Friday, November 25, 2011

The Second Law of Thermodynamics

Russ Hobbie and I discuss thermodynamics in Chapter 3 of the 4th edition of Intermediate Physics for Medicine and Biology. We take a statistical perspective (similar to that used so effectively by Frederick Reif in Statistical Physics, which is Volume 5 of the Berkeley Physics Course), and discuss many topics such as heat, temperature, entropy, the Boltzmann factor, Gibbs free energy, and the chemical potential. But only at the very end of the chapter do we mention the central concept of thermodynamics: The Second Law.
“In some cases, thermal energy can be converted into work. When gas in a cylinder is heated, it expands against a piston that does work. Energy can be supplied to an organism and it lives. To what extent can these processes, which apparently contradict the normal increase of entropy, be made to take place? The questions can be stated in a more basic form.

1. To what extent is it possible to convert internal energy distributed randomly over many molecules into energy that involves a change of a macroscopic parameter of the system? (How much work can be captured from the gas as it expands the piston?)

2. To what extent is it possible to convert a random mixture of simple molecules into complex and highly organized macromolecules?

Both these questions can be reformulated: under what conditions can the entropy of a system be made to decrease?

The answer is that the entropy of a system can be made to decrease if, and only if, it is in contact with one or more auxiliary systems that experience at least a compensating increase in entropy. Then the total entropy remains the same or increases. This is one form of the second law of thermodynamics. For a fascinating discussion of the second law, see Atkins (1994).”
The book by Peter Atkins, The Second Law, is published by the Scientific American Library, and is aimed at a general audience. It is a wonderful book, and provides the best non-mathematical description of thermodynamics I know of. Atkins’ preface begins
“No other part of science has contributed as much to the liberation of the human spirit as the Second Law of thermodynamics. Yet, at the same time, few other parts of science are held to be so recondite. Mention of the Second Law raises visions of lumbering steam engines, intricate mathematics, and infinitely incomprehensible entropy. Not many would pass C. P. Snow’s test of general literacy, in which not knowing the Second Law is equivalent to not having read a work of Shakespeare.

In this book I hope to go some way toward revealing the workings of the Law, and showing its span of application. I start with the steam engine, and the acute observations of the early scientists, and I end with a consideration of the processes of life. By looking under the classical formulation of the Law we see its mechanism. As soon as we do so, we realize how simple it is to comprehend, and how wide is its application. Indeed, the interpretation of the Second Law in terms of the behavior of molecules is not only straightforward (and in my opinion much easier to understand that the First Law, that of the conservation of energy), but also much more powerful. We shall see that the insight it provides lets us go well beyond the domain of classical thermodynamics, to understand all the processes that underlie the richness of the world.”
Atkins’s book is at the level of a Scientific American article, with many useful (and colorful) pictures and historical anecdotes. The writing is excellent. For instance, consider this excerpt:
“The Second Law recognizes that there is a fundamental dissymmetry in Nature…hot objects cool, but cool objects do not spontaneously become hot; a bouncing ball comes to rest, but a stationary ball does not spontaneously begin to bounce. Here is the feature of Nature that both Kelvin and Clausius disentangled from the conservation of energy: although the total quantity of energy must be conserved in any process…, the distribution of that energy changes in an irreversible manner….”
I particularly like Atkins’ analysis of the equivalence of two statements of the Second Law: No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work (Kelvin statement); and No process is possible in which the sole result is the transfer of energy from a cooler to a hotter body (Clausius statement). Atkins writes
‘The Clausius statement, like the Kelvin statement, identifies a fundamental dissymmetry in Nature, but ostensibly a different dissymmetry. In the Kelvin statement the dissymmetry is that between work and heat; in the Clausius statement there is no overt mention of work. The Clausius statement implies a dissymmetry in the direction of natural change: energy may flow spontaneously down the slope of temperature, not up. The twin dissymmetries are the anvils on which we shall forge the description of all natural change.”
Peter Atkins has written several books, including another of my favorites: Peter Atkins’ Molecules. Here is a video of Atkins discussing his book the Four Laws that Drive the Universe. Not surprisingly, the four laws are the laws of thermodynamics.

