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class="btn btn-default btn-block text-xs-center" data-toggle="modal" data-target="#modal-feedback" style="margin-bottom: 10px"><i class="fa fa-send"></i> Feedback</button> </div> </div> <div class="col-md-9" id="main-column"> <h1 class="page_title"> Electrical power <a href="#" class="mark-page-favorite pull-right" data-pid="488" title="Mark as favorite" onclick="return false;"><i class="fa fa-star-o"></i></a> </h1> <ol class="breadcrumb"> <li><a href="../../../physics.html"><i class="fa fa-home"></i></a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><a href="../445/electricity-and-magnetism.html">Electricity and magnetism</a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><a href="../181/electric-circuits.html">Electric circuits</a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><span class="gray">Electrical power</span></li> <span class="pull-right" style="color: #555" title="Suggested study time: 10 minutes"><i class="fa fa-clock-o"></i> 10'</span> </ol> <article id="main-article"> <p><img alt="" src="../../electricity/phone.jpg" style="width: 250px; height: 167px; float: left;">In everyday speech we refer to 'charging' our phone or 'having no charge'. This is incorrect in physics, as charge is conserved and not used up.</p> <p>Instead, we are using electrical energy (the ability to make charges move!). We get billed for the units we consume of electrical power over a given time.</p> <hr class="hidden-separator"> <div class="panel panel-has-colored-body panel-purple"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Summary</p> </div> </div> <div class="panel-body"> <div> <p>Power (in any form) is defined as the rate at which work is done or energy is changed:</p> <p style="text-align: center;"><span class="math-tex">\(P={W\over t}\)</span></p> <p>In electrical circuits, it's often simpler to use <span class="math-tex">\(P=IV\)</span> (to avoid going via three other equations!).</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/210014638"></iframe></div> <p>With practice, you will become fluent in deciding which of the following to use:</p> <p style="text-align: center;"><span class="math-tex">\(P=IV\)</span></p> <p style="text-align: center;"><span class="math-tex">\(P=I^2R\)</span></p> <p style="text-align: center;"><span class="math-tex">\(P={V^2 \over R}\)</span></p> </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><i>Use quizzes to practise application of theory.</i> </p> <br><a class="btn btn-primary btn-block text-center" data-toggle="modal" href="#49d57716"><i class="fa fa-play"></i> START QUIZ!</a><div class="modal fade modal-slide-quiz" id="49d57716"> <div class="modal-dialog" style="width: 95vw; max-width: 960px"> <div class="modal-content"> <div class="modal-header slide-quiz-title"> <h4 class="modal-title" style="width: 100%;"> Electrical power <strong class="q-number pull-right"> <span class="counter">1</span>/<span class="total">1</span> </strong> </h4> </div> <div class="modal-body p-xs-3"> <div class="slide-quiz" data-stats="6-131-488" style="opacity: 0"> <div class="exercise shadow-bottom"><div class="q-question"><p>Which circuit has the brightest bulb?</p><p style="text-align: center;"><img alt="" height="293" src="../../mc-images/img_0113.jpg" width="334"></p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> D</label></p></div><div class="q-explanation"><p>The pd across the bulb in C is 6V the others are all less.</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Which circuit has the brightest bulb?</p><p style="text-align: center;"><img alt="" height="272" src="../../img_0114.jpg" width="332"></p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> D</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> C</label></p></div><div class="q-explanation"><p>PD across the bulbs in D is 2x all the others.</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Which circuit has the brightest bulb?</p><p style="text-align: center;"><img alt="" height="298" src="../../img_0115.jpg" width="354"></p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> D</label></p></div><div class="q-explanation"><p>D has 2x the pd of C and A and C has 0V across the bulb</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>In which 2 circuits are the red bulbs of equal brightness</p><p style="text-align: center;"><img alt="" height="325" src="../../img_0116.jpg" width="365"></p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A and C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> C and D</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A and D</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> B and D</label></p></div><p>Bulbs in B and D have same pd</p><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>What is the order of brightness of these bulbs</p><p style="text-align: center;"><img alt="" height="351" src="../../img_0117.jpg" width="406"></p></div><div class="q-answer"><p><label class="radio"><input type="radio"> BCAD</label></p><p><label class="radio"><input type="radio"> ABCD</label></p><p><label class="radio"><input type="radio"> DACB</label></p><p><label class="radio"><input class="c" type="radio"> DABC</label></p></div><div class="q-explanation"><p>D takes all the current, 2/3 goes through A, 1/3 through B, 1/6 through C.</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Calculate the power dissipated in the bulb</p><p style="text-align: center;"><img alt="" height="276" src="../../img_0118.jpg" width="271"></p></div><div class="q-answer"><p><label class="radio"><input class="c" type="radio"> 72 W</label></p><p><label class="radio"><input type="radio"> 24 W</label></p><p><label class="radio"><input type="radio"> 6 W</label></p><p><label class="radio"><input type="radio"> 36 W</label></p></div><div class="q-explanation"><p>Power = V<sup>2</sup>/R</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Calculate the power dissipated in the bulb</p><p style="text-align: center;"><img alt="" height="204" src="../../img_0119.jpg" width="217"></p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 36 W</label></p><p><label class="radio"><input type="radio"> 2 W</label></p><p><label class="radio"><input class="c" type="radio"> 8 W</label></p><p><label class="radio"><input type="radio"> 18 W</label></p></div><div class="q-explanation"><p>I = 6/3 = 2A</p><p>P = I<sup>2</sup>R = 4 x 2</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Calculate the power dissipated in the bulb</p><p style="text-align: center;"><img alt="" height="257" src="../../img_0120.jpg" width="200"></p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 9 W</label></p><p><label class="radio"><input type="radio"> 3 W</label></p><p><label class="radio"><input type="radio"> 12 W</label></p><p><label class="radio"><input class="c" type="radio"> 18 W</label></p></div><div class="q-explanation"><p>P = V<sup>2</sup>/R = 36/2</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>If the switch is closed which bulbs get brighter?</p><p style="text-align: center;"><img alt="" height="224" src="../../img_0121.jpg" width="251"></p></div><div class="q-answer"><p><label class="radio"><input type="radio"> B only</label></p><p><label class="radio"><input type="radio"> C only</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> C and A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A and B</label></p></div><div class="q-explanation"><p>resistance in circuit gets less so current is more PD across C increases.</p><p>PD across B gets less</p><p>current through A increases</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div> </div> </div> <div class="modal-footer slide-quiz-actions"> <div class=""> <div class="pull-left pull-xs-none mb-xs-3"> <button class="btn btn-default d-xs-none btn-prev"> <i class="fa 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