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<a name="Introduction-to-ezunits"></a>
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Next: <a href="maxima_264.html#Introduction-to-physical_005fconstants" accesskey="n" rel="next">Introduction to physical_constants</a>, Previous: <a href="maxima_262.html#ezunits_002dpkg" accesskey="p" rel="previous">ezunits-pkg</a>, Up: <a href="maxima_262.html#ezunits_002dpkg" accesskey="u" rel="up">ezunits-pkg</a> [<a href="maxima_toc.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="maxima_423.html#Function-and-Variable-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Introduction-to-ezunits-1"></a>
<h3 class="section">57.1 Introduction to ezunits</h3>
<p><code>ezunits</code> is a package for working with dimensional quantities,
including some functions for dimensional analysis.
<code>ezunits</code> can carry out arithmetic operations on dimensional quantities and unit conversions.
The built-in units include Systeme Internationale (SI) and US customary units,
and other units can be declared.
See also <code><a href="maxima_264.html#physical_005fconstants">physical_constants</a></code>, a collection of physical constants.
</p>
<p><code>load("ezunits")</code> loads this package.
<code>demo("ezunits")</code> displays several examples.
The convenience function <code>known_units</code> returns a list of
the built-in and user-declared units,
while <code>display_known_unit_conversions</code> displays
the set of known conversions in an easy-to-read format.
</p>
<p>An expression <em>a ` b</em> represents a dimensional quantity,
with <code>a</code> indicating a nondimensional quantity and <code>b</code> indicating the dimensional units.
A symbol can be used as a unit without declaring it as such;
unit symbols need not have any special properties.
The quantity and unit of an expression <em>a ` b</em> can
be extracted by the <code>qty</code> and <code>units</code> functions, respectively.
</p>
<p>A symbol may be declared to be a dimensional quantity,
with specified quantity or specified units or both.
</p>
<p>An expression <em>a ` b `<!-- /@w -->` c</em> converts from unit <code>b</code> to unit <code>c</code>.
<code>ezunits</code> has built-in conversions for SI base units,
SI derived units, and some non-SI units.
Unit conversions not already known to <code>ezunits</code> can be declared.
The unit conversions known to <code>ezunits</code> are specified by the
global variable <code>known_unit_conversions</code>,
which comprises built-in and user-defined conversions.
Conversions for products, quotients, and powers of units are
derived from the set of known unit conversions.
</p>
<p>As Maxima generally prefers exact numbers (integers or rationals)
to inexact (float or bigfloat),
so <code>ezunits</code> preserves exact numbers when they appear
in dimensional quantities.
All built-in unit conversions are expressed in terms of exact numbers;
inexact numbers in declared conversions are coerced to exact.
</p>
<p>There is no preferred system for display of units;
input units are not converted to other units
unless conversion is explicitly indicated.
<code>ezunits</code> recognizes the prefixes m-, k-, M, and G-
(for milli-, kilo-, mega-, and giga-)
as applied to SI base units and SI derived units,
but such prefixes are applied only when indicated by an explicit conversion.
</p>
<p>Arithmetic operations on dimensional quantities are carried out by
conventional rules for such operations.
</p>
<ul>
<li> <em>(x ` a) * (y ` b)</em> is equal to <em>(x * y) ` (a * b)</em>.
</li><li> <em>(x ` a) + (y ` a)</em> is equal to <em>(x + y) ` a</em>.
</li><li> <em>(x ` a)^y</em> is equal to <em>x^y ` a^y</em> when <code>y</code> is nondimensional.
</li></ul>
<p><code>ezunits</code> does not require that units in a sum have the same dimensions;
such terms are not added together, and no error is reported.
</p>
<p><code>ezunits</code> includes functions for elementary dimensional analysis,
namely the fundamental dimensions and fundamental units
of a dimensional quantity,
and computation of dimensionless quantities and natural units.
The functions for dimensional analysis were adapted from similar
functions in another package, written by Barton Willis.
</p>
<p>For the purpose of dimensional analysis,
a list of fundamental dimensions and an associated list of fundamental units
are maintained;
by default the fundamental dimensions are
length, mass, time, charge, temperature, and quantity,
and the fundamental units are the associated SI units,
but other fundamental dimensions and units can be declared.
</p>
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Categories:<a href="maxima_424.html#Category_003a-Physical-units">Physical units</a>
·<a href="maxima_424.html#Category_003a-Share-packages">Share packages</a>
·<a href="maxima_424.html#Category_003a-Package-ezunits">Package ezunits</a>
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<a name="Item_003a-ezunits_002fnode_002fIntroduction-to-physical_005fconstants"></a><hr>
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<p>
Next: <a href="maxima_264.html#Introduction-to-physical_005fconstants" accesskey="n" rel="next">Introduction to physical_constants</a>, Previous: <a href="maxima_262.html#ezunits_002dpkg" accesskey="p" rel="previous">ezunits-pkg</a>, Up: <a href="maxima_262.html#ezunits_002dpkg" accesskey="u" rel="up">ezunits-pkg</a> [<a href="maxima_toc.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="maxima_423.html#Function-and-Variable-Index" title="Index" rel="index">Index</a>]</p>
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