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<meta name="Author" content="C. L. Davis">
<title>Electricty - Dielectric Materials - Physics 299</title>
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<center><img src="ULPhys1.gif" height="50" align="texttop"
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<h1>Dielectric Materials<br>
</h1>
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<div class="quotation"> <font color="#ff0000"><i>"Basic research
is like shooting an arrow into the air and, where it lands,
painting a target."</i></font><br>
</div>
<div class="quotename"> </div>
Homer Burton Adkins<br>
</center>
<img src="netbar.gif" height="40" align="middle" width="100%"> <br>
<ul>
<li>In all our discussions to date we have implicitly assumed that
our charges have been located in a vacuum or on the surface of
conductors.&nbsp; We now need to consider how to take into
account the presence of non-conducting material in the real
world.&nbsp; Dielectric material is simply another way of saying
non-conducting material.</li>
</ul>
<ul>
<li><img alt="elec dielec fig2" src="elec_dielec_fig2.jpg"
height="211" align="right" width="436">Imagine a parallel
plate capacitor in which a dielectric material is placed between
the plates (at right below).&nbsp; The dielectric is composed of
atoms/molecules which contain positive and negative
charges.&nbsp; The applied electric field between the plates, E<sub>0</sub>,
will cause the positive and negative charges of the constituent
atoms/molecules to move slightly in opposite directions
(right).&nbsp; Electric dipole moments will be "induced" in the
material, as shown.&nbsp; The net effect will be for charge to
appear on the surface of the dielectric material as shown.&nbsp;
The dielectric is said to have been polarized, leading to a
polarization electric field, E<sub>P</sub>. <br>
</li>
</ul>
<blockquote>
<div align="left"><img alt="exclamation"
src="exclamation-icon.gif" height="30" width="31">&nbsp; In
conductors (metals) there are (almost) free electrons which will
move through the material when an electric field is applied,
generating an electric current.<br>
</div>
</blockquote>
<ul>
</ul>
<ul>
<li><img alt="elec dielec fig1" src="elec_dielec_fig1.jpg"
height="311" align="right" width="495">The net <b>E</b> field
between the plates has been reduced, <br>
</li>
</ul>
<div align="center"><img alt="elec dielec eqn1"
src="elec_dielec_eqn1.png" height="30" width="386"><br>
<br>
<blockquote>
<div align="left">where &#954; is called the dielectric constant or
relative permittivity of the medium.<br>
<br>
Note that for a vacuum, since E<sub>P</sub> = 0,&nbsp; &#954; = 1
and since&nbsp; E<sub>P</sub> &lt; E<sub>0</sub> for all other
materials &#954; &gt; 1.<br>
</div>
</blockquote>
<div align="left">
<ul>
<li>It is easy to show that for the parallel plate capacitor
the voltage (p.d) between the plates and the energy stored
are reduced by a factor &#954;, whereas the capacitance is
increased by a factor of &#954;.</li>
</ul>
<ul>
<li>By application of Gauss's Law to a parallel plate
capacitor with a dielectric between the plates it can be
shown that to account for the presence of the dielectric
Gauss's Law becomes,</li>
</ul>
<div align="center"><img alt="elec dielec eqn2"
src="elec_dielec_eqn2.png" height="60" width="139"><br>
<blockquote>
<div align="left">As a general rule when dielectric media is
present wherever &#949;<sub>0</sub> appears it must be replaced
by &#949;<sub>0</sub>&#954;.</div>
</blockquote>
</div>
</div>
<blockquote> </blockquote>
</div>
<img src="netbar.gif" height="40" width="100%"> <br>
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<p class="MsoNormal"><span style="color: rgb(255, 0, 0);
font-style: italic;">
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charset=windows-1252">
</span></p>
<p><font color="#ff0000"><i>A chemist, a biologist and an
electrical engineer were on death row waiting to go in the
electric chair.</i></font></p>
<font color="#ff0000"><i> </i></font>
<p><font color="#ff0000"><i>The chemist was brought forward first.
"Do you have anything you want to say?" asked the
executioner, strapping him in. "No," replied the chemist.
The executioner flicked the switch and nothing happened.
Under this particular State's law, if an execution attempt
fails, the prisoner is to be released, so the chemist was
released.</i></font></p>
<font color="#ff0000"><i> </i></font>
<p><font color="#ff0000"><i>Then the biologist was brought
forward. "Do you have anything you want to say?" "No, just
get on with it." The executioner flicked the switch, and
again nothing happened, so the biologist was released.</i></font></p>
<font color="#ff0000"><i> </i></font>
<p><font color="#ff0000"><i> Then the electrical engineer was
brought forward. "Do you have anything you want to say?"
asked the executioner. "Yes," replied the engineer. "If you
swap the red and the blue wires over, you might make this
thing work."</i></font></p>
<img src="celticbar.gif" height="22" width="576"> <br>
&nbsp;
<p><i>Dr. C. L. Davis</i> <br>
<i>Physics Department</i> <br>
<i>University of Louisville</i> <br>
<i>email</i>: <a href="mailto:c.l.davis@louisville.edu">c.l.davis@louisville.edu</a>
<br>
&nbsp; </p>
<p><img src="header-index.gif" height="51" width="92"> </p>
</center>
<p><br>
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