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</ol> <article id="main-article"> <p><img alt="" src="../../electricity/1-charge-sm-1.jpg" style="float: left; width: 250px; height: 250px;">To understand electrical currents, we must first understand what an electric current consists of and what makes it 'go'.</p> <p>An electric current is the movement of <em>charges</em>. Charges move when they experience an <em>electric force</em>. While charge is the more axiomatic quantity, current is the fundamental quantity with an SI base unit. </p> <hr class="hidden-separator"> <div class="panel panel-turquoise panel-has-colored-body"> <div class="panel-heading"> <div> <p>Key Concepts</p> </div> </div> <div class="panel-body"> <div> <div class="panel panel-has-colored-body panel-has-border panel-turquoise"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Electric charge</p> </div> </div> <div class="panel-body"> <div> <p>Charge is a conserved quantity, like mass. It is measured in Coulombs. The smallest amount of charge that exists independently is 1.6 x 10 <sup>-19</sup> C; this is called the fundamental unit of charge and is the charge associated with the components of the atom. All matter contains charge and all things that are charged have mass; they are inseperable.</p> <p>However, unlike mass (which is always attractive across the universe), charges fall into one of two categories: <em>positive</em> and <em>negative</em>.</p> <p>An atom is uncharged as it contains equal numbers of positive charges and negative charges:</p> <ul> <li>The nucleus is charged by positive protons, with a charge of +1.6 x 10<sup>-19</sup> C.</li> <li>The outer shells are charged by negative electrons, with a charge of -1.6 x 10<sup>-19</sup> C.</li> <li>The number of protons = number of electrons.</li> <li>Neutrons, also contained in the nucleus, are uncharged.</li> <li>A particle that has electrons added or removed is called an <em>ion</em>, a charged particle.</li> </ul> <p style="text-align: center;"><img alt="" src="../../images/electricfield1.png" style="width: 270px; height: 209px;"></p> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-has-border panel-turquoise panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Electric fields and forces</p> </div> </div> <div class="panel-body"> <div> <p>An electric field is a region of space where a charge experiences a force. Electric fields cause a non-contact force between the charges:</p> <ul> <li>Objects with the same type of overall charge (e.g. positive) repel one another</li> <li>Objects with unlike charges (i.e. positive and negative) attract</li> </ul> <p><img class="sibico" src="../../../img/sibico/sl-blue.svg" style="height:1.25em;width: auto;vertical-align:text-bottom" title="SL"> As with graviatational forces (see <a href="../268/gravity.html" title="Gravity">Gravity</a>), electric forces are vector quantities. Just like with gravitational fields, electric fields can be <em>radial</em> or <em>uniform</em>. Here we will consider just radial fields (HL students encounter uniform fields later).The direction of the electric force depends on the electric field.</p> <p>Field lines are drawn to show the direction and strength of the field:</p> <ul> <li>Arrows show the direction of the force experienced by a small positive charge placed in the field</li> <li>The concentration of field lines (i.e. closer together next to the charge) represents the strength of a field</li> </ul> <p style="text-align: center;"><img alt="" src="../../images/efields1.png" style="width: 503px; height: 153px;"></p> <p><img class="sibico" src="../../../img/sibico/exam-style.svg" style="height:1.25em;width: auto;vertical-align:text-bottom" title="Exam-style question"> How could you use field lines to show the repulsion of two charges?</p> <button class="btn btn-xs bg-turquoise showhider"><i class="fa fa-fw fa-plus"></i></button><section class="hiddenbox hidden"> <p style="text-align: center;"><img alt="" src="../../electricity/repel.png" style="width: 257px; height: 193px;"></p> </section> <p>The electron volt is a convenient unit of energy. 1 eV is the amount of energy gained by an electron accelerated through a potential difference of 1 V.</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/173770235"></iframe></div> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-yellow"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Essentials</p> </div> </div> <div class="panel-body"> <div> <div class="panel panel-has-colored-body panel-has-border panel-yellow"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Coulomb's law</p> </div> </div> <div class="panel-body"> <div> <p>Coulombs law tells us which quantities affect the size of the electric force between two point charges (or between charges that can be modelled as having all of the charge at their centres).</p> <p style="text-align: center;"><img alt="" src="../../images/electricfield2.png" style="width: 296px; height: 90px;"></p> <div class="box"> <p>The force experienced by two point charges is directly proportional to the product of their charges and inversely proportional to their separation squared:</p> <h4 style="text-align: center;"><span class="math-tex">\(F_E \propto {{q_1 q_2}\over r^2}\)</span></h4> </div> <p>The proportional sign can be replaced with an equals sign by multiplying by a constant:</p> <div class="box"> <h4 style="text-align: center;"><span class="math-tex">\(F_E=k{{q_1 q_2}\over r^2}\)</span></h4> <p><img class="sibico" src="../../../img/sibico/calc.svg" style="height:1.25em;width: auto;vertical-align:text-bottom" title="Calculator"> <em>k</em> is known as Coulomb's constant or the electric force constant. Its value is 8.99×10<sup>9</sup> N m<sup>2</sup> C<sup>−2</sup>.