davisnotes/elec_dipexte.html

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<title>Electricity - Electric Dipole in an External Field - Physics
299</title>
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<h1> <img src="ULPhys1.gif" height="50" align="texttop"
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<h1>Electric Dipole in an External Electric Field <br>
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<center><img src="celticbar.gif" height="22" width="576"><br>
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<font color="#ff0000"><i>"</i></font><font color="#ff0000"><i>
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When you look at yourself from a universal standpoint,
something inside always reminds or informs you that there are
bigger and better things to worry about."</i></font><br>
Albert Einstein<br>
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<img src="netbar.gif" height="40" align="middle" width="100%"> <br>
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<li>We have already considered the electric field <i><b>created</b></i>
by an electric dipole.&nbsp; Now we consider the behavior of an
electric dipole placed in a uniform (constant) electric field.</li>
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<div align="center"><img alt="elec dip in external field"
src="elec_dip_exte.gif" height="157" width="323"><br>
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<li>Note that since the force on each of the charges are equal
in magnitude but opposite in direction there is <b>no net
force on the dipole</b>.</li>
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<li>However, since the two forces are not concurrent, there is
a non-zero torque about the center of the dipole given by,</li>
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<div align="center"><img alt="elec dip ext e eqn1"
src="elec_dipexte_eqn1.jpg" height="45" width="117"><br>
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<li>Using the definition of the work done by a torque
(rotational force), it can be shown that the&nbsp;
electrical potential energy stored by a dipole in an
external field is given by,</li>
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<div align="center"><img alt="elec dip in ext e eqn2"
src="elec_dipexte_eqn2.jpg" height="41" width="117"><br>
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<li><img alt="hot" src="hot.gif" height="43"
align="middle" width="79">A dipole placed in a
uniform electric field will rotate until it is
aligned "-" to "+" along the field - this is the
lowest energy configuration.</li>
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<li><img alt="exclamation" src="exclamation-icon.gif"
height="30" width="31">&nbsp; If the external
field is not uniform, the net force will not be
zero.<br>
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<font color="#ff0000"><i>What is a quantum particle?
The dreams that stuff is made of!</i></font><br>
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<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>
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