167 lines
5.7 KiB
HTML
167 lines
5.7 KiB
HTML
<!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"
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content="text/html; charset=ISO-8859-1">
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<meta name="GENERATOR"
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content="Mozilla/4.7 [en] (X11; U; OSF1 V4.0 alpha) [Netscape]">
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<meta name="Author" content="C. L. Davis">
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<title>Electricity - Electric Field - Physics 299</title>
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</head>
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<body style="color: rgb(0, 0, 0); background-color: rgb(255, 255, 255);"
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alink="#ff0000" link="#0000ee" vlink="#551a8b">
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<center>
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<h1> <img src="ULPhys1.gif" align="texttop" height="50" width="189"></h1>
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</center>
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<center>
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<h1>Electric Field</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>"Discovery consists of seeing what everybody
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has
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seen and thinking what nobody has thought"</i></font><br>
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Albert von Szent-Gyorgyi<br>
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</center>
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<img src="netbar.gif" align="middle" height="40" width="100%"> <br>
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<ul>
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<li>When two charges exert a force on each other, through what means
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is the force transmitted ?</li>
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</ul>
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<div style="margin-left: 40px;">The simplest assumption is "<span
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style="font-style: italic; font-weight: bold;">action-at-a-distance</span>".
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They just "<span style="font-weight: bold; font-style: italic;">know</span>"
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about each other's presence. If one of the charges moves the
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other charge is aware of this immediately. This sounds like
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"magic" with today's scientific understanding. <br>
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The mechanism to remove the magic was proposed by <a
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href="http://www.bbc.co.uk/history/historic_figures/faraday_michael.shtml">
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Michael Faraday </a> <a
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href="http://www.rigb.org/contentControl?action=detail&section=1391&pg=4&filter=pd"><img
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alt="faraday" src="faraday.jpg"
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style="border: 0px solid ; width: 80px; height: 121px;" align="middle"></a>
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- the <span style="font-weight: bold;">electric field</span>.
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Every charge creates its own electric field in the space around it
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(actually the space around it means all space); other charges then
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interact with this field. When a charge moves it creates a
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disturbance in its electric field which is propagated away from the
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charge at the speed of light.<br>
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<br>
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<img style="width: 31px; height: 30px;" alt="exclamation"
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src="exclamation-icon.gif"> In developing his theory of gravitation
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Newton was aware of the same "<span
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style="font-style: italic; font-weight: bold;">action-at-a-distance</span>"
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problem. To solve the problem, in a similar manner, we introduce
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the concept of the gravitational field.<br>
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<br>
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<img style="width: 31px; height: 30px;" alt="exclamation"
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src="exclamation-icon.gif"> Note that the concept of the electric
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field is a convenient construct to describe electromagnetic
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phenomena,
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but its true existence is neither proven nor essential.</div>
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<ul>
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<br>
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<li> The electric field vector, <span style="font-weight: bold;">E</span>,
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is defined in the following way. If a charge, q, feels a force <b><u>F</u></b>,
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then
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the
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electric field, <b><u>E</u></b>, at the location of the charge
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is
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given by</li>
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<center>
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<p><br>
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<b><u><img alt="" src="elec_field_eqn1.gif"
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style="width: 43px; height: 44px;"></u></b><br>
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</p>
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</center>
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<br>
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<li> Units of electric field are Newtons/Coulomb (N/C) or (as
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we shall see later) Volts/metre (V/m).</li>
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<br>
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<li> If the electric field felt by the charge q is due to
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a point
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charge Q, located a distance R from q, then its magnitude is given by</li>
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<br>
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<center>
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<p><img alt="" src="elec_field_eqn2.gif"
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style="width: 55px; height: 41px;"><br>
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</p>
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</center>
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<br>
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<li> The electric field due to many point charges is given by the
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vector
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sum of the fields due to the individual charges</li>
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<br>
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<center>
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<p><b><u>E</u></b> = <b><u>E</u></b><sub>1</sub> +
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<b><u>
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E</u></b><sub> 2</sub> + <b><u>E</u></b> <sub>3</sub> +
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.....</p>
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</center>
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<br>
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<p>In performing this sum the direction of each <b><u>E</u></b>
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is
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the
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same as the force felt by a <b><i>positive</i></b> charge. <br>
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</p>
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<li> The electric field in a region of space can be represented by
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electric field lines otherwise known as "<b><i>lines of force</i></b>".</li>
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<br>
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<center>
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<p><img src="efield_lines1.gif" align="texttop" height="221"
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width="370"> </p>
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</center>
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<br>
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<p>The direction of the electric field line is the same as that of
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the force
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felt by a positive charge. The density of the field lines
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provides a
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measure of the magnitude of the field. FIeld lines alway begin on
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positive
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charges and end on negative charges; they cannot be left 'hanging' in
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empty
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space. <br>
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</p>
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</ul>
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<div style="text-align: center;"><img
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style="width: 745px; height: 254px;" alt="efield_lines3.jpg"
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src="efield_lines3.jpg"><br>
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</div>
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<ul>
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<p><br>
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</p>
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<center>
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<p>The diagrams above display the electric field lines in the
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vicinity of
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two equal point charges.</p>
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</center>
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</ul>
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<div style="text-align: left;"><img src="netbar.gif" height="40"
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width="100%">
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</div>
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<center><br>
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<span style="font-size: 12pt; font-family: "Times New Roman";"><span
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style="color: rgb(255, 0, 0); font-style: italic;">"Two things are
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infinite: the universe and human
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stupidity; and I'm not sure about the universe."</span><br style="">
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<!--[if !supportLineBreakNewLine]-->Albert Einstein</span><br>
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<br>
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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">
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</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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