5.7: Odds and Ends
- Page ID
- 91867
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)We’ll end this chapter with a few miscellaneous notes about SI units:
- In a few special cases, we customarily drop the ending vowel of a prefix when combining with a unit that begins with a vowel: it’s megohm (not “megaohm”); kilohm (not “kiloohm”); and hectare (not “hectoare”). In all other cases, keep both vowels (e.g. microohm, kiloare, etc.). There’s no particular reason for this—it’s just customary.
- In pharmacology (on bottles of vitamins or prescription medicine, for example), it is usual to indicate micrograms with “mcg” rather than “µ,g”. While this is technically incorrect, it is done to avoid mis- reading the units. Using “mc” for “micro” is not done outside pharmacology, and you should not use it in physics. Always use µ, for “micro”.
- Sometimes in electronics work the SI prefix symbol may be used in place of the decimal point. For example, 24.9 MQ may be written “24M9”. This saves space on electronic diagrams and when print- ing values on electronic components, and also avoids problems with the decimal point being nearly invisible when the print is tiny. This is unofficial use, and is only encountered in electronics.
- One sometimes encounters older metric units of length called the micron (µ, now properly called the micrometer, 10-6 meter) and the millimicron (mµ, now properly called the nanometer, 10 -9 meter). The micron and millimicron are now obsolete.
- At one time there was a metric prefix myria- (my) that meant 104. This prefix is obsolete and is no longer used.
- In computer work, the SI prefixes are often used with units of bytes, but may refer to powers of 2 that are near the SI values. For example, the term " \(1 \mathrm{kB}\) " may mean 1000 bytes, or it may mean \(2^{10}=1024\) bytes. Similarly, a 100 GB hard drive may have a capacity of \(100,000,000,000\) bytes, or it may mean \(100 \times 2^{30}=107,374,182,400\) bytes. To help resolve these ambiguities, a set of binary prefixes has been introduced (Table 4 of Appendix 63.8). These prefixes have not yet entirely caught on in the computing industry, though.