</p> </div> <p> NB: This is similar to <a href="../269/newtons-law-of-gravitation.html" title="Newton's law of gravitation">Newton's law of gravitation</a>.</p> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-has-border panel-yellow panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Field strength</p> </div> </div> <div class="panel-body"> <div> <p>Electric field strength (<em>E</em>) is defined as the the electric force per unit charge experienced by a small positive <em>test charge</em> placed at the point.</p> <div class="box"> <p style="text-align: center;"><span class="math-tex">\(E={F_E\over q}\)</span></p> </div> <p>In this case, let us assume that the charges in the equation are <em>Q</em> (the charge creating the original field) and <em>q </em>(the charge placed in the existing field), in order to distinguish more easily between them:</p> <h4 style="text-align: center;"><span class="math-tex">\(E = {k{Qq \over r^2} \over q}\)</span></h4> <h4 style="text-align: center;"><span class="math-tex">\(E = {k{Q \over r^2}}\)</span></h4> <p>Electric field strength has the units N C<sup>-1</sup> (since it is defined as force per unit charge).</p> <p><img class="sibico" src="../../../img/sibico/tok.svg" style="height:1.25em;width: auto;vertical-align:text-bottom" title="Theory if Knowledge"> The significance of the <em>test charge</em> is that the electric field strength is calculated as though the placed charge were not there (and indeed, the size of this charge cancels out!). However, if a charge were actually placed there, it would change the shape of the field in that location.</p> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-green"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Test Yourself</p> </div> </div> <div class="panel-body"> <div> <p><em>Use flashcards to practise your recall.</em></p> <div class="tib-flashcard"><a class="show-flashcards btn btn-success btn-xs-block btn-block " data-levels="1" data-mode="Normal" data-topics="537" data-subject-id="6" data-n-flashcards="14" style="text-align:center">Show flashcards</a></div><hr> </div> </div> </div> <div class="page-container panel-self-assessment" data-id="180"> <div class="panel-heading">MY PROGRESS</div> <div class="panel-body understanding-rate"> <div class="msg"></div> <label class="label-lg">Self-assessment</label><p>How much of <strong>Electric fields</strong> have you understood?</p><div class="slider-container text-center"><div id="self-assessment-slider" class="sib-slider self-assessment " data-value="1" data-percentage=""></div></div> <label class="label-lg">My notes</label> <textarea name="page-notes" class="form-control" rows="3" placeholder="Write your notes here..."></textarea> </div> <div class="panel-footer text-xs-center"> <span id="last-edited" class="mb-xs-3"> </span> <div class="actions mt-xs-3"> <button id="save-my-progress" type="button" class="btn btn-sm btn-primary text-center btn-xs-block"> <i class="fa fa-fw fa-floppy-o"></i> Save </button> </div> </div></div> <div id="modal-feedback" class="modal fade" tabindex="-1" role="dialog"> <div class="modal-dialog" role="document"> <div class="modal-content"> <div class="modal-header"> <h4 class="modal-title">Feedback</h4> <button type="button" class="close hidden-xs hidden-sm" data-dismiss="modal" aria-label="Close"> <span aria-hidden="true">×</span> </button> </div> <div class="modal-body"> <div class="errors"></div> <p><strong>Which of the following best describes your feedback?</strong></p> <form method="post" style="overflow: hidden"> <div class="form-group"> <div class="radio"><label style="color: #121212;"><input type="radio" name="feedback-type" value="Recommendation"> Recommend</label></div><div class="radio"><label style="color: #121212;"><input type="radio" name="feedback-type" value="Problem"> Report a problem</label></div><div class="radio"><label style="color: #121212;"><input type="radio" name="feedback-type" value="Improvement"> Suggest an improvement</label></div><div class="radio"><label style="color: #121212;"><input type="radio" name="feedback-type" value="Other"> Other</label></div> </div> <hr> <div class="row"> <div class="col-md-6"> <div class="form-group"> <label for="feedback-name">Name</label> <input type="text" class="form-control" name="feedback-name" placeholder="Name" value=" "> </div> </div> <div class="col-md-6"> <div class="form-group"> <label for="feedback-email">Email address</label> <input type="email" class="form-control" name="feedback-email" placeholder="Email" value="@airmail.cc"> </div> </div> </div> <div class="form-group"> <label for="feedback-comments">Comments</label> <textarea class="form-control" name="feedback-comments" style="resize: vertical;"></textarea> </div> <input type="hidden" name="feedback-ticket" value="082b9c9c4ae3624d"> <input type="hidden" name="feedback-url" value="https://studyib.net/physics/page/180/electric-fields"> <input type="hidden" name="feedback-subject" value="6"> <input type="hidden" name="feedback-subject-name" value="Physics"> <div class="pull-left"> </div> </form> </div> <div class="modal-footer"> <button type="button" class="btn btn-primary btn-xs-block feedback-submit mb-xs-3 pull-right"> <i class="fa fa-send"></i> Send </button> <button type="button" class="btn btn-default btn-xs-block m-xs-0 pull-left" data-dismiss="modal"> Close </button> </div> </div> </div></div> <div id="fc-viewer" class="modal fade modal-flashcard" tabindex="-1" role="dialog"><div class="modal-dialog" role="document"><div class="modal-content"><div class="modal-header" style="background-color: #fafafa;"><div class="row" style="width: 100%;"><div class="col-md-12 tags-heading"><div style="display: flex; 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