davisnotes/elec_dipole.html

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<title>Electricity - Electric Dipole - Physics 299</title>
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<h1> <img src="ULPhys1.gif" height="50" align="texttop"
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<h1>Electric Field due to a Dipole<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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To punish me for my contempt for authority, fate made me an
authority myself"</i></font><br>
Albert Einstein<br>
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<li>An electric dipole consists of two point charges of equal
magnitude, but opposite sign, separated by a short distance.</li>
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<li>The dipole&nbsp; is electrically neutral, but due to the
separation of its charges gives rise to an electric field in its
vicinity.</li>
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<div align="center"><img alt="electric dipole" src="elec_dipole.jpg"
height="347" width="461"><br>
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<li>The electric field at the "field point" is given by&nbsp;
<b>E</b> = <b>E</b><sub>+q</sub> + <b>E</b><sub>-q</sub>.&nbsp;
Note that in adding the two electric fields the y-component
cancels leaving only an x-component given by,</li>
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<div align="center"><img alt="elec dipole eqn1"
src="elec_dipole_eqn1.jpg" height="55" width="261"><br>
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<div align="left">where R is the distance from the centre of
the dipole to the field point and the approximation is
valid when r and R are almost equal.&nbsp; In this case
the dimension of the dipole (a) is small compared to the
field point distance.<br>
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<li>p (=2aq) is called the electric dipole moment.&nbsp;
It's actually a vector pointing from the negative to the
positive charge in the dipole so that,</li>
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<div align="center"><img alt="elec dipole eqn 2"
src="elec_dipole_eqn2.jpg" height="59" width="126"><br>
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<li><img alt="hot" src="hot.gif" height="43"
align="middle" width="79">Many molecules have
charge distributions which can be approximated as an
electric dipole, water being one of the most common.</li>
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<div align="center"><img alt="water molecule"
src="elec_water-molecule-and-dipole-moment.jpg"
height="214" width="603"><br>
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<span style="font-size: 12pt; font-family: &quot;Times New
Roman&quot;;"><span style="color: rgb(255, 0, 0); font-style:
italic;">"How many Astronomers does it take to change a light
bulb ?<br>
None, astronomers prfeer the dark"</span></span><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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