the material, but you'll hopefully do that based on an intuition for the subject.


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