Friday, November 18, 2011

Plessey Semiconductor Electric Potential Integrated Circuit

The electrocardiogram, or ECG, is one of the most common and useful tools for diagnosing heart arrhythmias. Russ Hobbie and I discuss the ECG in Chapter 7 (The Exterior Potential and the Electrocardiogram) of the 4th edition of Intermediate Physics for Medicine and Biology. The November issue of the magazine IEEE Spectrum contains an article by Willie D. Jones about new instrumentation for measuring the ECG. Jones writes
“In October, Plessey Semiconductors of Roborough, England, began shipping samples of its Electric Potential Integrated Circuit (EPIC), which measures minute changes in electric fields. In videos demonstrating the technology, two sensors placed on a person’s chest delivered electrocardiogram (ECG) readings. No big deal, you say? The sensors were placed on top of the subject’s sweater, and in future iterations, the sensors could be integrated into clothes or hospital gurneys so that vital signs could be monitored continuously—without cords, awkward leads, hair-pulling sticky tape, or even the need to remove the patient’s clothes.”
Apparently the Plessey device is an ultra high input impedance voltmeter. The electrode is capacitively coupled to the body, so no electrical contact is necessary. You can learn more about it by watching this video. I don’t want to sound like an advertisement for Plessey Semiconductors, but I think this device is neat. (I have no relationship with Plessey, and I have no knowledge of the quality of their product, other than what I saw in the IEEE Spectrum article and the video that Plessey produced.)

According to the Plessey press release, “most places on earth have a vertical electric field of about 100 Volts per metre. The human body is mostly water and this interacts with the electric field. EPIC technology is so sensitive that it can detect these changes at a distance and even through a solid wall.”

I don’t have any inside information about this device, but let me guess how it can detect a person at a distance. The body would perturb a surrounding electric field because it is mostly saltwater, and therefore a conductor. In Section 9.10 of Intermediate Physics for Medicine and Biology, Russ and I explain how a conductor interacts with applied electric fields. For the case of a dc field, the conducting tissue completely shields the interior of the body from the field. To understand how a body could affect an electric field, try solving the following new homework problem

Section 9.10

Problem 34 ½ Consider how a spherical conductor, of radius a, perturbs an otherwise uniform electric field, Eo. The conductor is at a uniform potential, which we take as zero. As in Problem 34, assume that the electric potential V outside the conductor is V = A cosθ/r2Eo r cosθ.
(a) Use the boundary condition that the potential is continuous at r=a to determine the constant A.
(b) In the direction θ=0, determine the upward component of the electric field, - dV/dr.
(c) The perturbation of the electric field by the conductor is the difference between the fields with and without the conductor present. Calculate this difference. How does it depend on r?
(d) Suppose you measure the voltage in two locations separated by 10 cm, and that your detector can reliably detect voltage differences of 1 mV. How far from the center of a 1 m radius conductor can you be (assuming θ=0) and still detect the perturbation caused by the conductor?
You may be wondering why there is a 100 V/m electric field at the earth’s surface. The Feynman Lectures (Volume 2, Chapter 9) has a nice discussion about electricity in the atmosphere. The reason that this electric field exists is complicated, and has to do with 1) charging of the earth by lightning, and 2) charge separation in falling raindrops.

Friday, November 11, 2011

The Making of the Pacemaker: Celebrating a Lifesaving Invention

I’m still thinking about Wilson Greatbatch, one of the inventors of the implantable pacemaker, who died a few weeks ago (see my September 30 blog entry honoring him). Here is an interesting excerpt from his book The Making of the Pacemaker: Celebrating a Lifesaving Invention, about how he created the circuit in the first pacemaker.
“My marker oscillator used a 10k basebias resistor. I reached into my resistor box for one but misread the colors and got a brown-black-green (one megohm) instead of a brown-black-orange. The circuit started to ‘squeg’ with a 1.8 ms pulse, followed by a one second quiescent interval. During the interval, the transistor was cut off and drew practically no current. I stared at the thing in disbelief and then realized that this was exactly what was needed to drive a heart. I built a few more. For the next five years, most of the world’s pacemakers used a blocking oscillator with a UTC DOT-1 transformer, just because I grabbed the wrong resistor.”
Here is another story from the book about how he met William Chardack, his primary collaborator in developing the first pacemaker.
“In Buffalo we had the first local chapter in the world of the Institute of Radio Engineers, Professional Group in Medical Electronics (the IRE/PBME, now the Biomedical Engineering Society of the Institute of Electrical and Electronic Engineers [IEEE]). Every month twenty-five to seventy-five doctors and engineers met for a technical program. We strove to attract equal numbers of doctors and engineers. We had a standing offer to send an engineering team to assist any doctor who had an instrumentation problem. I went with one team to visit Dr. Chardack on a problem deadline with a blood oximeter. Imagine my surprise to find that his assistant was my old high school classmate, Dr. Andrew Gage. We couldn’t help Dr. Chardack much with his oximeter problem, but when I broached my pacemaker idea to him, he walked up and down the lab a couple times, looked at me strangely, and said, ‘If you can do that, you can save ten thousand lives a year.’ Three weeks later we had our first model implanted in a dog."
This excerpt is interesting:
“I had $2,000 in cash and enough set aside to feed my family for two years. I put it to the Lord in prayer and felt led to quit all my jobs and devote my time to the pacemaker. I gave the family money to my wife. I then took the $2,000 and went up into my wood-heated barn workshop. In two years I built fifty pacemakers, forty of which went into animals and ten into patients. We had no grant funding and asked for none. The program was successful. We got fifty pacemakers for $2,000. Today, you can’t buy one for that.”
This one may be my favorite. You gotta love Eleanor. They were married in 1945 and stayed together until her death in January of this year.
“Many of the early Medtronic programs were first worked out in Clarence, New York, and then taken to Minneapolis. I had two ovens set up in my bedroom. My wife did much of the testing. The shock test consisted of striking the transistor with a wooden pencil while measuring beta (current gain). We found that a metal pencil could wreck the transistor, but a wooden pencil could not. Many mornings I would awake to the cadence of my wife Eleanor tap, tap, tapping the transistors with her calibrated pencil. For some months every transistor that was used worldwide in Medtronic pacemakers got tapped in my bedroom.”
You can learn more about pacemakers and defibrillators in the 4th edition of Intermediate Physics for Medicine and Biology.

