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70 lines
2.7 KiB
70 lines
2.7 KiB
<sect1 id="ai-flux">
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<sect1info>
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<author>
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<firstname>Jasem</firstname>
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<surname>Mutlaq</surname>
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<affiliation><address>
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</address></affiliation>
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</author>
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</sect1info>
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<title>Flux</title>
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<indexterm><primary>Flux</primary>
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<seealso>Luminosity</seealso>
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</indexterm>
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<para>
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The <firstterm>flux</firstterm> is the amount of energy that passes through a unit area each second.
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</para>
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<para>
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Astronomers use flux to denote the apparent brightness of a celestial body. The apparent brightness is defined as the the amount of light received from a star
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above the earth atmosphere passing through a unit area each second. Therefore, the apparent brightness is simply the flux we receive from a star.
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</para>
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<para>
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The flux measures the <emphasis>rate of flow</emphasis> of energy that passes through each cm^2 (or any unit area) of an object's surface each second.
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The detected flux depends on the distance from the source that radiates the energy. This is because the energy has to spread over a volume of space before it reaches us.
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Let us assume that we have an imaginary balloon that encloses a star. Each dot on the balloon represents a unit of energy emitted from the star. Initially, the dots in an area
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of one cm^2 are in close proximity to each other and the flux (energy emitted per square centimeter per second) is high. After a distance d, the volume and surface area of the
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balloon increased causing the dots to <emphasis>spread away</emphasis> from each. Consequently, the number of dots (or energy) enclosed in one cm^2 has decreased as illustrated in Figure 1.
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</para>
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<para>
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<mediaobject>
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<imageobject>
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<imagedata fileref="flux.png" format="PNG"/>
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</imageobject>
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<caption><para><phrase>Figure 1</phrase></para></caption>
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</mediaobject>
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</para>
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<para>
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The flux is inversely proportional to distance by a simple r^2 relation. Therefore, if the distance is doubled, we receive 1/2^2 or 1/4th of the original flux.
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From a fundamental standpoint, the flux is the <link linkend="ai-luminosity">luminosity</link> per unit area:
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<mediaobject>
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<imageobject>
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<imagedata fileref="flux1.png" format="PNG"/>
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</imageobject>
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</mediaobject>
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</para>
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<para>
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where (4 * PI * R^2) is the surface area of a sphere (or a balloon!) with a radius R.
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Flux is measured in Watts/m^2/s or as commonly used by astronomers: Ergs/cm^2/s.
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For example, the luminosity of the sun is L = 3.90 * 10^26 W. That is, in one second the sun radiates 3.90 * 10^26 joules of energy into space. Thus, the flux we receive
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passing through one square centimeter from the sun at a distance of one AU (1.496 * 10^13 cm) is:
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</para>
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<para>
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<mediaobject>
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<imageobject>
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<imagedata fileref="flux2.png" format="PNG"/>
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</imageobject>
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</mediaobject>
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</para>
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</sect1>
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