153 lines
6.7 KiB
HTML
153 lines
6.7 KiB
HTML
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<!DOCTYPE html PUBLIC "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html;
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charset=windows-1252">
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<meta name="GENERATOR" content="Mozilla/4.7 [en] (X11; U; OSF1 V4.0
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alpha) [Netscape]">
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<meta name="Author" content="C. L. Davis">
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<title>Electricty - Dielectric Materials - Physics 299</title>
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</head>
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<body style="color: rgb(0, 0, 0); background-color: rgb(255, 255,
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255);" link="#0000ee" alink="#ff0000" bgcolor="#ff0000"
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text="#000000" vlink="#551a8b">
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<center><img src="ULPhys1.gif" height="50" align="texttop"
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width="189"></center>
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<center>
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<h1>Dielectric Materials<br>
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</h1>
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</center>
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<center><img src="celticbar.gif" height="22" width="576"><br>
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<br>
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<font color="#ff0000"><i>
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<meta http-equiv="content-type" content="text/html;
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charset=windows-1252">
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</i></font>
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<div class="quotation"> <font color="#ff0000"><i>"Basic research
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is like shooting an arrow into the air and, where it lands,
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painting a target."</i></font><br>
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</div>
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<div class="quotename"> </div>
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Homer Burton Adkins<br>
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</center>
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<img src="netbar.gif" height="40" align="middle" width="100%"> <br>
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<ul>
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<li>In all our discussions to date we have implicitly assumed that
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our charges have been located in a vacuum or on the surface of
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conductors. We now need to consider how to take into
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account the presence of non-conducting material in the real
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world. Dielectric material is simply another way of saying
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non-conducting material.</li>
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</ul>
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<ul>
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<li><img alt="elec dielec fig2" src="elec_dielec_fig2.jpg"
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height="211" align="right" width="436">Imagine a parallel
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plate capacitor in which a dielectric material is placed between
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the plates (at right below). The dielectric is composed of
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atoms/molecules which contain positive and negative
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charges. The applied electric field between the plates, E<sub>0</sub>,
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will cause the positive and negative charges of the constituent
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atoms/molecules to move slightly in opposite directions
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(right). Electric dipole moments will be "induced" in the
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material, as shown. The net effect will be for charge to
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appear on the surface of the dielectric material as shown.
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The dielectric is said to have been polarized, leading to a
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polarization electric field, E<sub>P</sub>. <br>
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</li>
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</ul>
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<blockquote>
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<div align="left"><img alt="exclamation"
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src="exclamation-icon.gif" height="30" width="31"> In
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conductors (metals) there are (almost) free electrons which will
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move through the material when an electric field is applied,
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generating an electric current.<br>
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</div>
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</blockquote>
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<ul>
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</ul>
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<ul>
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<li><img alt="elec dielec fig1" src="elec_dielec_fig1.jpg"
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height="311" align="right" width="495">The net <b>E</b> field
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between the plates has been reduced, <br>
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</li>
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</ul>
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<div align="center"><img alt="elec dielec eqn1"
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src="elec_dielec_eqn1.png" height="30" width="386"><br>
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<br>
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<blockquote>
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<div align="left">where κ is called the dielectric constant or
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relative permittivity of the medium.<br>
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<br>
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Note that for a vacuum, since E<sub>P</sub> = 0, κ = 1
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and since E<sub>P</sub> < E<sub>0</sub> for all other
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materials κ > 1.<br>
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</div>
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</blockquote>
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<div align="left">
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<ul>
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<li>It is easy to show that for the parallel plate capacitor
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the voltage (p.d) between the plates and the energy stored
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are reduced by a factor κ, whereas the capacitance is
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increased by a factor of κ.</li>
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</ul>
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<ul>
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<li>By application of Gauss's Law to a parallel plate
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capacitor with a dielectric between the plates it can be
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shown that to account for the presence of the dielectric
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Gauss's Law becomes,</li>
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</ul>
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<div align="center"><img alt="elec dielec eqn2"
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src="elec_dielec_eqn2.png" height="60" width="139"><br>
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<blockquote>
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<div align="left">As a general rule when dielectric media is
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present wherever ε<sub>0</sub> appears it must be replaced
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by ε<sub>0</sub>κ.</div>
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</blockquote>
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</div>
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</div>
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<blockquote> </blockquote>
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</div>
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<img src="netbar.gif" height="40" width="100%"> <br>
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<center>
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<p class="MsoNormal"><span style="color: rgb(255, 0, 0);
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font-style: italic;">
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<meta http-equiv="content-type" content="text/html;
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charset=windows-1252">
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</span></p>
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<p><font color="#ff0000"><i>A chemist, a biologist and an
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electrical engineer were on death row waiting to go in the
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electric chair.</i></font></p>
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<font color="#ff0000"><i> </i></font>
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<p><font color="#ff0000"><i>The chemist was brought forward first.
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"Do you have anything you want to say?" asked the
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executioner, strapping him in. "No," replied the chemist.
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The executioner flicked the switch and nothing happened.
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Under this particular State's law, if an execution attempt
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fails, the prisoner is to be released, so the chemist was
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released.</i></font></p>
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<font color="#ff0000"><i> </i></font>
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<p><font color="#ff0000"><i>Then the biologist was brought
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forward. "Do you have anything you want to say?" "No, just
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get on with it." The executioner flicked the switch, and
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again nothing happened, so the biologist was released.</i></font></p>
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<font color="#ff0000"><i> </i></font>
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<p><font color="#ff0000"><i> Then the electrical engineer was
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brought forward. "Do you have anything you want to say?"
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asked the executioner. "Yes," replied the engineer. "If you
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swap the red and the blue wires over, you might make this
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thing work."</i></font></p>
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<img src="celticbar.gif" height="22" width="576"> <br>
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<p><i>Dr. C. L. Davis</i> <br>
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<i>Physics Department</i> <br>
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<i>University of Louisville</i> <br>
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<i>email</i>: <a href="mailto:c.l.davis@louisville.edu">c.l.davis@louisville.edu</a>
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<br>
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</p>
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<p><img src="header-index.gif" height="51" width="92"> </p>
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</center>
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<p><br>
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</p>
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</body>
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</html>
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