Friday, August 31, 2012

Edward Purcell (1912-1997)

Yesterday, August 30, was the 100-year anniversary of the birth of physicist Edward Purcell (1912-1997). Purcell appears several times in the 4th edition of Intermediate Physics for Medicine and Biology. He first shows up in Chapter 1, when discussing fluid dynamics and the Reynolds number.
“When the Reynolds number is small, viscous effects are important. The fluid is not accelerated, and external forces that cause the flow are balanced by viscous forces. Since viscosity is a form of internal friction in the fluid, work done on the system by the external forces is transformed into thermal energy. The low-Reynolds number regime is so different from our everyday experience that the effects often seem counterintuitive. They are nicely described by Purcell (1977).”
The reference is to Purcell’s wonderful American Journal of Physics paper Life at Low Reynolds Number (Volume 45, Pages 3-11, 1977). It is a classic that I always hand out to my students when I teach PHY 325, Biological Physics (a class based on the textbook….you guessed it….Intermediate Physics for Medicine and Biology).

Purcell makes his second appearance in Chapter 4
“The analysis [of diffusion] can also be applied to the problem of bacterial chemotaxis—the movement of bacteria along concentration gradients. This problem has been discussed in detail by Berg and Purcell (1977).”
In this case, the reference is to his article with Howard Berg
Berg, H. C., and E. M. Purcell (1977). Physics of chemoreception. Biophys. J. 20: 193–219.
In Chapter 8, Russ Hobbie and I often cite Purcell’s excellent textbook Electricity and Magnetism (1985), which is Volume 2 of the renowned Berkeley Physics Course. Our Figure 8.10 is a reproduction of one of his figures. Purcell’s book is unusual for an introductory text in that it develops magnetism as an implication of special relativity. Russ and I write
“We now know that magnetism results from electric forces that moving charges exert on other moving charges and that the appearance of the magnetic force is a consequence of special relativity. An excellent development of magnetism from this perspective is found in Purcell (1985).”
I’m not sure that this is the best way to teach magnetostatics to college freshman taking introductory physics, but it does provide exceptional insight into the ultimate origin of the magnetic force, especially when described in Purcell’s prose.

In Chapter 18 Russ and I describe the work that earned Purcell the Nobel Prize that he shared with Felix Bloch: nuclear magnetic resonance. We describe the Carr-Purcell pulse sequence, which is a set of radio-frequency magnetic pulses that result in a series of spin-echos, allowing the measurement of the NMR T2 relaxation time. We then consider the improved Carr-Purcell-Meiboom-Gill pulse sequence, which is like the Carr-Purcell sequence except that it avoids cumulative errors if the radio-frequency pulse does not have exactly the correct duration or amplitude.

I’m a loyal reader of Time Magazine, and to me it is impressive that Purcell has appeared on the cover of Time when the magazine selected 15 scientists—including Purcell--as men of the year for 1960.

You can learn more about Edward Purcell in an oral history transcript prepared by the Neils Bohr Library and Archives, part of the American Institute of Physics Center for History of Physics. Also, see his New York Times obituary here.

1 comment:

  1. Russ Hobbie tells me that he was Ed Purcell's teaching assistant during his second year in graduate school at Harvard, refining some new experiments Purcell had made for a junior E&M course. Russ says it was a wonderful experience.

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