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207 lines
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HTML
207 lines
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HTML
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<article id="content" role="main">
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<!-- #BeginEditable "Body" -->
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<h1 class="center">Power</h1>
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<p class="center"><img src="images/lightning.jpg" alt="lightning" height="249" width="480"></p>
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<p>Power is energy flowing! It is measured as <b>energy per unit of time</b></p>
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<ul>
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<li>Energy is in <b>Joules</b> (J)</li>
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<li>Power is in <b>Joules per second</b> (J/s) which has the special unit of <b>Watts</b> (W)</li>
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</ul>
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<p class="def center large">1 Watt = 1 Joule of energy per second<br>
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<br>
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1 W = 1 J/s</p>
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<h2>Power Formulas</h2>
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<p class="center"><img src="images/power-drink.jpg" alt="power drink" height="200" width="143"><br>
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<b>No</b>, not that type of Power Formula!</p>
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<p>Power is the <b>rate of energy per time</b>:</p>
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<p class="center large">P = <span class="intbl">
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<em>Energy</em>
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<strong>Time
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</strong></span></p>
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<div class="example">
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<h3>Example: 3000 J of energy is used in 20 seconds, what is the power?</h3>
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<p class="center larger">P = <span class="intbl">
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<em>3000 J</em>
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<strong>20 s
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</strong></span> = 150 W</p>
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</div>
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<p>We can also change the formula this way:</p>
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<table align="center" width="90%" border="0">
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<tbody>
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<tr>
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<td align="right"><br></td>
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<td> </td>
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<td class="large">P = <span class="intbl">
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<em>Energy</em>
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<strong>Time
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</strong></span></td>
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</tr>
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<tr>
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<td align="right">Energy is also Force × Distance, so:</td>
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<td> </td>
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<td class="large">P = <span class="intbl"><em>Force × Distance</em>
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<strong>Time
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</strong></span></td>
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</tr>
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<tr>
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<td align="right">Distance/Time is Velocity, so:</td>
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<td> </td>
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<td class="large">P = Force × Velocity</td>
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</tr>
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</tbody></table>
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<div class="example">
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<p class="center"><img src="images/guys-pull.jpg" alt="guys pull" height="165" width="360"></p>
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<h3>Example: These guys pull with a combined force of 4000 N, and manage a velocity of 0.4 m/s, what power are they using?</h3>
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<p class="center large">P = 4000 N x 0.4 m/s<br>
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= 1600 N m/s<br>
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= 1600 W</p>
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</div>
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<div class="def">
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<p>Hang on! Is <b>N m/s</b> (Newton meter per second) really the same as <b>W</b> (Watt)?</p>
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<p>In fact all these are the same:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<td align="right"> </td>
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<td> </td>
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<td align="center"> Power is:</td>
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</tr>
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<tr>
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<td align="right">Start with:</td>
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<td width="30"> </td>
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<td class="large" align="center">N m/s</td>
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</tr>
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<tr>
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<td align="right">A Newton is <b>kg m/s<sup>2</sup></b>:</td>
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<td> </td>
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<td class="large" align="center">kg m/s<sup>2</sup> m/s</td>
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</tr>
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<tr>
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<td align="right" height="38">Simplify:</td>
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<td> </td>
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<td class="large" align="center">kg m<sup>2</sup>/s<sup>3</sup></td>
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</tr>
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<tr>
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<td align="right">A Joule is <b>kg m<sup>2</sup>/s<sup>2</sup></b>:</td>
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<td> </td>
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<td class="large" align="center">J/s</td>
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</tr>
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<tr>
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<td align="right">A Watt is J/s:</td>
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<td> </td>
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<td class="large" align="center">W</td>
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</tr>
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</tbody></table>
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</div>
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</div>
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<h2>Horsepower</h2>
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<p><b>1 horsepower</b> (hp) is 745.7 W (or 746 W for electrical motors)</p>
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<div class="example">
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<h3>Example: Sam's car has a 200 hp engine. What is that in watts?</h3>
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<p>200 × 745.7 = 149,140 W = <b>149 kW</b> (to nearest kW)</p>
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</div>
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<h2>What is a kiloWatt Hour?</h2>
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<p><span class="center"><b>1 kWh</b> is the <b>Energy</b> used when the Power is 1 kW for 1 hour. </span></p>
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<div class="example">
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/bar-radiator.jpg" alt="bar radiator" height="134" width="200"></p>
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<h3>This is a little <b>1 kW</b> radiator. How much energy does it use in an hour?</h3>
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<ul>
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<li>1 kW is 1000 Watts which is <b>1000 J/s</b></li>
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<li>There are 3600 seconds in an hour</li>
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</ul>
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<p>So in an hour it uses:</p>
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<p class="center larger">1000 J/s × 3600 s = 3,600,000 J = <b>3.6 MJ</b></p>
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And so <b>1 kWh</b> (kiloWatt hour) is <span class="center"><b>3.6 MJ</b> of <b>Energy</b></span></div>
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<p> </p>
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<div class="questions">
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<script>getQ(17609, 17611, 17613, 17615, 17619, 17616, 17618, 17622, 17620, 17628);</script> </div>
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<div class="related">
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<a href="index.html">Physics Index</a>
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