Friday, November 4, 2011

Countercurrent Heat Exchange

Problem 17 in Chapter 5 of the 4th edition of Intermediate Physics for Medicine and Biology considers a countercurrent heat exchanger. Countercurrent transport in general is discussed in Section 5.8 in terms of the movement of particles. However, Russ Hobbie and I conclude the section by applying the concept to heat exchange.
“The principle [of countercurrent exchange] is also used to conserve heat in the extremities—such as a person’s arms and legs, whale flippers, or the leg of a duck. If a vein returning from an extremity runs closely parallel to the artery feeding the extremity, the blood in the artery will be cooled and the blood in the vein warmed. As a result, the temperature of the extremity will be lower and the heat loss to the surroundings will be reduced."
Problem 17 provides an example of this behavior, and cites Knut Schmidt-Nielsen’s book How Animals Work (1972, Cambridge University Press), which describes countercurrent exchange in more detail. (His comments below about the nose refer to an earlier section of the book, in which Schmidt-Nielsen discusses heat exchange in the nose of the kangaroo rat).
“The heat exchange in the nose has a great similarity to the well-known countercurrent heat exchange which takes place, for example, in the extremities of many aquatic animals, such as in the flippers of whales and the legs of wading birds. The body of a whale that swims in water near the freezing point is well insulated with blubber, but the thin streamlined flukes and flippers are uninsulated and highly vascularized and would have an excessive heat loss if it were not for the exchange of heat between arterial and venous blood in these structures. As the cold venous blood returns to the body from the flipper, the vessels run in close proximity to the arteries, in fact, they completely surround the artery, and heat from the arterial blood flows into the returning venous blood, which is thus reheated before it returns to the body (figure 3). Similarly, in the limbs of many animals both arteries and veins split up into a large number of parallel, intermingled vessels each with a diameter of about 1 mm or so, forming a discrete vascular bundle known as a rete…Whether the blood vessels form such a rete system, or in some other way run in close proximity, as in the flipper of the whale, is a question of design and does not alter the principle of the heat recovery mechanism. The blood flows in opposite directions in the arteries and veins, and heat exchange takes place between the two parallel sets of tubes; the system is therefore known as a countercurrent heat exchanger.”
Schmidt-Nielsen also wrote Scaling: Why is Animal Size So Important?, which Russ and I cite often in Chapter 2 and which I included in my top ten list of biological physics books. I have also read Schmidt-Nielsen's autobiography The Camel’s Nose: Memoirs of a Curious Scientist. (See the review of this book in the New England Journal of Medicine.) His Preface begins
“This is a personal story of a life spent in science. It tells about curiosity, about finding out and finding answers. The questions I have tried to answer have been very straightforward, perhaps even simple. Do marine birds drink sea water? How do camels in hot deserts manage for days without drinking when humans could not survive without water for more than a day? How can kangaroo rats live in the desert without any water to drink? How can snails find water and food in the most barren deserts? Can crab-eating frogs really survive in sea water?

These are important questions. The answers not only tell us how animals overcome seemingly insurmountable obstacles in hostile environments; they also give us insight into general principles of life and survival.”
Schmidt-Nielsen died in 2007, and Steven Vogel (who I quoted in last week’s blog entry) wrote an article about him for the Biographical Memoirs of Fellows of the Royal Society (volume 54, pages 319-331, 2008). See also his obituary in the Journal of Experimental Biology. A statue of Schmidt-Nielsen with a camel (which he famously studied) graces the Duke University campus.