For some applications, knowing the electron mass to, say, nine significant figures is important. But in biology and medicine, most quantities are not known with such precision. If a number is known to one percent, that is impressive. When I teach biological and medical physics, I would much rather see my students have an approximate feel for the size of important constants, without having them bother to memorize more than one or two significant figures. To know that the speed of light is 299,792,458 m/s is nice, but what I really want them to remember (forever!) is that the speed of light is about 3 × 10

^{8}m/s. If they need more precision, they can look it up.

Edward Purcell, one of the great ones, published his “Round-Number Handbook of Physics” in the January 1983 issue of the American Journal of Physics. He presented a list of important physical constants, but only to one or two significant figures. It was meant not as a reference to look up precise values, but as a list of approximate values that every physicist should know without needing to consult a reference. Unfortunately, Purcell used cgs units, which are becoming more and more obsolete.

Below I present my version of a “Round-Number Handbook of Physics for Medicine and Biology”. I take the constants from Appendix O and approximate them as round numbers in (mostly) mks units. These are the numbers you should remember.

c | Speed of light | 3 × 10^{8} m/s |

e | Elementary charge | 1.6 × 10^{-19} C |

F | Faraday constant | 10^{5} C/mole |

g | Acceleration of gravity | 10 m/s^{2} |

h | Planck’s constant | 2π × 10^{-34} J s |

ℏ | Planck’s constant (reduced) | 10^{-34} J s |

k_{B} | Boltzmann’s constant | 1.4 × 10^{-23} J/K |

m_{e} | Electron mass | 9 × 10^{-31} kg |

m_{e}c^{2} | Electron rest energy | 0.5 MeV |

m_{p} | Proton mass | 1.7 × 10^{-27} kg |

m_{p}c^{2} | Proton rest energy | 1000 MeV |

N_{A} | Avogadro’s number | 6 × 10^{23} 1/mole |

r_{e} | Classical radius of the electron | 3 × 10^{-15} m |

R | Gas constant | 8 J/(mole K) |

2 cal/(mole °C) | ||

ε_{0} | Electrical permittivity | 9 × 10^{-12} F/m |

1/4πε_{0} | Coulomb’s law constant | 9 × 10^{9} N m^{2}/C^{2} |

σ_{SB} | Stefan Boltzmann constant | 6 × 10^{-8} W/(m^{2} K^{4}) |

λ_{C} | Compton wavelength of the electron | 2.4 pm |

μ_{B} | Bohr magneton | 10^{-23} J/T |

μ_{0} | Magnetic permeability | 4π × 10^{-7} H/m |

μ_{N} | Nuclear magneton | 5 × 10^{-27} J/T |

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